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PubMed1978

Influence of seed and root exudations on the rhizosphere effect in Sorghum vulgare and Crotalaria juncea.

The influence of seed and root exudations on the rhizosphere effect of Sorghum vulgare and Crotalaria juncea was examined. While the quantitative occurrence of certain morphological and physiological groups of bacteria on the seeds and rhizospheres of the two plant species differed, there were similarities in the occurrence of certain physiological and nutritional groups of bacteria on the seed as well as on the rhizosphere. The occurrence of some genera of fungi on the seeds and in the rhizosphere indicated the influence of both seeds and root exudations on the rhizosphere effect. Increase in root exudations of amino acids and sugars correlated with a concomitant increase in microbial activity in the rhizosphere on the 15th day of growth in both plant species but there was a delayed effect of the exuded chemicals on the rhizosphere microflora after 30 d of plant growth.

Amino Acids↗

PubMed1997

Comprehensive analysis of organic ligands in whole root exudates using nuclear magnetic resonance and gas chromatography-mass spectrometry.

Root exudates in the rhizosphere are vital to the normal life cycle of plants. A key factor is phytometallophores, which function in the nutritional acquisition of iron and zinc and are likely to be important in the uptake of pollutant metals by plants. Unraveling the biochemistry of these compounds is tedious using traditional analyses, which also fall short in providing the overall chemical composition or in detecting unknown or unexpected organic ligands in the exudates. Here, we demonstrate a comprehensive analysis of the exudate composition directly by 1H and 13C multidimensional NMR and silylation GC-MS. The advantages are (a) minimal sample preparation, with no loss of unknown compounds, and reduced net analysis time; (b) structure-based analysis for universal detection and identification; and (c) simultaneous analysis of a large number of constituents in a complex mixture. Using barley root exudates, a large number of common organic and amino acids were identified. Three derivatives of mugineic acid phytosiderophores were also determined, the major one being 3-epihydroxymugineic acid, for which complete 1H and 13C NMR assignments were obtained. Quantification of all major components using these methods revealed a sevenfold increase in total exudation under moderate iron deficiency, with 3-epihydroxymugineic acid comprising approximately 22% of the exudate mixture. As iron deficiency increased, total quantities of exudate per gram of root remained unchanged, but the relative quantity of carbon allocated to phytosiderophore increased to approximately 50% of the total exudate in response to severe iron deficiency.

Gas Chromatography-Mass Spectrometry↗

PubMed2003

Activation of the nodA promoter by the nodD genes of Rhizobium galegae induced by synthetic flavonoids or Galega orientalis root exudate.

Rhizobial nodD genes produce transcriptional regulators that, together with appropriate inducer compounds, activate the other symbiotic nodulation (nod) genes and initiate the nodule formation process. Two nodD homologues, nodD1 and nodD2, are present in the Rhizobium galegae strain HAMBI 1174. In this work we analysed their ability to induce the nodA promoter with synthetic inducers known to activate nod genes in other rhizobia. According to phylogenetic analysis, the inducer-specific carboxy-terminal part of the R. galegae nodD protein sequence groups together with those of Rhizobium leguminosarum and Sinorhizobium meliloti. However, the respective inducer compounds for their NodD proteins are not highly effective with R. galegae nodD products. The best inducer discovered with R. galegae nodD1 was the root exudate of the host plant of R. galegae, Galega orientalis. HPLC analyses revealed the presence of many divergent flavonoid compounds in the G. orientalis root exudate. The most effective HPLC fractions induced R. galegae nodD1 up to the level obtained by intact G. orientalis root exudate while apigenin and luteolin, which were also present in the root exudate, were only moderate inducers. A UV-Vis diode array spectrum of the most active peak indicated that the main inducer present in the G. orientalis root exudate is an unidentified chalcone-type compound. In the Galega-R. galegae interaction the first recognition between the NodD protein and the flavonoid inducer secreted from the roots of Galega is specific for these organisms, and thus partly responsible of the strict host specificity of this symbiosis.

Acyltransferases↗

PubMed2005

Effects of model root exudates on structure and activity of a soil diazotroph community.

Nitrogen fixation is often enhanced in the rhizosphere as compared with bulk soil, due to asymbiotic microorganisms utilizing root exudates as an energy source. We have studied the activity and composition of asymbiotic soil diazotrophs following pulse additions of artificial root exudates and single carbon sources, simulating the situation of bulk soil coming into contact with exudates from growing roots. Artificial root exudates and single sugars rapidly induced nitrogen fixation. The population of potential diazotrophs was studied using universal and group-specific nifH polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis. Reverse transcription PCR of nifH mRNA confirmed that phylotypes with an apparently increasing population size also expressed the nitrogenase system. According to our results, the actively nitrogen-fixing population represents only a fraction of the total diazotroph diversity, and the results of group-specific nifH PCR and phylogenetic analysis of cloned nifH and 16S rRNA gene fragments identified active species that belonged to the genus Azotobacter. Rapid changes of transcriptional activity over time were observed, indicating different growth and activation strategies in different Azotobacter strains. Only sugar-containing substrates were able to induce nitrogen fixation, but substrate concentration and the presence of organic acids may have additional selective effects on the active diazotroph population.

Acetic Acid↗

PubMed2004

Contribution of current carbon assimilation in supplying root exudates of Lolium perenne measured using steady-state C labelling.

Coupling growth of Lolium perenne L. in sterile solution culture with steady-state (13)CO(2) labelling allowed quantification of the contribution of C, assimilated either before or after a specific time point, both to plant compartments and root exudates. Plants were grown for 27 days in atmospheres containing CO(2) with delta(13)C signatures of either -13.5 or -36.1 per thousand. Air supplies to plants were then reciprocally switched to the opposing signature (day 0), plants were destructively harvested and root exudates collected over the next 8 days. Following the switch, C assimilated after day 0 and transported to the roots initially only appeared in root tips, later appearing in both tip and non-tip material. The delta(13)C signature of the root exudate changed exponentially with time. Assimilation pre- and post-day 0 contributed equally to exudate C at 4.5 days. Beyond day 8, assimilation pre-day 0 still contributed 41.7% of exudate C. Over all 8 days, a linear relationship existed between the delta(13)C signatures of root tips and exudate, suggesting that all newly assimilated C in the exudate was from root tips. Results imply pulse-labelling approaches to study root exudates are discriminative in the sources of exudates labelled and in the sites from which exudation occurs.

Journal Article↗

PubMed2005

Oxalate and root exudates enhance the desorption of p,p'-DDT from soils.

The abiotic desorption of p,p'-DDT from seven Chinese soils spiked with p,p'-DDT and the effects of oxalate at 0.001-0.1M and the root exudates of maize, wheat, and ryegrass were evaluated using batch experiments. Soil organic carbon played a predominant role in the retention of DDT. Oxalate significantly increased the desorption of p,p'-DDT, with the largest increments ranging from 11% to 54% for different soils. Oxalate addition also resulted in the increased release of dissolved organic carbon and inorganic ions from soils. Root exudates had similar effects to those of oxalate. Root exudates significantly increased DDT desorption from the soils, and the general trend was similar among the plant species studied for all the soils (p > 0.05). Low molecular weight dissolved organic carbon amendments caused partial dissolution of the soil structure, such as the organo-mineral linkages, resulting in the release of organic carbon and metal ions and thus the subsequent enhanced desorption of DDT from the soils. The enhancing effects of oxalate and root exudates on DDT desorption were influenced by the contents of soil organic carbon and dissolved organic carbon in soils.

Adsorption↗

PubMed2005

[Cytokinin changes of root exudates, leaves and grains in inter-subspecific hybrid rice].

Using inter-subspecific hybrid rice of IIyou 2070 and IIyou 419 , their restorer lines 2070 and Zhong419, the various forms and levels of root-derived cytokinins(CTKs) in root exudates, and changes of various forms of cytokinins in rice flag leaves and grains were analyzed by HPLC and ELISA after initial heading stage. The results indicated that zeatin had 62.8% to 89.1% of total cytokinins and was the dominant form of cytokinins in root exudates during early grain filling, whereas, the contents of diHZ and diHZR greatly increased during milk ripening. This change was consistent with the dynamic changes of ZRs (Z+ZR)and diHZRs (diHZ+diHZR)in flag leaves of IIyou 2070 and 2070. Different from main cytokinin composition in root exudates, IPAs took high proportion of total CTKs in leaf and grain. We suggested that there were changes of various forms and activities of root-derived cytokinins in root exudates, and this change was related to catabolic enzymes of cytokinins and the different expression of enzyme genes. Meanwhile, the relationship between cytokinin changes of vigorous and weak spikelets and grain filling was disscused.

Chromatography, High Pressure Liquid↗

PubMed2004

[Expression and root exudation of Cry1Ab toxin protein in cry1Ab transgenic rice and its residue in rhizosphere soil].

The expression and root exudation of Cry1Ab toxin protein in cry1Ab transgenic rice (KMD) and its residue in rhizosphere soil were investigated. The results showed that content of Cry1Ab toxin protein in the shoot and root of KMD were 3.23-8.22 and 0.68-0.89 microg/g from early tillering to maturing stage, respectively. The content of the Cry1Ab toxin proteins via root exudation were 1.66-48.02 ng/(individual x d) at that phase. The results also demonstrated that the residue of the Cry1Ab toxin protein in rhizosphere soil was less than detectable level (0.5 ng/g air-dried soil). In addition, insecticidal bioassay indicated that rhizosphere soil and its extraction solution had no significant effect on mortality of first- and third-instar larvae of cotton bollworm (Heliochis armigera). It suggested that the KMD straws returned to the field might have the higher potential environmental risk than the root exudation.

Animals↗

PubMed2000

[Continuous cropping obstacle and rhizospheric microecology. II. Root exudates and phenolic acids].

This paper discussed the effect of main crop root exudates the relationship between the kinds and amounts of root exudates and the growth of different kinds of crops and their environments. From the aspects of the decompostion of crop residues and the excretion of root systems, the source, form and adsorption mechanism of soil phenolic acids and their effect on crop growth and soil bio-activity were also elaborated.

Agriculture↗

PubMed1984

Host recognition in the Rhizobium-soybean symbiosis: detection of a protein factor in soybean root exudate which is involved in the nodulation process.

The mechanism of host-symbiont recognition in the soybean-Rhizobium symbiosis was investigated utilizing mutants of R. japonicum defective in nodulation. Soybeans were grown in clear plastic growth pouches allowing the identification of the area on the root most susceptible to Rhizobium nodulation; the area between the root tip (RT) and smallest emergent root hair (SERH). The location of nodules in relation to this developing zone is an indication of the rate of nodule initiation. Nodules were scored as to the distance from the RT mark made at the time of inoculation. Seventy-eight per cent of the plants nodulate above the RT mark when inoculated with the wild type R. japonicum strain 3I1b110 with the average distance of the uppermost nodule being approximately 2 millimeters above the RT mark. These data indicate that the wild type strain initiates nodulation rapidly within the RT-SERH zone following inoculation. However, inoculation with the slow-to-nodulate mutant strain HS111 resulted in 100% of the plants nodulating only below the RT mark with the average distance of the uppermost nodule being approximately 56 millimeters below the RT mark. Thus, mutant strain HS111 is defective in the ability to rapidly initiate infection leading to nodulation within the RT-SERH zone. The location of the nodules suggest that stain HS111 must ;adapt' to the root environment before nodulation can occur. To test this, strain HS111 was incubated in soybean root exudate prior to inoculation. In this case, 68% of the plants nodulated above the RT mark with the average distance of the uppermost nodule being approximately 1 millimeter below the RT mark. Experiments indicated that the change in nodule initiation by strain HS111 brought about by incubation in soybean root exudate was due to a phenotypic, rather than a genotypic change. The half-time of root exudate incubation for strain HS111 necessary for optimal nodulation enhancement was less than 6 hours. Heat sensitivity and trypsin sensitivity of the nodulation enhancement factor(s) in soybean root exudate indicate a protein was involved in the reversal of the delay in nodulation by mutant strain HS111.

Journal Article↗

PubMedDate not supplied

Sesquiterpenoids in root exudates of Solanum aethiopicum.

Five known sesquiterpenoids, solavetivone, lubimin, lubiminoic acid, aethione and lubiminol were isolated from the root exudates recovered from Solanum aethiopicum by a newly proposed method using charcoal. Quantitative analysis of the sesquiterpenoids in the root exudates of S. aethiopicum and S. melongena suggested that relatively large amounts of the sesquiterpenoids were exuded from the roots. Antifungal activity of the sesquiterpenoids against Fusarium oxysporum and Verticillium dahliae was also examined.

Antifungal Agents↗

PubMed2006

Two mire species respond differently to enhanced ultraviolet-B radiation: effects on biomass allocation and root exudation.

Increased ultraviolet-B (UV-B) radiation arising from stratospheric ozone depletion may influence soil microbial communities via effects on plant carbon allocation and root exudation. Eriophorum angustifolium and Narthecium ossifragum plants, grown in peatland mesocosms consisting of Sphagnum peat, peat pore water and natural microbial communities, were exposed outdoors to enhanced UV-B radiation simulating 15% ozone depletion in southern Scandinavia for 8 wk. Enhanced UV-B increased rhizome biomass and tended to decrease the biomass of the largest root fraction of N. ossifragum and furthermore decreased dissolved organic carbon (DOC) and monocarboxylic acid concentration, which serves as an estimate of net root exudation, in the pore water of the N. ossifragum mesocosms. Monocarboxylic acid concentration was negatively related to the total carbon concentration of N. ossifragum leaves, which was increased by enhanced UV-B. By contrast, enhanced UV-B tended to increase monocarboxylic acid concentration in the rhizosphere of E. angustifolium and its root : shoot ratio. Microbial biomass carbon was increased by enhanced UV-B in the surface water of the E. angustifolium mesocosms. Increased UV-B radiation appears to alter below-ground biomass of the mire plants in species-specific patterns, which in turn leads to a change in the net efflux of root exudates.

Analysis of Variance↗

PubMed2006

The role of root exudates in rhizosphere interactions with plants and other organisms.

The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

Host-Parasite Interactions↗

PubMed1988

Root Exudates of Various Host Plants of Rhizobium leguminosarum Contain Different Sets of Inducers of Rhizobium Nodulation Genes.

Rhizobium promoters involved in the formation of root nodules on leguminous plants are activated by flavonoids in plant root exudate. A series of Rhizobium strains which all contain the inducible Rhizobium leguminosarum nodA promoter fused to the Escherichia coli lacZ gene, and which differ only in the source of the regulatory nodD gene, were recently used to show that the regulatory nodD gene determines which flavonoids are able to activate the nodA promoter (HP Spaink, CA Wijffelman, E Pees, RJH Okker, BJJ Lugtenberg 1987 Nature 328: 337-340). Since these strains therefore are able to discriminate between various flavonoids, they were used to determine whether or not plants that are nodulated by R. leguminosarum produce different inducers. After chromatographic separation of root exudate constituents from Vicia sativa L. subsp. nigra (L.), V. hirsuta (L.) S.F. Gray, Pisum sativum L. cv Rondo, and Trifolium subterraneum L., the fractions were tested with a set of strains containing a nodD gene of R. leguminosarum, R. trifolii, or Rhizobium meliloti, respectively. It appeared that the source of nodD determined whether, and to what extent, the R. leguminosarum nodA promoter was induced. Lack of induction could not be attributed to the presence of inhibitors. Most of the inducers were able to activate the nodA promoter in the presence of one particular nodD gene only. The inducers that were active in the presence of the R. leguminosarum nodD gene were different in each root exudate.

Journal Article↗

PubMed2003

Metabolic profiling of root exudates of Arabidopsis thaliana.

In addition to accumulating biologically active chemicals, plant roots continuously produce and secrete compounds into their immediate rhizosphere. However, the mechanisms that drive and regulate root secretion of secondary metabolites are not fully understood. To enlighten two neglected areas of root biology, root secretion and secondary metabolism, an in vitro system implementing root-specific elicitation over a 48-day time course was developed. After roots of Arabidopsis thaliana had been elicited with salicylic acid, jasmonic acid, chitosan, and two fungal cell wall elicitors, the secondary metabolites subsequently secreted were profiled. High-performance liquid chromatography was used to metabolically profile compounds in the root exudates, and 289 possible secondary metabolites were quantified. The chemical structures of 10 compounds were further characterized by (1)H and (13)C NMR: butanoic acid, trans-cinnamic acid, o-coumaric acid, p-coumaric acid, ferulic acid, p-hydroxybenzamide, methyl p-hydroxybenzoate, 3-indolepropanoic acid, syringic acid, and vanillic acid. Several of these compounds exhibited a wide range of antimicrobial activity against both soil-borne bacteria and fungi at the concentration detected in the root exudates.

Anti-Bacterial Agents↗

PubMed1987

Carbon allocation, root exudation and mycorrhizal colonization of Pinus echinata seedlings grown under CO(2) enrichment.

Increased exudation of carbon compounds from roots may provide a mechanism for enhancement of nutrient availability to plants growing in a CO(2)-enriched atmosphere. Therefore, the effect of atmospheric CO(2) concentration on carbon allocation and root exudation was investigated in Pinus echinata Mill. (shortleaf pine) seedlings. After 34 and 41 weeks, seedlings growing in 695 microl l(-1) CO(2) allocated proportionately more (14)C-labeled photosynthate to fine roots than did seedlings growing in ambient air. This was associated with greater fine root mass and mycorrhizal density in CO(2)-enriched plants after 34 weeks. Exudation of soluble, (14)C-labeled compounds from roots also was greater in these plants at 34 weeks, but the effect of CO(2) concentration on exudation did not persist at 41 weeks.

Journal Article↗

PubMed2006

Effects of the tomato pathogen Fusarium oxysporum f. sp. radicis-lycopersici and of the biocontrol bacterium Pseudomonas fluorescens WCS365 on the composition of organic acids and sugars in tomato root exudate.

The effects of the pathogenic fungus Fusarium oxysporum f. sp. radicis-lycopersici and of the bacterial biocontrol strain Pseudomonas fluorescens WCS365, and of both microbes, on the amounts and composition of root exudate components of tomato plants grown in a gnotobiotic stonewool substrate system were studied. Conditions were selected under which introduction of F. oxysporum f. sp. radicis-lycopersici caused severe foot and root rot, whereas inoculation of the seed with P. fluorescens WCS365 decreased the percentage of diseased plants from 96 to 7%. This is a much better disease control level than was observed in potting soil. Analysis of root exudate revealed that the presence of F. oxysporum f. sp. radicis-lycopersici did not alter the total amount of organic acids, but that the amount of citric acid decreased and that of succinic acid increased compared with the nontreated control. In contrast, in the presence of the P. fluorescens biocontrol strain WCS365, the total amount of organic acid increased, mainly due to a strong increase of the amount of citric acid, whereas the amount of succinic acid decreased dramatically. Under biocontrol conditions, when both microbes are present, the content of succinic acid decreased and the level of citric acid was similar to that in the nontreated control. The amount of sugar was approximately half that of the control sample when either one of the microbes was present alone or when both were present. Analysis of the interactions between the two microbes grown together in sterile tomato root exudate showed that WCS365 inhibited multiplication of F. oxysporum f. sp. radicis-lycopersici, whereas the fungus did not affect the number of CFU of the bacterium.

Carbohydrate Metabolism↗

PubMed2005

Transcriptome profiling of bacterial responses to root exudates identifies genes involved in microbe-plant interactions.

Molecules exuded by plant roots are thought to act as signals to influence the ability of microbial strains to colonize the roots and to survive in the rhizosphere. Differential bacterial responses to signals from different plant species may mediate the selection of specific rhizosphere populations. Very little, however, is known about the effects of plant exudates on patterns of bacterial gene expression. Here, we have tested the concept that plant root exudates modulate expression of bacterial genes involved in establishing microbe-plant interactions. We have examined the influence on the Pseudomonas aeruginosa PA01 transcriptome of exudates from two varieties of sugarbeet that select for genetically distinct pseudomonad populations in the rhizosphere. The response to the two exudates showed only a partial overlap; the majority of those genes with altered expression was regulated in response to only one of the two exudates. Genes with altered expression included those with functions previously implicated in microbe-plant interactions, such as aspects of metabolism, chemotaxis and type III secretion, and a subset with putative or unknown function. Use of a panel of mutants with targeted disruptions allowed us to identify previously uncharacterized genes with roles in the competitive ability of P. aeruginosa in the rhizosphere within this subset. No genes with host-specific effects were identified. Homologues of the genes identified occur in the genomes of both beneficial and pathogenic root-associated bacteria, suggesting that this strategy may help to elucidate molecular interactions that are important for biocontrol, plant growth promotion, and plant pathogenesis.

Beta vulgaris↗