cannawizard
Well-Known Member
**wow.. lovely dankness you got going there
CannaWizard's (AMC) Lounge
- Thread starter cannawizard
- Start date
cannawizard
Well-Known Member
[video=youtube;tw0LMkkJRRc]http://www.youtube.com/watch?v=tw0LMkkJRRc[/video]
--some night time action
--some night time action
cannawizard
Well-Known Member
**this is the full spectrum medical growroom , its a mixture of hps/mh/leds/..plasma
--strains that dont score high on the CBD charts will be dropped, slow process of elimination..
, its a mixture of hps/mh/leds/..plasma --strains that dont score high on the CBD charts will be dropped, slow process of elimination..
cannawizard
Well-Known Member
*current genepool is all THC+ hybrids... finding high CBD strain is harder than it looksbut usefull information..
do you look for anything other than cbd.do you get all your strain testd,if so what strain has scored highest cbd
---so far only harlequin & blueberry(dj shorts) got a decent CBD score
that og branding really gets me especially since the original og is really chemdawg which is a more sativa not an indica. because of the OG and Kush together the original genetic lineage is confused. whats worse is when you know a strain has nothing to do with chemdawg but the still slap the OG on it.
cannawizard
Well-Known Member
Root Exudation and Rhizosphere Biology1
+ Author Affiliations
Fig. 1.A, Representation of the complex interactions mediated by root exudates that take place in the rhizosphere between plant roots and other organisms. Organisms are not drawn to scale. QS, quorum sensing. B, In vitro culture of oca (Oxalis tuberosa) grown in sterile liquid medium under UV light exposure. C, Chemical structure of harmine as determined by1H and C13 NMR analysis. D, Fluorescent root exudates from O. tuberosa were observed bound to the blue germination paper under UV light exposure. E, Soil samples showing fluorescence obtained from greenhouse-grown oca plants. Samples were taken 5 cm from the stem girth of the plant, and the numbers (1
denote the depth by every 1 cm toward the top-layer soil. In vitro-grown oca plants and soil samples collected from oca's rhizosphere were visualized for blue-purplish fluorescence under UV light exposure with a short wave of UV approximately 254 nm.
Although root exudation clearly represents a significant carbon cost to the plant, the mechanisms and regulatory processes controlling root secretion are just now beginning to be examined. Root exudates have traditionally been grouped into low- and high-Mr compounds. However, a systematic study to determine the complexity and chemical composition of root exudates from diverse plant species has not been undertaken. Low-Mr compounds such as amino acids, organic acids, sugars, phenolics, and various other secondary metabolites are believed to comprise the majority of root exudates, whereas high-Mr exudates primarily include mucilage (high-Mr polysaccharides) and proteins.
The rhizosphere is a densely populated area in which the roots must compete with the invading root systems of neighboring plant species for space, water, and mineral nutrients, and with soil-borne microorganisms, including bacteria, fungi, and insects feeding on an abundant source of organic material (Ryan and Delhaize, 2001). Thus, root-root, root-microbe, and root-insect communications are likely continuous occurrences in this biologically active soil zone, but due to the underground nature of roots, these intriguing interactions have largely been overlooked. Root-root and root-microbe communication can either be positive (symbiotic) to the plant, such as the association of epiphytes, mycorrhizal fungi, and nitrogen-fixing bacteria with roots; or negative to the plant, including interactions with parasitic plants, pathogenic bacteria, fungi, and insects. Thus, if plant roots are in constant communication with symbiotic and pathogenic organisms, how do roots effectively carry out this communication process within the rhizosphere?
A large body of knowledge suggests that root exudates may act as messengers that communicate and initiate biological and physical interactions between roots and soil organisms. This update will focus on recent advancements in root exudation and rhizosphere biology.
Next Section
ROOT-RHIZOSPHERE COMMUNICATION
Survival of any plant species in a particular rhizosphere environment depends primarily on the ability of the plant to perceive changes in the local environment that require an adaptive response. Local changes within the rhizosphere can include the growth and development of neighboring plant species and microorganisms. Upon encountering a challenge, roots typically respond by secreting certain small molecules and proteins (Stintzi and Browse, 2000;Stotz et al., 2000). Root secretions may play symbiotic or defensive roles as a plant ultimately engages in positive or negative communication, depending on the other elements of its rhizosphere. In contrast to the extensive progress in studying plant-plant, plant-microbe, and plant-insect interactions that occur in aboveground plant organs such as leaves and stems, very little research has focused on root-root, root-microbe, and root-insect interactions in the rhizosphere. The following sections will examine the communication process between plant roots and other organisms in the rhizosphere.
Root-Root Communication
In natural settings, roots are in continual communication with surrounding root systems of neighboring plant species and quickly recognize and prevent the presence of invading roots through chemical messengers. Allelopathy is mediated by the release of certain secondary metabolites by plant roots and plays an important role in the establishment and maintenance of terrestrial plant communities. It also has important implications for agriculture; the effects may be beneficial, as in the case of natural weed control, or detrimental, when allelochemicals produced by weeds affect the growth of crop plants (Callaway and Aschehoug, 2000). A secondary metabolite secreted by the roots of knapweed (Centaurea maculosa) provides a classic example of root exudates exhibiting negative root-root communication in the rhizosphere. Recently, Bais et al. (2002c) identified (±)-catechin as the root-secreted phytotoxin responsible for the invasive behavior of knapweed in the rhizosphere. Interestingly, (−-catechin was shown to account for the allelochemical activity, whereas (+)-catechin was inhibitory to soil-borne bacteria (Bais et al., 2002c ). In addition to racemic catechin being detected in the exudates of in vitro-grown plants, the compound was also detected in soil extracts from knapweed-invaded fields, which strongly supported the idea that knapweed's invasive behavior is due to the exudation of (−-catechin. Moreover, this study established the biological significance of the exudation of a racemic compound such as catechin, demonstrating that one enantiomer can be responsible for the invasive nature of the plant, whereas the other enantiomer can contribute to plant defense.
Although studies have reported the biosynthesis of the common enantiomer (+)-catechin, little is known regarding the synthesis of (−-catechin or (±)-catechin as natural products. One possibility is that (+)-catechin production is followed by racemization in the root or during the exudation process. Alternatively, there could be a deviation from the normally observed stereo- and enantiospecific biosynthesis steps. The flavonols kaempferol and quercetin are generally perceived as final products, rather than intermediates, in the pathway (Winkel-Shirley, 2001). The correlation of these experiments to the root exudation process has yet to be determined, but the data should provide a starting point for further studies on the characterization of specific committed steps in the synthesis of racemic catechin in knapweed roots.
The above example demonstrates how plants use root-secreted secondary metabolites to regulate the rhizosphere to the detriment of neighboring plants. However, parasitic plants often use secondary metabolites secreted from roots as chemical messengers to initiate the development of invasive organs (haustoria) required for heterotrophic growth (Keyes et al., 2000). Some of the most devastating parasitic plants of important food crops such as maize (Zea mays), sorghum (Sorghum bicolor), millet (Panicum milaceum), rice (Oryza sativa), and legumes belong to the Scrophulariaceae, which typically invade the roots of surrounding plants to deprive them of water, minerals, and essential nutrients (Yoder, 2001). It has been reported that certain allelochemicals such as flavonoids,p-hydroxy acids, quinones, and cytokinins secreted by host roots induce haustorium formation (Estabrook and Yoder, 1998; Yoder, 2001), but the exact structural requirements of the secreted compounds for haustorium induction is not fully understood.
Root-Microbe Communication
Root-microbe communication is another important process that characterizes the underground zone. Some compounds identified in root exudates that have been shown to play an important role in root-microbe interactions include flavonoids present in the root exudates of legumes that activate Rhizobium meliloti genes responsible for the nodulation process (Peters et al., 1986). Although the studies are not yet conclusive, these compounds may also be responsible for vesicular-arbuscular mycorrhiza colonization (Becard et al., 1992, 1995; Trieu et al., 1997). In contrast, survival of the delicate and physically unprotected root cells under continual attack by pathogenic microorganisms depends on a continuous underground chemical warfare mediated by secretion of phytoalexins, defense proteins, and other as yet unknown chemicals (Flores et al., 1999).
The unexplored chemodiversity of root exudates is an obvious place to search for novel biologically active compounds, including antimicrobials. For instance, Bais et al. (2002b)recently identified rosmarinic acid (RA) in the root exudates of hairy root cultures of sweet basil (Ocimum basilicum) elicited by fungal cell wall extracts from Phytophthora cinnamoni. Basil roots were also induced to exude RA by fungal in situ challenge withPythium ultimum, and RA demonstrated potent antimicrobial activity against an array of soil-borne microorganisms includingPseudomonas aeruginosa (Bais et al., 2002b). Similar studies by Brigham et al. (1999) withLithospermum erythrorhizon hairy roots reported cell-specific production of pigmented naphthoquinones upon elicitation, and other biological activity against soil-borne bacteria and fungi. Given the observed antimicrobial activity of RA and naphthoquinones, these findings strongly suggest the importance of root exudates in defending the rhizosphere against pathogenic microorganisms. Moreover, the aforementioned studies complement earlier research that mainly focused on the regulation and production of these compounds by providing valuable insights into the biological importance of RA and shikonin.
Both Gram-negative and -positive bacteria, including important plant pathogenic bacteria such as Erwinia spp.,Pseudomonas spp., and Agrobacterium spp., possess quorum-sensing systems that control the expression of several genes required for pathogenicity (for review, see Fray, 2002). Quorum sensing is a form of cell-cell communication between bacteria mediated by small diffusible signaling molecules (autoinducers); these are generally acylated homo-Ser lactones (AHLs) for Gram-negative bacteria and peptide-signaling molecules for Gram-positive bacteria. Upon reaching a threshold concentration at high-population densities, an auto-inducer then activates transcriptional activator proteins that induce specific genes. Thus, intercellular signals enable a bacterial population to control the expression of genes in response to cell density. A recent review by Fray (2002) reported that AHL-producing transgenic tobacco plants restored pathogenicity to an avirulent AHL-deficient Erwinia carotovora mutant. Root exudates from pea (Pisum sativum) seedlings were found to contain several bioactive components that mimicked AHL signals in well-characterized bacterial reporter strains, stimulating AHL-regulated behaviors in some strains while inhibiting such behaviors in others. The chemical nature of such active mimic secondary metabolites is currently unknown (Teplitski et al., 2000; Knee et al., 2001). However, it was also reported that crude aqueous extracts from several plant species exhibited AHL inhibitory activity. Thus, it is possible that roots may have developed defense strategies by secreting compounds into the rhizosphere that interfere with bacterial quorum-sensing responses such as signal mimics, signal blockers, and/or signal-degrading enzymes, but future studies are required to isolate and characterize these compounds.
Root-Insect Communication
The study of plant-insect interactions mediated by chemical signals has largely been confined to leaves and stems, whereas the study of root-insect communication has remained largely unexplored due to the complexity of the rhizosphere and a lack of suitable experimental systems. However, root herbivory by pests such as aphids can cause significant decreases in yield and quality of important crops including sugar beet (Beta vulgaris), potato (Solanum tuberosum), and legumes (Hutchison and Campbell, 1994). One attempt to study root-insect communication was developed by Wu et al. (1999) using an in vitro coculture system with hairy roots and aphids. In this study, it was observed that aphid herbivory reduced vegetative growth and increased the production of polyacetylenes, which have been reported to be part of the phytoalexin response (Flores et al., 1988). In a more recent study, Bais et al. (2002a) reported the characterization of fluorescent β-carboline alkaloids from the root exudates of O. tuberosa (oca). The main fluorescent compounds were identified as harmine (7-methoxy-1-methyl-β-carboline) and harmaline (3, 4-dihydroharmine; Bais et al., 2002a; Fig. 1, BE). In addition to their fluorescent nature, these alkaloids exhibit strong phototoxicity against a polyphagous feeder,Trichoplusia ni, suggesting their insecticidal activity may be linked to photoactivation (Larson et al., 1988). The Andean highlands, where O. tuberosa is primarily cultivated, are subjected to a high incidence of UV radiation, and it was observed that the strongest fluorescence intensity occurred with oca varieties that showed resistance to the larvae ofMycrotrypes spp., the Andean tuber weevil (Flores et al., 1999). These data suggest that UV light penetrating soil layers could photoactivate fluorescent β-carboline alkaloids secreted by oca roots to create an insecticidal defense response.
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ALTERATION OF SOIL CHARACTERISTICS THROUGH EXUDATION
As a consequence of normal growth and development, a large range of organic and inorganic substances are secreted by roots into the soil, which inevitably leads to changes in its biochemical and physical properties (Rougier, 1981). Various functions have been attributed to root cap exudation including the maintenance of root-soil contact, lubrication of the root tip, protection of roots from desiccation, stabilization of soil micro-aggregates, and selective adsorption and storage of ions (Griffin et al., 1976;Rougier, 1981; Bengough and McKenzie, 1997; Hawes et al., 2000). Root mucilage is a reasonably studied root exudate that is believed to alter the surrounding soil as it is secreted from continuously growing root cap cells (Vermeer and McCully, 1982; Ray et al., 1988; McCully, 1995; Sims et al., 2000). Soil at field capacity typically possesses a matric potential of −5 to −10 kPa (Chaboud and Rougier, 1984). It has been speculated that as the soil dries and its hydraulic potential decreases, exudates will subsequently begin to lose water to soil. When this occurs, the surface tension of the exudates decreases and its viscosity increases. As the surface tension decreases, the ability of the exudates to wet the surrounding soil particles will become greater. In addition, as viscosity increases, the resistance to movement of soil particles in contact with exudates will increase, and a degree of stabilization within the rhizosphere will be achieved. For instance, McCully and Boyer (1997)reported that mucilage from the aerial nodal roots of maize has a water potential of −11 Mpa, indicating a large capacity for water storage when fully hydrated, whereas the mucilage loses water to the soil as it begins to dry.
This speculation supports the idea that root exudates could play a major role in the maintenance of root-soil contact, which is especially important to the plant under drought and drying conditions, when hydraulic continuity will be lost. The largest, most coherent soil rhizosheaths are formed on the roots of grasses in dry soil (Watt et al., 1994). However, sheath formation requires fully hydrated exudates to permeate the surrounding soil particles that are then bonded to the root and each other as the mucilage dries. Young (1995) found that rhizosheath soil was significantly wetter than bulk soil and suggested that exudates within the rhizosheath increase the water-holding capacity of the soil. Furthermore, it has recently been proposed that in dry soil, the source of water to hydrate and expand exudates is the root itself. Modern cryo-scanning microscopy has helped researchers determine that the rhizosheath of a plant is more hydrated in the early morning hours compared with the midday samplings (McCully and Boyer, 1997). This implies that the exudates released from the roots at night allow the expansion of the roots into the surrounding soil. When transpiration resumes, the exudates begin to dry and adhere to the adjacent soil particles. Thus, the rhizosheath is a dynamic region, with cyclic fluctuations in hydration content controlled to some extent by roots.
Taken together, these studies indicate that root exudation plays a major role in maintaining root-soil contact in the rhizosphere by modifying the biochemical and physical properties of the rhizosphere and contributing to root growth and plant survival. However, the exact fate of exuded compounds in the rhizosphere, and the nature of their reactions in the soil, remains poorly understood.
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CELLULAR MECHANISMS OF ROOT EXUDATION
Subcellular Trafficking of Exuded Metabolites
Despite the ecophysiological significance of plant-secreted compounds and the large number of compounds that plant cells produce, very little is currently known about the molecular mechanisms for the trafficking of phytochemicals. In at least some plants, channels are likely to be involved in the secretion of organic acids normally present at high levels in the cytoplasm. A good example is provided by the exudation of citrate, malate, and related organic acids by maize and wheat (Triticum aestivum) in response to high Al3+ concentrations (Ma et al., 2001). However, plants have the potential to express 100,000 compounds, primarily derived from secondary metabolism (Verpoorte, 2000), many of them with cytotoxic activities that would prevent their accumulation in the cytoplasm. The speculation that phytochemicals are transported from the site of synthesis to the site of storage by vesicles or specialized organelles is gaining momentum as evidence accumulates regarding the presence of intracellular bodies in plant cells induced to accumulate large quantities of secondary metabolites (Grotewold, 2001). For example, it has long been known that specific steps of the isoquinoline alkaloid biosynthetic pathway are sequestered in alkaloid vesicles and that pathway intermediates must traffic from one subcellular compartment to another by mechanisms that prevent their free diffusion in the cytosol (Facchini, 2001). Subcellular inclusions that accumulate 3-deoxy anthocyanidin flavonoid phytoalexins are observed in sorghum leaves infected by the fungusColletotrichum graminicola (Snyder and Nicholson, 1990). These inclusions are similar to the anthocyanoplasts observed in maize cells expressing the C1 and R regulators of anthocyanin accumulation (Grotewold et al., 1998).
Root exudates often include phenylpropanoids and flavonoids, presumably synthesized on the cytoplasmic surface of the endoplasmic reticulum (ER; Winkel-Shirley, 2001). For example, the flavone luteolin, secreted by alfalfa (Medicago sativa) seedlings and seed coats, provides one of the signals that induces the nodulation genes in R. meliloti (Peters et al., 1986). Cytotoxic and antimicrobial catechin flavonoids are secreted by the roots of knapweed plants (Bais et al., 2002c). Although the mechanisms by which these compounds are transported from the ER to the plasma membrane are not known, it is possible that they are transported by ER-originating vesicles that fuse to the cell membrane and release their contents.
Vesicles with the above-described properties and containing green autofluorescent compounds have been identified in maize cells ectopically expressing the P regulator of 3-deoxy flavonoid biosynthesis (Grotewold et al., 1998). These vesicles are likely to originate from the ER, as suggested by the presence of green fluorescence inside specific regions of the ER after treatment with brefeldin A. The vesicles fuse and form large green fluorescent bodies that migrate to the surface of the cell and fuse to the cell membrane and release the green fluorescent compound to the cell wall (Grotewold et al., 1998). Interestingly, the accumulation of the green fluorescence in the cell wall is increased by treatment with Golgi-disrupting agents, such as brefeldin A or monensin, suggesting a trans-Golgi network-independent pathway for the secretion of these compounds. Cultured cells of maize ectopically expressing P also accumulate increased quantities of yellow autofluorescent compounds that are targeted to the central vacuole by subcellular structures that resemble anthocyanoplasts (Grotewold et al., 1998). The use of these autofluorescent compounds, or the fluorescent β-carbolines present in exudates ofO. tuberosa roots (Bais et al., 2002a), should greatly increase the opportunities available to study the molecular mechanisms underlying the secretion of phytochemicals.
ATP-Binding Cassette (ABC) Transporter as an Alternative to Vesicular Trafficking
The previous section highlighted the possibility of vesicular trafficking and fusion as a cellular mechanism responsible for root exudation, but could other mechanisms also be responsible once the compounds reach the membrane? For example, the involvement of membrane transporters such as the ABC transporters might be responsible for the secretion of root-secreted compounds. The ABC superfamily of membrane transporters is one of the largest protein families, and its members can be found in animals, bacteria, fungi, and plants. ABC transporters use ATP hydrolysis to actively transport chemically and structurally unrelated compounds from cells (Martinoia et al., 2002). The recent completion of the Arabidopsis genome research project (Arabidopsis Genome Initiative, 2000) revealed that Arabidopsis contains 53 putative ABC transporter genes. However, the protein localization and function of most of these genes are largely unknown (Martinoia et al., 2002). Most of the plant ABC transporters characterized to date have been localized in the vacuolar membrane and are believed to be responsible for the intracellular sequestration of cytotoxins (Theodoulou, 2000).
Currently, very little is known about plant plasma membrane ABC transporters, but the Arabidopsis AtPGP1, localized to the plasma membrane (Sidler et al., 1998), has been shown to be involved in cell elongation by actively pumping auxin from its site of synthesis in the cytoplasm to appropriate cells (Noh et al., 2001). Working on the assumption that plasma membrane ABC transporters might be involved in the secretion of defense metabolites, and their expression may be regulated by the concentration of these metabolites, Jasinski et al. (2002) identified a plasma membrane ABC transporter (NpABC1) from Nicotiana plumbaginifolia by treating cell cultures with various secondary metabolites. Interestingly, addition of sclareolide, an antifungal diterpene produced at the leaf surface of Nicotiana spp. (Baily et al., 1975), resulted in the expression of NpABC1 (Jasinski et al., 2002). These findings suggest that NpABC1 and likely other plasma membrane ABC transporters are involved in the secretion of secondary metabolites involved in plant defense, but further studies are required to positively identify plasma membrane ABC transporters involved in root exudation of specific compounds.
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Spatial Localization of Root Exudates
Major differences in root architecture exist among plant species (Fitter, 1996), and because different root classes of the same plant exploit different portions of the soil and are subject to different external signals, it has been speculated that they may have different metabolic activity. In accordance, it has been observed that nutrient influx by plant roots is heterogeneous in time and space. In the common bean (Phaseolus vulgaris), the basal roots have a consistently higher influx rate of nutrients than the other root classes (i.e. adventitious, lateral, and tap; Liao et al., 2001; Rubio et al., 2001). This characteristic could be beneficial for the plant because basal roots generally explore the topsoil, where the majority of available nutrients are located (Lynch and Brown, 2001). Furthermore, Russell and Sanderson (1967) found a large variation in the phosphorus influx rate among seminal, nodal, and lateral roots of barley (Hordeum vulgare).Kuhllmann and Barraclough (1987) observed that the rates of nitrogen uptake by nodal roots of wheat were up to 6 times higher than those of seminal roots, but the uptake ratio of potassium differed to a much smaller extent among root classes. Despite this large body of evidence linking root architecture with root absorption of nutrients, the effect of root architecture on root exudation has been virtually unexplored.
Another long-standing question is related to the pattern of root exudation along the longitudinal root axis. From the base to the tip, most root classes can be clearly divided into different sections based on marked dissimilarities in their anatomical characteristics (Gilroy and Jones, 2000). These sections are typically the root tip, the elongation zone, the maturation zone, and the matured zone. The root tip includes two subsections: the root cap and the meristematic region. In the elongation zone, located right behind the root tip, no cell division occurs, but there is vigorous cell elongation activity. The next section is the maturation zone, where xylem vessels are completely differentiated. Here, some epidermal cells elongate perpendicularly toward the rhizosphere; these cells are known as the root hairs. After a short period of life, root hairs die and this region becomes the mature zone of the root. The degree of cell vacuolization increases from the root tip (where no cell vacuoles are present) to the base of the root. How this anatomical heterogeneity along the root axis relates to the metabolic activity of the roots has concerned researchers for decades (Prevot and Steward, 1936).
Although the stages of aging correlate well with the metabolic activity of the root, it is widely recognized that the gradual maturation of root tissues along the root axis is not the only source of variation of metabolic activity (Eshel and Waisel, 1996). Although the large carbon demand in the apical zone has been traditionally attributed to high biosynthesis rates, it may also be due to an active root exudation process. In the case of the influx processes, the absorption of sulfur is highest in the elongation zone immediately behind the meristematic region (Holobrada, 1977) and that of iron at the apical zones of the roots. In the case of nitrogen or phosphorus, contrasting results have been found (Colmer and Bloom, 1998).
Much less attention has been focused on the spatial localization of the root exudation process. The scarce information available suggests that the pattern of exudation is not homogeneous along the root axis. Release of phytosiderophores in response to iron deficiencies appears to be concentrated in the apical zones of the root (Marschner et al., 1987). Release of organic anions would also follow a heterogeneous pattern along the root (Hoffland et al., 1989), which is consistent with the presence of a pH gradient from the tip to the base of the root (Fischer et al., 1989). On the other hand, based on the type of soil and its surface resistance, root tips may secrete a battery of compounds to soften the soil to facilitate root growth (Morel et al., 1991). Although such a mechanism has been hypothesized for decades, the chemicals involved in this phenomenon have yet to be identified. An understanding of the spatial and physical localization of the sites of exudation in the roots will facilitate the elucidation of plant-microbe and plant-plant interactions. For instance, external signals from pathogens and invasive plants may determine the zone of the root where the release of exudates takes place. If there is any relationship between the presence of pathogens and invasive plants with the localization of root exudation process, it is virtually unknown at the present time.
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FINAL REMARKS
Due to significant advances in root biology and current National Science Foundation-funded projects on genomics of root-specific traits, roots are no longer considered an unexplored biological frontier. In contrast, knowledge of rhizospheric processes mediated by root exudates has not developed at the same pace. As highlighted in this update, several lines of evidence indicate that root exudates in their various forms may regulate plant and microbial communities in the rhizosphere. It is worth mentioning that most microbes live in the soil, but just a few of these organisms have developed compatible interactions with specific plants to become successful plant pathogens. Instead, the vast majority of microbes exhibit incompatible interactions with plants, which could be explained by the constant and diverse secretion of antimicrobial root exudates. The understanding of the biology of root exudation processes may contribute to devising novel strategies for improving plant fitness and the isolation of novel value-added compounds found in the root exudates.
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Footnotes
LITERATURE CITED
+ Author Affiliations
- 1 Department of Horticulture and Landscape Architecture (T.S.W., H.P.B., J.M.V.),
- 2 Cellular and Molecular Biology Graduate Program (J.M.V.), and
- 3 Graduate Degree Program in Ecology (J.M.V.), Colorado State University, Fort Collins, Colorado 80523; and
- 4 Department of Plant Biology and Plant Biotechnology Center, The Ohio State University, Columbus, Ohio 43210 (E.G.)
Fig. 1.A, Representation of the complex interactions mediated by root exudates that take place in the rhizosphere between plant roots and other organisms. Organisms are not drawn to scale. QS, quorum sensing. B, In vitro culture of oca (Oxalis tuberosa) grown in sterile liquid medium under UV light exposure. C, Chemical structure of harmine as determined by1H and C13 NMR analysis. D, Fluorescent root exudates from O. tuberosa were observed bound to the blue germination paper under UV light exposure. E, Soil samples showing fluorescence obtained from greenhouse-grown oca plants. Samples were taken 5 cm from the stem girth of the plant, and the numbers (1
denote the depth by every 1 cm toward the top-layer soil. In vitro-grown oca plants and soil samples collected from oca's rhizosphere were visualized for blue-purplish fluorescence under UV light exposure with a short wave of UV approximately 254 nm. Although root exudation clearly represents a significant carbon cost to the plant, the mechanisms and regulatory processes controlling root secretion are just now beginning to be examined. Root exudates have traditionally been grouped into low- and high-Mr compounds. However, a systematic study to determine the complexity and chemical composition of root exudates from diverse plant species has not been undertaken. Low-Mr compounds such as amino acids, organic acids, sugars, phenolics, and various other secondary metabolites are believed to comprise the majority of root exudates, whereas high-Mr exudates primarily include mucilage (high-Mr polysaccharides) and proteins.
The rhizosphere is a densely populated area in which the roots must compete with the invading root systems of neighboring plant species for space, water, and mineral nutrients, and with soil-borne microorganisms, including bacteria, fungi, and insects feeding on an abundant source of organic material (Ryan and Delhaize, 2001). Thus, root-root, root-microbe, and root-insect communications are likely continuous occurrences in this biologically active soil zone, but due to the underground nature of roots, these intriguing interactions have largely been overlooked. Root-root and root-microbe communication can either be positive (symbiotic) to the plant, such as the association of epiphytes, mycorrhizal fungi, and nitrogen-fixing bacteria with roots; or negative to the plant, including interactions with parasitic plants, pathogenic bacteria, fungi, and insects. Thus, if plant roots are in constant communication with symbiotic and pathogenic organisms, how do roots effectively carry out this communication process within the rhizosphere?
A large body of knowledge suggests that root exudates may act as messengers that communicate and initiate biological and physical interactions between roots and soil organisms. This update will focus on recent advancements in root exudation and rhizosphere biology.
Next Section
ROOT-RHIZOSPHERE COMMUNICATION
Survival of any plant species in a particular rhizosphere environment depends primarily on the ability of the plant to perceive changes in the local environment that require an adaptive response. Local changes within the rhizosphere can include the growth and development of neighboring plant species and microorganisms. Upon encountering a challenge, roots typically respond by secreting certain small molecules and proteins (Stintzi and Browse, 2000;Stotz et al., 2000). Root secretions may play symbiotic or defensive roles as a plant ultimately engages in positive or negative communication, depending on the other elements of its rhizosphere. In contrast to the extensive progress in studying plant-plant, plant-microbe, and plant-insect interactions that occur in aboveground plant organs such as leaves and stems, very little research has focused on root-root, root-microbe, and root-insect interactions in the rhizosphere. The following sections will examine the communication process between plant roots and other organisms in the rhizosphere.
Root-Root Communication
In natural settings, roots are in continual communication with surrounding root systems of neighboring plant species and quickly recognize and prevent the presence of invading roots through chemical messengers. Allelopathy is mediated by the release of certain secondary metabolites by plant roots and plays an important role in the establishment and maintenance of terrestrial plant communities. It also has important implications for agriculture; the effects may be beneficial, as in the case of natural weed control, or detrimental, when allelochemicals produced by weeds affect the growth of crop plants (Callaway and Aschehoug, 2000). A secondary metabolite secreted by the roots of knapweed (Centaurea maculosa) provides a classic example of root exudates exhibiting negative root-root communication in the rhizosphere. Recently, Bais et al. (2002c) identified (±)-catechin as the root-secreted phytotoxin responsible for the invasive behavior of knapweed in the rhizosphere. Interestingly, (−
-catechin was shown to account for the allelochemical activity, whereas (+)-catechin was inhibitory to soil-borne bacteria (Bais et al., 2002c ). In addition to racemic catechin being detected in the exudates of in vitro-grown plants, the compound was also detected in soil extracts from knapweed-invaded fields, which strongly supported the idea that knapweed's invasive behavior is due to the exudation of (−-catechin. Moreover, this study established the biological significance of the exudation of a racemic compound such as catechin, demonstrating that one enantiomer can be responsible for the invasive nature of the plant, whereas the other enantiomer can contribute to plant defense. Although studies have reported the biosynthesis of the common enantiomer (+)-catechin, little is known regarding the synthesis of (−
-catechin or (±)-catechin as natural products. One possibility is that (+)-catechin production is followed by racemization in the root or during the exudation process. Alternatively, there could be a deviation from the normally observed stereo- and enantiospecific biosynthesis steps. The flavonols kaempferol and quercetin are generally perceived as final products, rather than intermediates, in the pathway (Winkel-Shirley, 2001). The correlation of these experiments to the root exudation process has yet to be determined, but the data should provide a starting point for further studies on the characterization of specific committed steps in the synthesis of racemic catechin in knapweed roots. The above example demonstrates how plants use root-secreted secondary metabolites to regulate the rhizosphere to the detriment of neighboring plants. However, parasitic plants often use secondary metabolites secreted from roots as chemical messengers to initiate the development of invasive organs (haustoria) required for heterotrophic growth (Keyes et al., 2000). Some of the most devastating parasitic plants of important food crops such as maize (Zea mays), sorghum (Sorghum bicolor), millet (Panicum milaceum), rice (Oryza sativa), and legumes belong to the Scrophulariaceae, which typically invade the roots of surrounding plants to deprive them of water, minerals, and essential nutrients (Yoder, 2001). It has been reported that certain allelochemicals such as flavonoids,p-hydroxy acids, quinones, and cytokinins secreted by host roots induce haustorium formation (Estabrook and Yoder, 1998; Yoder, 2001), but the exact structural requirements of the secreted compounds for haustorium induction is not fully understood.
Root-Microbe Communication
Root-microbe communication is another important process that characterizes the underground zone. Some compounds identified in root exudates that have been shown to play an important role in root-microbe interactions include flavonoids present in the root exudates of legumes that activate Rhizobium meliloti genes responsible for the nodulation process (Peters et al., 1986). Although the studies are not yet conclusive, these compounds may also be responsible for vesicular-arbuscular mycorrhiza colonization (Becard et al., 1992, 1995; Trieu et al., 1997). In contrast, survival of the delicate and physically unprotected root cells under continual attack by pathogenic microorganisms depends on a continuous underground chemical warfare mediated by secretion of phytoalexins, defense proteins, and other as yet unknown chemicals (Flores et al., 1999).
The unexplored chemodiversity of root exudates is an obvious place to search for novel biologically active compounds, including antimicrobials. For instance, Bais et al. (2002b)recently identified rosmarinic acid (RA) in the root exudates of hairy root cultures of sweet basil (Ocimum basilicum) elicited by fungal cell wall extracts from Phytophthora cinnamoni. Basil roots were also induced to exude RA by fungal in situ challenge withPythium ultimum, and RA demonstrated potent antimicrobial activity against an array of soil-borne microorganisms includingPseudomonas aeruginosa (Bais et al., 2002b). Similar studies by Brigham et al. (1999) withLithospermum erythrorhizon hairy roots reported cell-specific production of pigmented naphthoquinones upon elicitation, and other biological activity against soil-borne bacteria and fungi. Given the observed antimicrobial activity of RA and naphthoquinones, these findings strongly suggest the importance of root exudates in defending the rhizosphere against pathogenic microorganisms. Moreover, the aforementioned studies complement earlier research that mainly focused on the regulation and production of these compounds by providing valuable insights into the biological importance of RA and shikonin.
Both Gram-negative and -positive bacteria, including important plant pathogenic bacteria such as Erwinia spp.,Pseudomonas spp., and Agrobacterium spp., possess quorum-sensing systems that control the expression of several genes required for pathogenicity (for review, see Fray, 2002). Quorum sensing is a form of cell-cell communication between bacteria mediated by small diffusible signaling molecules (autoinducers); these are generally acylated homo-Ser lactones (AHLs) for Gram-negative bacteria and peptide-signaling molecules for Gram-positive bacteria. Upon reaching a threshold concentration at high-population densities, an auto-inducer then activates transcriptional activator proteins that induce specific genes. Thus, intercellular signals enable a bacterial population to control the expression of genes in response to cell density. A recent review by Fray (2002) reported that AHL-producing transgenic tobacco plants restored pathogenicity to an avirulent AHL-deficient Erwinia carotovora mutant. Root exudates from pea (Pisum sativum) seedlings were found to contain several bioactive components that mimicked AHL signals in well-characterized bacterial reporter strains, stimulating AHL-regulated behaviors in some strains while inhibiting such behaviors in others. The chemical nature of such active mimic secondary metabolites is currently unknown (Teplitski et al., 2000; Knee et al., 2001). However, it was also reported that crude aqueous extracts from several plant species exhibited AHL inhibitory activity. Thus, it is possible that roots may have developed defense strategies by secreting compounds into the rhizosphere that interfere with bacterial quorum-sensing responses such as signal mimics, signal blockers, and/or signal-degrading enzymes, but future studies are required to isolate and characterize these compounds.
Root-Insect Communication
The study of plant-insect interactions mediated by chemical signals has largely been confined to leaves and stems, whereas the study of root-insect communication has remained largely unexplored due to the complexity of the rhizosphere and a lack of suitable experimental systems. However, root herbivory by pests such as aphids can cause significant decreases in yield and quality of important crops including sugar beet (Beta vulgaris), potato (Solanum tuberosum), and legumes (Hutchison and Campbell, 1994). One attempt to study root-insect communication was developed by Wu et al. (1999) using an in vitro coculture system with hairy roots and aphids. In this study, it was observed that aphid herbivory reduced vegetative growth and increased the production of polyacetylenes, which have been reported to be part of the phytoalexin response (Flores et al., 1988). In a more recent study, Bais et al. (2002a) reported the characterization of fluorescent β-carboline alkaloids from the root exudates of O. tuberosa (oca). The main fluorescent compounds were identified as harmine (7-methoxy-1-methyl-β-carboline) and harmaline (3, 4-dihydroharmine; Bais et al., 2002a; Fig. 1, BE). In addition to their fluorescent nature, these alkaloids exhibit strong phototoxicity against a polyphagous feeder,Trichoplusia ni, suggesting their insecticidal activity may be linked to photoactivation (Larson et al., 1988). The Andean highlands, where O. tuberosa is primarily cultivated, are subjected to a high incidence of UV radiation, and it was observed that the strongest fluorescence intensity occurred with oca varieties that showed resistance to the larvae ofMycrotrypes spp., the Andean tuber weevil (Flores et al., 1999). These data suggest that UV light penetrating soil layers could photoactivate fluorescent β-carboline alkaloids secreted by oca roots to create an insecticidal defense response.
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ALTERATION OF SOIL CHARACTERISTICS THROUGH EXUDATION
As a consequence of normal growth and development, a large range of organic and inorganic substances are secreted by roots into the soil, which inevitably leads to changes in its biochemical and physical properties (Rougier, 1981). Various functions have been attributed to root cap exudation including the maintenance of root-soil contact, lubrication of the root tip, protection of roots from desiccation, stabilization of soil micro-aggregates, and selective adsorption and storage of ions (Griffin et al., 1976;Rougier, 1981; Bengough and McKenzie, 1997; Hawes et al., 2000). Root mucilage is a reasonably studied root exudate that is believed to alter the surrounding soil as it is secreted from continuously growing root cap cells (Vermeer and McCully, 1982; Ray et al., 1988; McCully, 1995; Sims et al., 2000). Soil at field capacity typically possesses a matric potential of −5 to −10 kPa (Chaboud and Rougier, 1984). It has been speculated that as the soil dries and its hydraulic potential decreases, exudates will subsequently begin to lose water to soil. When this occurs, the surface tension of the exudates decreases and its viscosity increases. As the surface tension decreases, the ability of the exudates to wet the surrounding soil particles will become greater. In addition, as viscosity increases, the resistance to movement of soil particles in contact with exudates will increase, and a degree of stabilization within the rhizosphere will be achieved. For instance, McCully and Boyer (1997)reported that mucilage from the aerial nodal roots of maize has a water potential of −11 Mpa, indicating a large capacity for water storage when fully hydrated, whereas the mucilage loses water to the soil as it begins to dry.
This speculation supports the idea that root exudates could play a major role in the maintenance of root-soil contact, which is especially important to the plant under drought and drying conditions, when hydraulic continuity will be lost. The largest, most coherent soil rhizosheaths are formed on the roots of grasses in dry soil (Watt et al., 1994). However, sheath formation requires fully hydrated exudates to permeate the surrounding soil particles that are then bonded to the root and each other as the mucilage dries. Young (1995) found that rhizosheath soil was significantly wetter than bulk soil and suggested that exudates within the rhizosheath increase the water-holding capacity of the soil. Furthermore, it has recently been proposed that in dry soil, the source of water to hydrate and expand exudates is the root itself. Modern cryo-scanning microscopy has helped researchers determine that the rhizosheath of a plant is more hydrated in the early morning hours compared with the midday samplings (McCully and Boyer, 1997). This implies that the exudates released from the roots at night allow the expansion of the roots into the surrounding soil. When transpiration resumes, the exudates begin to dry and adhere to the adjacent soil particles. Thus, the rhizosheath is a dynamic region, with cyclic fluctuations in hydration content controlled to some extent by roots.
Taken together, these studies indicate that root exudation plays a major role in maintaining root-soil contact in the rhizosphere by modifying the biochemical and physical properties of the rhizosphere and contributing to root growth and plant survival. However, the exact fate of exuded compounds in the rhizosphere, and the nature of their reactions in the soil, remains poorly understood.
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CELLULAR MECHANISMS OF ROOT EXUDATION
Subcellular Trafficking of Exuded Metabolites
Despite the ecophysiological significance of plant-secreted compounds and the large number of compounds that plant cells produce, very little is currently known about the molecular mechanisms for the trafficking of phytochemicals. In at least some plants, channels are likely to be involved in the secretion of organic acids normally present at high levels in the cytoplasm. A good example is provided by the exudation of citrate, malate, and related organic acids by maize and wheat (Triticum aestivum) in response to high Al3+ concentrations (Ma et al., 2001). However, plants have the potential to express 100,000 compounds, primarily derived from secondary metabolism (Verpoorte, 2000), many of them with cytotoxic activities that would prevent their accumulation in the cytoplasm. The speculation that phytochemicals are transported from the site of synthesis to the site of storage by vesicles or specialized organelles is gaining momentum as evidence accumulates regarding the presence of intracellular bodies in plant cells induced to accumulate large quantities of secondary metabolites (Grotewold, 2001). For example, it has long been known that specific steps of the isoquinoline alkaloid biosynthetic pathway are sequestered in alkaloid vesicles and that pathway intermediates must traffic from one subcellular compartment to another by mechanisms that prevent their free diffusion in the cytosol (Facchini, 2001). Subcellular inclusions that accumulate 3-deoxy anthocyanidin flavonoid phytoalexins are observed in sorghum leaves infected by the fungusColletotrichum graminicola (Snyder and Nicholson, 1990). These inclusions are similar to the anthocyanoplasts observed in maize cells expressing the C1 and R regulators of anthocyanin accumulation (Grotewold et al., 1998).
Root exudates often include phenylpropanoids and flavonoids, presumably synthesized on the cytoplasmic surface of the endoplasmic reticulum (ER; Winkel-Shirley, 2001). For example, the flavone luteolin, secreted by alfalfa (Medicago sativa) seedlings and seed coats, provides one of the signals that induces the nodulation genes in R. meliloti (Peters et al., 1986). Cytotoxic and antimicrobial catechin flavonoids are secreted by the roots of knapweed plants (Bais et al., 2002c). Although the mechanisms by which these compounds are transported from the ER to the plasma membrane are not known, it is possible that they are transported by ER-originating vesicles that fuse to the cell membrane and release their contents.
Vesicles with the above-described properties and containing green autofluorescent compounds have been identified in maize cells ectopically expressing the P regulator of 3-deoxy flavonoid biosynthesis (Grotewold et al., 1998). These vesicles are likely to originate from the ER, as suggested by the presence of green fluorescence inside specific regions of the ER after treatment with brefeldin A. The vesicles fuse and form large green fluorescent bodies that migrate to the surface of the cell and fuse to the cell membrane and release the green fluorescent compound to the cell wall (Grotewold et al., 1998). Interestingly, the accumulation of the green fluorescence in the cell wall is increased by treatment with Golgi-disrupting agents, such as brefeldin A or monensin, suggesting a trans-Golgi network-independent pathway for the secretion of these compounds. Cultured cells of maize ectopically expressing P also accumulate increased quantities of yellow autofluorescent compounds that are targeted to the central vacuole by subcellular structures that resemble anthocyanoplasts (Grotewold et al., 1998). The use of these autofluorescent compounds, or the fluorescent β-carbolines present in exudates ofO. tuberosa roots (Bais et al., 2002a), should greatly increase the opportunities available to study the molecular mechanisms underlying the secretion of phytochemicals.
ATP-Binding Cassette (ABC) Transporter as an Alternative to Vesicular Trafficking
The previous section highlighted the possibility of vesicular trafficking and fusion as a cellular mechanism responsible for root exudation, but could other mechanisms also be responsible once the compounds reach the membrane? For example, the involvement of membrane transporters such as the ABC transporters might be responsible for the secretion of root-secreted compounds. The ABC superfamily of membrane transporters is one of the largest protein families, and its members can be found in animals, bacteria, fungi, and plants. ABC transporters use ATP hydrolysis to actively transport chemically and structurally unrelated compounds from cells (Martinoia et al., 2002). The recent completion of the Arabidopsis genome research project (Arabidopsis Genome Initiative, 2000) revealed that Arabidopsis contains 53 putative ABC transporter genes. However, the protein localization and function of most of these genes are largely unknown (Martinoia et al., 2002). Most of the plant ABC transporters characterized to date have been localized in the vacuolar membrane and are believed to be responsible for the intracellular sequestration of cytotoxins (Theodoulou, 2000).
Currently, very little is known about plant plasma membrane ABC transporters, but the Arabidopsis AtPGP1, localized to the plasma membrane (Sidler et al., 1998), has been shown to be involved in cell elongation by actively pumping auxin from its site of synthesis in the cytoplasm to appropriate cells (Noh et al., 2001). Working on the assumption that plasma membrane ABC transporters might be involved in the secretion of defense metabolites, and their expression may be regulated by the concentration of these metabolites, Jasinski et al. (2002) identified a plasma membrane ABC transporter (NpABC1) from Nicotiana plumbaginifolia by treating cell cultures with various secondary metabolites. Interestingly, addition of sclareolide, an antifungal diterpene produced at the leaf surface of Nicotiana spp. (Baily et al., 1975), resulted in the expression of NpABC1 (Jasinski et al., 2002). These findings suggest that NpABC1 and likely other plasma membrane ABC transporters are involved in the secretion of secondary metabolites involved in plant defense, but further studies are required to positively identify plasma membrane ABC transporters involved in root exudation of specific compounds.
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Spatial Localization of Root Exudates
Major differences in root architecture exist among plant species (Fitter, 1996), and because different root classes of the same plant exploit different portions of the soil and are subject to different external signals, it has been speculated that they may have different metabolic activity. In accordance, it has been observed that nutrient influx by plant roots is heterogeneous in time and space. In the common bean (Phaseolus vulgaris), the basal roots have a consistently higher influx rate of nutrients than the other root classes (i.e. adventitious, lateral, and tap; Liao et al., 2001; Rubio et al., 2001). This characteristic could be beneficial for the plant because basal roots generally explore the topsoil, where the majority of available nutrients are located (Lynch and Brown, 2001). Furthermore, Russell and Sanderson (1967) found a large variation in the phosphorus influx rate among seminal, nodal, and lateral roots of barley (Hordeum vulgare).Kuhllmann and Barraclough (1987) observed that the rates of nitrogen uptake by nodal roots of wheat were up to 6 times higher than those of seminal roots, but the uptake ratio of potassium differed to a much smaller extent among root classes. Despite this large body of evidence linking root architecture with root absorption of nutrients, the effect of root architecture on root exudation has been virtually unexplored.
Another long-standing question is related to the pattern of root exudation along the longitudinal root axis. From the base to the tip, most root classes can be clearly divided into different sections based on marked dissimilarities in their anatomical characteristics (Gilroy and Jones, 2000). These sections are typically the root tip, the elongation zone, the maturation zone, and the matured zone. The root tip includes two subsections: the root cap and the meristematic region. In the elongation zone, located right behind the root tip, no cell division occurs, but there is vigorous cell elongation activity. The next section is the maturation zone, where xylem vessels are completely differentiated. Here, some epidermal cells elongate perpendicularly toward the rhizosphere; these cells are known as the root hairs. After a short period of life, root hairs die and this region becomes the mature zone of the root. The degree of cell vacuolization increases from the root tip (where no cell vacuoles are present) to the base of the root. How this anatomical heterogeneity along the root axis relates to the metabolic activity of the roots has concerned researchers for decades (Prevot and Steward, 1936).
Although the stages of aging correlate well with the metabolic activity of the root, it is widely recognized that the gradual maturation of root tissues along the root axis is not the only source of variation of metabolic activity (Eshel and Waisel, 1996). Although the large carbon demand in the apical zone has been traditionally attributed to high biosynthesis rates, it may also be due to an active root exudation process. In the case of the influx processes, the absorption of sulfur is highest in the elongation zone immediately behind the meristematic region (Holobrada, 1977) and that of iron at the apical zones of the roots. In the case of nitrogen or phosphorus, contrasting results have been found (Colmer and Bloom, 1998).
Much less attention has been focused on the spatial localization of the root exudation process. The scarce information available suggests that the pattern of exudation is not homogeneous along the root axis. Release of phytosiderophores in response to iron deficiencies appears to be concentrated in the apical zones of the root (Marschner et al., 1987). Release of organic anions would also follow a heterogeneous pattern along the root (Hoffland et al., 1989), which is consistent with the presence of a pH gradient from the tip to the base of the root (Fischer et al., 1989). On the other hand, based on the type of soil and its surface resistance, root tips may secrete a battery of compounds to soften the soil to facilitate root growth (Morel et al., 1991). Although such a mechanism has been hypothesized for decades, the chemicals involved in this phenomenon have yet to be identified. An understanding of the spatial and physical localization of the sites of exudation in the roots will facilitate the elucidation of plant-microbe and plant-plant interactions. For instance, external signals from pathogens and invasive plants may determine the zone of the root where the release of exudates takes place. If there is any relationship between the presence of pathogens and invasive plants with the localization of root exudation process, it is virtually unknown at the present time.
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FINAL REMARKS
Due to significant advances in root biology and current National Science Foundation-funded projects on genomics of root-specific traits, roots are no longer considered an unexplored biological frontier. In contrast, knowledge of rhizospheric processes mediated by root exudates has not developed at the same pace. As highlighted in this update, several lines of evidence indicate that root exudates in their various forms may regulate plant and microbial communities in the rhizosphere. It is worth mentioning that most microbes live in the soil, but just a few of these organisms have developed compatible interactions with specific plants to become successful plant pathogens. Instead, the vast majority of microbes exhibit incompatible interactions with plants, which could be explained by the constant and diverse secretion of antimicrobial root exudates. The understanding of the biology of root exudation processes may contribute to devising novel strategies for improving plant fitness and the isolation of novel value-added compounds found in the root exudates.
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Footnotes
- ↵1 This work was supported by the Colorado State University Agricultural Experiment Station (to J.M.V.), by National Science Foundation-Faculty Early Career Development Award (CAREER) (grant no. MCB 0093014 to J.M.V.), by the Invasive Weeds Initiative of the State of Colorado (to J.M.V.), by the Lindbergh Foundation (to J.M.V.), by the Environmental Protection Agency (to J.M.V.), by the U.S. Department of Agriculture-National Research Initiative Competitive Grants Program (grant no. 200201267 to E.G.), and by the National Science Foundation (grant no. MCB 0130062 to E.G.).
- ↵* Corresponding author; email [email protected]; fax 9704917745.
- www.plantphysiol.org/cgi/doi/10.1104/pp.102.019661.
- Received December 23, 2002.
- Revision received February 4, 2003.
- Accepted February 25, 2003.
LITERATURE CITED
- ↵
- Arabidopsis Genome Initiative
CrossRefMedline
- ↵
- Baily JA,
- Carter GA,
- Burden RS,
- Wain RL
CrossRef
- ↵
- Bais HP,
- Loyola Vargas VM,
- Flores HE,
- Vivanco JM
CrossRefWeb of Science
- ↵
- Bais HP,
- Park S-W,
- Stermitz FR,
- Halligan KM,
- Vivanco JM
CrossRefMedlineWeb of Science
- ↵
- Bais HP,
- Walker TS,
- Schweizer HP,
- Vivanco JM
- ↵
- Bais HP,
- Walker TS,
- Stermitz FR,
- Hufbauer RA,
- Vivanco JM
FREE Full Text
- ↵
- Becard G,
- Douds DD,
- Pfeffer PE
Abstract/FREE Full Text
- ↵
- Becard G,
- Taylor LP,
- Douds DD,
- Pfeffer PE,
- Doner LW
- ↵
- Benfey PN,
- Scheres B
- ↵
- Bengough AG,
- McKenzie BM
- ↵
- Brigham LA,
- Michaels PJ,
- Flores HE
Abstract/FREE Full Text
- ↵
- Callaway RM,
- Aschehoug ET
- ↵
- Chaboud A,
- Rougier M
- ↵
- Colmer TD,
- Bloom AJ
A comparison of net NH4+ and NO3- fluxes along roots of rice and maize. Plant Cell Environ 21:240246.
CrossRef
- ↵
- Waisel Y,
- Eshel A,
- Kafkafi U
- Eshel A,
- Waisel Y
- ↵
- Estabrook EM,
- Yoder JI
Plant-plant communications: rhizosphere signaling between parasitic angiosperms and their hosts. Plant Physiol 116:17.
FREE Full Text
- ↵
- Facchini PJ
CrossRefMedlineWeb of Science
- ↵
- Fischer KS,
- Edmeades GO,
- Johnson EC
CrossRef
- ↵
- Waisel EAY,
- Kafkafi U
- Fitter A
- ↵
- Flores HE,
- Pickard JJ,
- Hoy MW
Production of polyacetylenes and thiophenes in heterotrophic and photosynthetic root cultures of Asteraceae. Biol Mol 7:233254.
- ↵
- Flores HE,
- Vivanco JM,
- Loyola-Vargas VM
CrossRefMedline
- ↵
- Fray RG
Abstract/FREE Full Text
- ↵
- Gilroy S,
- Jones DL
CrossRefMedlineWeb of Science
- ↵
- Griffin GJ,
- Hale MG,
- Shay FJ
CrossRef
- ↵
- Grotewold E
- ↵
- Grotewold E,
- Chamberlain M,
- St. Claire G,
- Swenson J,
- Siame BA,
- Butler LG,
- Snook M,
- Bowen B
Engineering secondary metabolism in maize cells by ectopic expression of transcription factors. Plant Cell 10:721740.
Abstract/FREE Full Text
- ↵
- Hawes MC,
- Gunawardena U,
- Miyasaka S,
- Zhao X
CrossRefMedlineWeb of Science
- ↵
- Hoffland E,
- Findenegg GR,
- Nelemans JA
CrossRef
- ↵
- Holobrada M
- ↵
- Hutchison WD,
- Campbell CD
- ↵
- Jasinski M,
- Stukkens Y,
- Degand H,
- Purnell B
- ↵
- Keyes WJ,
- O'Malley RC,
- Kim D,
- Lynn DG
Medline
- ↵
- Knee EM,
- Gong FC,
- Gao M,
- Teplitski M,
- Jones AR,
- Foxworthy A,
- Mort AJ,
- Bauer WD
MedlineWeb of Science
- ↵
- Kuhllmann H,
- Barraclough PB
- ↵
- Larson RA,
- Marley KA,
- Tuveson RW,
- Berenbaum MR
β-Carboline alkaloids: mechanisms of phototoxicity to bacteria and insects. Photochem Photobiol 48:665674.
Medline
- ↵
- Liao H,
- Rubio G,
- Yan X,
- Cao A,
- Brown KM,
- Lynch JP
CrossRefMedlineWeb of Science
- ↵
- Lynch JP,
- Brown KM
CrossRef
- ↵
- Ma JF,
- Ryan PR,
- Delhaize E
CrossRefMedlineWeb of Science
- ↵
- Marschner H
- ↵
- Marschner H,
- Romheld V,
- Kissel M
- ↵
- Martinoia E,
- Klein M,
- Geisler M,
- Bovet L,
- Forestier C,
- Kolukisaoglu U,
- Muller-Rober B,
- Schulz B
CrossRefMedlineWeb of Science
- ↵
- McCully ME
CrossRef
- ↵
- McCully ME,
- Boyer JS
- ↵
- Morel J,
- Habib L,
- Plantureux S,
- Guckert A
- ↵
- Nardi S,
- Concheri G,
- Pizzeghello D,
- Sturaro A,
- Rella R,
- Parvoli G
CrossRef
- ↵
- Noh B,
- Murphy AS,
- Spalding EP
Abstract/FREE Full Text
- ↵
- Peters NK,
- Frost JW,
- Long SR
Abstract/FREE Full Text
- ↵
- Prevot P,
- Steward FC
FREE Full Text
- ↵
- Ray TC,
- Callow JA,
- Kennedy JF
Composition of root mucilage polysaccharide from Lepidium sativum. J Exp Bot 39:12491261.
Abstract/FREE Full Text
- ↵
- Tanner W,
- Loews FA
- Rougier M
- ↵
- Rubio G,
- Walk T,
- Ge Z,
- Yan X,
- Liao H,
- Lynch JP
Abstract/FREE Full Text
- ↵
- Russell RS,
- Sanderson J
Abstract/FREE Full Text
- ↵
- Ryan PR,
- Delhaize E
CrossRefMedlineWeb of Science
- ↵
- Sidler M,
- Hassa P,
- Hasan S,
- Ringli C,
- Dudler R
Involvement of an ABC transporter in a developmental pathway regulating hypocotyls cell elongation in the light. Plant Cell 10:16231636.
Abstract/FREE Full Text
- ↵
- Sims IM,
- Middleton K,
- Lane AG,
- Cairns AJ,
- Bacic A
Medline
- ↵
- Smith DL,
- Fedoroff NV
Abstract/FREE Full Text
- ↵
- Snyder BA,
- Nicholson RL
Abstract/FREE Full Text
- ↵
- Stintzi A,
- Browse J
Abstract/FREE Full Text
- ↵
- Stotz HU,
- Pittendrigh BR,
- Kroymann J,
- Weniger K,
- Fritsche J,
- Bauke A,
- Mitchell-Olds T
Abstract/FREE Full Text
- ↵
- Teplitski M,
- Robinson JB,
- Bauer WD
MedlineWeb of Science
- ↵
- Theodoulou FL
Medline
- ↵
- Flores HE,
- Lynch JP,
- Eissentat D
- Trieu AT,
- Van Buuren ML,
- Harrison MJ
- ↵
- Vermeer J,
- McCully ME
CrossRef
- ↵
- Verpoorte R,
- Alfermann AW
- Verpoorte R
- ↵
- Watt M,
- McCully ME,
- Canny MJ
Abstract
- ↵
- Winkel-Shirley B
FREE Full Text
- ↵
- Wu T,
- Wittkamper J,
- Flores HE
- ↵
- Yoder JI
CrossRefMedlineWeb of Science
- ↵
- Young IM
- P for Two, Sharing a Scarce Resource: Soil Phosphorus Acquisition in the Rhizosphere of Intercropped Species Plant Physiol. 2011 156: 1078-1086.
- Amino Acid Export in Plants: A Missing Link in Nitrogen Cycling Mol Plant 2011 4: 453-463.
- Mrt, a Gene Unique to Fungi, Encodes an Oligosaccharide Transporter and Facilitates Rhizosphere Competency in Metarhizium robertsii Plant Physiol. 2010 154: 1549-1557.
- Influence of Forest Trees on the Distribution of Mineral Weathering-Associated Bacterial Communities of the Scleroderma citrinum Mycorrhizosphere Appl. Environ. Microbiol. 2010 76: 4780-4787.
- Genetic and Metabolic Divergence within a Rhizobium leguminosarum bv. trifolii Population Recovered from Clover Nodules Appl. Environ. Microbiol. 2010 76: 4593-4600.
- Regulation of flagellar, motility and chemotaxis genes in Rhizobium leguminosarum by the VisN/R-Rem cascade Microbiology 2010 156: 1673-1685.
- Multiple adaptive responses of Australian native perennial legumes with pasture potential to grow in phosphorus- and moisture-limited environments Ann Bot 2010 105: 755-767.
- Root Secretion of Phytochemicals in Arabidopsis Is Predominantly Not Influenced by Diurnal Rhythms Mol Plant 2010 3: 491-498.
- Utricularia carnivory revisited: plants supply photosynthetic carbon to traps J Exp Bot 2010 61: 99-103.
- Altered Profile of Secondary Metabolites in the Root Exudates of Arabidopsis ATP-Binding Cassette Transporter Mutants Plant Physiol. 2008 146: 762-771.
- Metabolomics Reveals Novel Pathways and Differential Mechanistic and Elicitor-Specific Responses in Phenylpropanoid and Isoflavonoid Biosynthesis in Medicago truncatula Cell Cultures Plant Physiol. 2008 146: 387-402.
- Progressive Inhibition by Water Deficit of Cell Wall Extensibility and Growth along the Elongation Zone of Maize Roots Is Related to Increased Lignin Metabolism and Progressive Stelar Accumulation of Wall Phenolics Plant Physiol. 2006 140: 603-612.
- Paenibacillus polymyxa Invades Plant Roots and Forms Biofilms Appl. Environ. Microbiol. 2005 71: 7292-7300.
- Structural Complexity, Differential Response to Infection, and Tissue Specificity of Indolic and Phenylpropanoid Secondary Metabolism in Arabidopsis Roots Plant Physiol. 2005 138: 1058-1070.
- Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria Microbiol. Mol. Biol. Rev. 2005 69: 155-194.
- Enhanced Mineralization of [U-14C]2,4-Dichlorophenoxyacetic Acid in Soil from the Rhizosphere of Trifolium pratense Appl. Environ. Microbiol. 2004 70: 4766-4774.
- Characterization of a Root-Specific Arabidopsis Terpene Synthase Responsible for the Formation of the Volatile Monoterpene 1,8-Cineole Plant Physiol. 2004 135: 1956-1966.
cannawizard
Well-Known Member
cannawizard
Well-Known Member
***http://www.erowid.org/plants/cannabis/cannabis_writings4.shtml
Marijuana Resin & THC
An Excerpt from Marijuana Grower's Insider's Guide
Resin itself, or an abundance of resin glands, is not a good indictator of potency. A mj variety may have copious amounts of resin and still be nearly worthless for smoking; another variety may have little apparent resin, but it may be super-potent. This is because much of the resin is made up of inactive ingredients, and the resin might contain mostly inactive cannaboids. Also, the active cannaboids (THC,THCV, etc.) are only necessary in amounts small enough that they may not be major components in the resin. Surprisingly, the few scientific studies that have compared the concentration of resin glands or resin to potency have found that the concentration of resin glands was negatively correlated with the potency (in other words, the fact that a variety of mj is resinous or has a lot of resin glands has little to do with its potency) Experience connoissers actually may look for a non-resinous but very potent variety - who needs cough inducing resin if it doesn't add to the high?
Marijuana Resin & THC
An Excerpt from Marijuana Grower's Insider's Guide
Resin itself, or an abundance of resin glands, is not a good indictator of potency. A mj variety may have copious amounts of resin and still be nearly worthless for smoking; another variety may have little apparent resin, but it may be super-potent. This is because much of the resin is made up of inactive ingredients, and the resin might contain mostly inactive cannaboids. Also, the active cannaboids (THC,THCV, etc.) are only necessary in amounts small enough that they may not be major components in the resin. Surprisingly, the few scientific studies that have compared the concentration of resin glands or resin to potency have found that the concentration of resin glands was negatively correlated with the potency (in other words, the fact that a variety of mj is resinous or has a lot of resin glands has little to do with its potency) Experience connoissers actually may look for a non-resinous but very potent variety - who needs cough inducing resin if it doesn't add to the high?
cannawizard
Well-Known Member
**cool info
Cannabis Neuroprotective?
by Earth Erowid
Nov 2003
Erowid Extracts #5
Citation: Erowid E. "Cannabis Neuroprotective?" Erowid Extracts. Nov 2003;5:7.
Research has begun to accumulate over the past few years showing that cannabinoids are neuroprotective against brain injury resulting from toxins, hypoxia, and head trauma. Cannabinoids are, loosely, chemicals that are similar in structure to the psychoactive components in cannabis and/or chemicals that activate the cannabinoid receptor system in the body. Researchers have found protective effects not only from the plant-derived cannabinoids such as THC, but also from endogenous cannabinoids (those occurring naturally in the body, such as anandamide) and some synthetic pharmaceutical cannabinoids.
The research with the cannabis-source cannabinoids, conducted in mice, rats, and in vitro, has shown remarkable effectiveness in reducing brain damage from injected toxins, hypoxia, and head trauma.1 Other research has found that anandamide levels in the brains of rats naturally rise after brain injury or death and the cannabinoid system may play a primary role in limiting brain damage.2
Because psychoactivity is considered an unwanted side effect, much of the current research is being done with synthetic cannabinoid system agonists. One synthetic cannabinoid, Dexanabinol (HU-211), is already in phase 3 trials (medium scale, involving humans) headed towards governmental approval as a neuroprotective pharmaceutical. Research conducted in Israel that gave 67 patients with serious head trauma either Dexanabinol or placebo confirms similar research in rats showing reduced damage and faster recovery among those receiving the cannabinoids.3 Although other promising head trauma treatments have failed in the demanding and complex phase 3 research trials, many interested in the field of neuroinjury are excited about the findings to date.
The mechanisms by which the cannabinoids reduce damage from both toxic and traumatic injury to the brain are not yet fully understood. Although some researchers have suggested that the cannabinoids may offer protection through a strong antioxidant effect, this is now considered unlikely to account for much of the protection, since cannabinoid-receptor antagonists block the beneficial effects and the doses of the cannabinoids given are very low.
Perhaps the current best guess for how these chemicals provide their protective effects is that their general dampening of neural activity reduces excitotoxicity (damage caused by overly excited neurons). One of the specific ways this happens is through the inhibition of the glutamate system in the brain. The glutamatergic neurons are part of the excitatory system in the brain; inhibiting glutamate reduces the activity of other neurons. At least in some parts of the brain, activation of the CB1 cannabinoid receptor (a specific type of cannabinoid receptor) has been shown to block pre-synaptic release of glutamate. CB1 receptor activation is also known to inhibit certain calcium channels, directly reducing the production of nitric oxide and other potentially damaging reactive oxygen species.1,4
For more information and a more complete list of references, see erowid.org/extracts/n5/cannabinoids.shtml and read the summary article "Cannabinoids and brain injury: therapeutic implications" by Mechoulam et al. 2002.5
References #
Cannabis Neuroprotective?
by Earth Erowid
Nov 2003
Erowid Extracts #5
Citation: Erowid E. "Cannabis Neuroprotective?" Erowid Extracts. Nov 2003;5:7.
Research has begun to accumulate over the past few years showing that cannabinoids are neuroprotective against brain injury resulting from toxins, hypoxia, and head trauma. Cannabinoids are, loosely, chemicals that are similar in structure to the psychoactive components in cannabis and/or chemicals that activate the cannabinoid receptor system in the body. Researchers have found protective effects not only from the plant-derived cannabinoids such as THC, but also from endogenous cannabinoids (those occurring naturally in the body, such as anandamide) and some synthetic pharmaceutical cannabinoids.
The research with the cannabis-source cannabinoids, conducted in mice, rats, and in vitro, has shown remarkable effectiveness in reducing brain damage from injected toxins, hypoxia, and head trauma.1 Other research has found that anandamide levels in the brains of rats naturally rise after brain injury or death and the cannabinoid system may play a primary role in limiting brain damage.2
Because psychoactivity is considered an unwanted side effect, much of the current research is being done with synthetic cannabinoid system agonists. One synthetic cannabinoid, Dexanabinol (HU-211), is already in phase 3 trials (medium scale, involving humans) headed towards governmental approval as a neuroprotective pharmaceutical. Research conducted in Israel that gave 67 patients with serious head trauma either Dexanabinol or placebo confirms similar research in rats showing reduced damage and faster recovery among those receiving the cannabinoids.3 Although other promising head trauma treatments have failed in the demanding and complex phase 3 research trials, many interested in the field of neuroinjury are excited about the findings to date.
The mechanisms by which the cannabinoids reduce damage from both toxic and traumatic injury to the brain are not yet fully understood. Although some researchers have suggested that the cannabinoids may offer protection through a strong antioxidant effect, this is now considered unlikely to account for much of the protection, since cannabinoid-receptor antagonists block the beneficial effects and the doses of the cannabinoids given are very low.
Perhaps the current best guess for how these chemicals provide their protective effects is that their general dampening of neural activity reduces excitotoxicity (damage caused by overly excited neurons). One of the specific ways this happens is through the inhibition of the glutamate system in the brain. The glutamatergic neurons are part of the excitatory system in the brain; inhibiting glutamate reduces the activity of other neurons. At least in some parts of the brain, activation of the CB1 cannabinoid receptor (a specific type of cannabinoid receptor) has been shown to block pre-synaptic release of glutamate. CB1 receptor activation is also known to inhibit certain calcium channels, directly reducing the production of nitric oxide and other potentially damaging reactive oxygen species.1,4
For more information and a more complete list of references, see erowid.org/extracts/n5/cannabinoids.shtml and read the summary article "Cannabinoids and brain injury: therapeutic implications" by Mechoulam et al. 2002.5
References #
- van der Stelt M, Veldhuis WB, Bar PR, Veldink GA, et al. "Neuroprotection by Delta9-tetrahydrocannabinol, the main active compound in marijuana, against ouabain-induced in vivo excitotoxicity." J Neurosci. 2001; 21(17):6475-6479.
- Marsicano G, Goodenough S, Monory K, Hermann H, et al. "CB1 cannabinoid receptors and on-demand defense against excitotoxicity." Science. Oct 3, 2003; 302(5642):84-88.
- Knoller N, Levi L, Shoshan I, Reichenthal E, et al. "Dexanabinol (HU-211) in the treatment of severe closed head injury: a randomized, placebo-controlled, phase II clinical trial." Crit Care Med. Mar 2002; 30(3):548-554.
- Shen M, Piser TM, Seybold VS, Thayer SA. "Cannabinoid receptor agonists inhibit glutamatergic synaptic transmission in rat hippocampal cultures." J Neurosci. Jul 15, 1996; 16(14):4322-4334.
- Mechoulam R, Panikashvili D, Shohami E. "Cannabinoids and brain injury: therapeutic implications." Trends Mol Med. Feb 2002; 8(2):58-61.
Wolverine97
Well-Known Member
**..brainfart...
--if THC+ enzmatically converts the uv waves filtered/absorb towards other uses (inner functions yet unknown, unstudied).. wtf are those "uses"..
--cheers
View attachment 1825369
phase one: absorb UV-b
Phase two: ???
phase three: profit
cannawizard
Well-Known Member
**phase 2: (imho) after the wave is absorbed, the enzymes then assimilate those "particles" into other uses.. (ex: raw energy of some sorts)... which still boggles my mind .. to what...---real crazy science behind that inner workings
**phase three: free to any patient w/ cancer or (any of the 7major dibs listed under SB420).. non-profit thru hard work, Only.
Wolverine97
Well-Known Member
Yeah, sorry it was a South Park reference. Underpants Gnomes, they're a bitch.
cannawizard
Well-Known Member
**haha, i get it now
cannawizard
Well-Known Member
**well im leaning towards a pure sat to do it, lets see if barneys dr. grinspoon can do itim relaxed home slice, i just sayins i wanna see u grow some trichless dank, your the one with a bajillion lights and pots lol
cannawizard
Well-Known Member
**finally got the kessils going, had to make some make-shift poles; then use the clamps to stick'em in place ..upload the pics in a bit.
..upload the pics in a bit.