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PubMed2003

Pseudomonas aeruginosa-plant root interactions. Pathogenicity, biofilm formation, and root exudation.

Pseudomonas aeruginosa is an opportunistic human pathogen capable of forming a biofilm under physiological conditions that contributes to its persistence despite long-term treatment with antibiotics. Here, we report that pathogenic P. aeruginosa strains PAO1 and PA14 are capable of infecting the roots of Arabidopsis and sweet basil (Ocimum basilicum), in vitro and in the soil, and are capable of causing plant mortality 7 d postinoculation. Before plant mortality, PAO1 and PA14 colonize the roots of Arabidopsis and sweet basil and form a biofilm as observed by scanning electron microscopy, phase contrast microscopy, and confocal scanning laser microscopy. Upon P. aeruginosa infection, sweet basil roots secrete rosmarinic acid (RA), a multifunctional caffeic acid ester that exhibits in vitro antibacterial activity against planktonic cells of both P. aeruginosa strains with a minimum inhibitory concentration of 3 microg mL(-1). However, in our studies RA did not attain minimum inhibitory concentration levels in sweet basil's root exudates before P. aeruginosa formed a biofilm that resisted the microbicidal effects of RA and ultimately caused plant mortality. We further demonstrated that P. aeruginosa biofilms were resistant to RA treatment under in vivo and in vitro conditions. In contrast, induction of RA secretion by sweet basil roots and exogenous supplementation of Arabidopsis root exudates with RA before infection conferred resistance to P. aeruginosa. Under the latter conditions, confocal scanning laser microscopy revealed large clusters of dead P. aeruginosa on the root surface of Arabidopsis and sweet basil, and biofilm formation was not observed. Studies with quorum-sensing mutants PAO210 (DeltarhlI), PAO214 (DeltalasI), and PAO216 (DeltalasI DeltarhlI) demonstrated that all of the strains were pathogenic to Arabidopsis, which does not naturally secrete RA as a root exudate. However, PAO214 was the only pathogenic strain toward sweet basil, and PAO214 biofilm appeared comparable with biofilms formed by wild-type strains of P. aeruginosa. Our results collectively suggest that upon root colonization, P. aeruginosa forms a biofilm that confers resistance against root-secreted antibiotics.

Anti-Bacterial Agents↗

PubMed2001

GC-MS analysis of hydrophobic root exudates of sorghum and implications on the parasitic plant Striga asiatica.

Striga asiatica is a parasitic angiosperm that responds to germination stimulants produced by host plants, including many grasses. GC-MS analyses of hydrophobic root exudates of sorghum revealed the root exudates to be composed of fatty acids, resorcinol, and a series of structurally related hydroquinones, three of which were previously unknown. High yields of resorcinol and the hydroquinone series were detected in sorghum. At least one of the hydroquinones induces germination in Striga, and the resorcinol is thought to stabilize the hydroquinones in the soil. The previously unknown series of hydroquinones offers insight into the possible biosynthesis of the components of the exudate and their possible importance in initiating Striga germination.

Gas Chromatography-Mass Spectrometry↗

PubMed1987

Hyphal Elongation of Glomus fasciculatus in Response to Root Exudates.

The spore germination rates on water agar of the vesicular-arbuscular mycorrhizal fungus Glomus fasciculatus were highest at water potentials of -4 to -6 bars. Root exudates from plants grown in a sterile nutrient solution, with or without phosphorus, did not affect germination. Root exudates collected from 2-, 4-, and 6-week-old Trifolium repens cv. ;Ladino' seedlings that were deprived of P enabled hyphal growth from germinated Glomus fasciculatus spores of 21.4, 14.7, and 7.6 mm, respectively. Hyphal elongation in the presence of exudates from plants grown with P, or in the absence of exudates, was negligible (<1 mm). Root P at 2 weeks was not significantly different between plants grown with and without P. There were no significant differences between the quantities of exudates from plants grown with or without P at 2, 4, and 6 weeks. The data suggest that it is the quality of exudates from plants experiencing P deprivation that is important in stimulating vesicular-arbuscular mycorrhizal hyphal elongation.

Journal Article↗

PubMed2006

Influence of plant root exudates, germ tube orientation and passive conidia transport on biological control of fusarium wilt by strains of nonpathogenic Fusarium oxysporum.

In earlier studies, biological control of Fusarium wilt of cucumber induced by Fusarium oxysporum f. sp. cucumerinum was demonstrated using nonpathogenic strains C5 and C14 of Fusarium oxysporum. Strain C14 induced resistance and competed for infection sites whether roots were wounded or intact, whereas strain C5 required wounds to achieve biocontrol. In the current work, additional attributes involved in enhanced resistance by nonpathogenic biocontrol agents strains to Fusarium wilt of cucumber and pea were further investigated. In pre-penetration assays, pathogenic formae specials exhibited a significantly higher percentage of spore germination in 4-day-old root exudates of cucumber and pea than nonpathogens. Also, strain C5 exhibited the lowest significant reduction in spore germination in contrast to strain C14 or control. One-day-old cucumber roots injected with strain C14 resulted in significant reduction in germ tube orientation towards the root surface, 48-96 h after inoculation with F. o. cucumerinum spores, whereas strain C5 induced significantly lower spore orientation of the pathogen and only at 72 and 96 h after inoculation. In post-penetration tests, passive transport of microconidia of pathogenic and nonpathogens in stems from base to apex were examined when severed plant roots were immersed in spore suspension. In repeated trials, strain C5, F. o. cucumerinum and F. o. pisi were consistently isolated from stem tissues of both cucumber and pea at increasing heights over a 17 days incubation period. Strain C14 however, was recovered at a maximum translocation distance of 4.6 cm at day 6 and later height of isolation significantly declined thereafter to 1.2 cm at day 17. In pea stem, the decline was even less. Significant induction of resistance to challenge inoculation by the pathogen in cucumber occurred 72 and 96 h after pre-inoculation with biocontrol agents. Nonetheless, strain C14 induced protection as early as 48 h and the maximum resistance was reached at 96 h. The presented data confirm the previous findings that attributes important for nonpathogenic fusaria to induce resistant are: rapid spore germination and orientation in response to root exudate; active root penetration and passive conidia transport in stem to initiate defence reaction without pathogenicity and enough lag period between induction and challenge inoculation. Strain C14 possesses all these qualifications and hence its ability to enhance host resistance is superior than strain C5.

Biological Transport↗

PubMed1999

Chemotaxis of Rhizobium sp.S2 towards Cajanus cajan root exudate and its major components.

The chemotactic response of Rhizobium sp. S2, a slow-growing Cajanus cajan isolate, towards its host root exudate was examined. Two classes of mutants, one nonchemotactic towards nutrients (amino acids and sugars) and signal compounds like flavonoids and the other, nonchemotactic towards amino acids and sugars but positive towards naringenin, the flavonoid present in Cajanus cajan root exudate, were obtained. The plasmid-cured derivative of the parent showed positive response towards amino acids and sugars but was nonchemotactic towards naringenin. A possible presence of dual chemotaxis pathways, one towards nutrients and the other for sensing signal compounds, was thus demonstrated. The possible involvement of naringenin as a chemoattractant in the preliminary stages of this Rhizobium-legume interaction was also established.

Amino Acids↗

PubMed1981

Membrane-mediated decrease in root exudation responsible for phorphorus inhibition of vesicular-arbuscular mycorrhiza formation.

The mechanism responsible for phosphorus inhibition of vesicular-arbuscular mycorrhiza formation in sudangrass (Sorghum vulgare Pers.) was investigated in a phosphorus-deficient sandy soil (0.5 micrograms phosphorus per gram soil) amended with increasing levels of phosphorus as superphosphate (0, 28, 56, 228 micrograms per gram soil). The root phosphorus content of 4-week-old plants was correlated with the amount of phosphorus added to the soil. Root exudation of amino acids and reducing sugars was greater for plants grown in phosphorus-deficient soil than for those grown in the phosphorus-treated soils. The increase in exudation corresponded with changes in membrane permeability of phosphorus-deficient roots, as measured by K(+) ((86)Rb) efflux, rather than with changes in root content of reducing sugars and amino acids. The roots of phosphorus-deficient plants inoculated at 4 weeks with Glomus fasciculatus were 88% infected after 9 weeks as compared to less than 25% infection in phosphorus-sufficient roots; these differences were correlated with root exudation at the time of inoculation. For plants grown in phosphorus-deficient soil, infection by vesicular-arbuscular mycorrhizae increased root phosphorus which resulted in a decrease in root membrane permeability and exudation compared to nonmycorrhizal plants. It is proposed that, under low phosphorus nutrition, increased root membrane permeability leads to net loss of metabolites at sufficient levels to sustain the germination and growth of the mycorrhizal fungus during pre- and postinfection. Subsequently, mycorrhizal infection leads to improvement of root phosphorus nutrition and a reduction in membrane-mediated loss of root metabolites.

Journal Article↗

PubMed1989

Analysis of the major inducers of the Rhizobium nodA promoter from Vicia sativa root exudate and their activity with different nodD genes.

Root exudate of Vicia sativa contains 7 inducers for the nodA promoter of Rhizobium leguminosarum biovar viciae. Six of these inducers are flavanones. One inducer was identified as 3,5,7,3'-tetrahydroxy-4'-methoxyflavanone, and a second inducer most likely is 7,3'-dihydroxy-4'-methoxyflavanone. The inducing activity of these compounds and the other inducers depends on the nodD gene present in the test strains, which originated either from R. leguminosarum biovars viciae or trifolii, or from R. meliloti. Three inducers are 'common', three others almost exclusively induce the nodA promoter in the presence of the R. leguminosarum biovar viciae nodD gene, and the last one is active with the noD genes of either R. leguminosarum biovar viciae or that of R. meliloti. Testing of a large number of flavonoids revealed two classes of structural features required for inducing ability: (i) features required for induction in general, and (ii), features restricting the inducing ability to (a) specific nodD gene(s). These features are discussed in relation to current models of the process of nodD-mediated transcription activation of the inducible nod genes.

Fabaceae↗

PubMed2002

Isolation and identification of a potent allelopathic substance in rice root exudates.

A search for growth inhibitors in rice root exudates was undertaken in order to clarify the allelopathic system in rice (Oryza sativa L.). Rice seedlings inhibited the growth of cress (Lepidium sativum L.) and lettuce (Lactuca sativa L.) seedlings when the cress and lettuce were grown with rice seedlings. The putative compound causing the inhibitory effect of rice seedlings was isolated from their culture solution, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. Momilactone B inhibited the growth of cress and lettuce seedlings at concentrations greater than 3 and 30 microM, respectively. The concentration of momilactone B was 3.4 and 1.1 nmol per seedling in the culture solutions of husked and non-husked rice seedlings, respectively. These results suggest that rice seedlings may release momilactone B into the environment and the stress caused by the husk-treatment may increase the amount of momilactone B released. Thus, momilactone B may play an important role in rice allelopathy.

Journal Article↗

PubMed2026

Rhizosphere Dialogue: Microorganisms Mediated by Root Exudates Alleviate Drought Stress in Grasses.

Drought stress threatens the ecological functions and economic value of grasses, posing a major challenge to their sustainable production. Plants co-evolve with rhizosphere microbial communities, sometimes described as the plant's second genome, that can contribute to drought adaptation. Drought alters root architecture, hormonal and redox regulation and belowground carbon allocation, thereby modifying the quantity and composition of root exudation and reshaping the rhizosphere environment. This review uses the rhizosphere dialogue as an integrative framework to link these plant responses with microbial recruitment and subsequent feedback to the host. We summarise three linked stages of this dialogue: drought-induced changes in root exudation; microbial recruitment and colonisation through chemotaxis, attachment, biofilm formation, and root colonisation; and microbiome-mediated feedback that improves plant water relations, hormonal and redox homoeostasis, nutrient acquisition, and root function. We highlight microbial extracellular polymeric substances, 1-aminocyclopropane-1-carboxylate deaminase, and microbial volatile organic compounds as key mediators of drought alleviation. We then discuss how this framework may inform rational synthetic microbial community (SynCom) design, microbiome-informed breeding, artificial intelligence and machine-learning assisted strain prioritisation, rhizosphere legacy effects, and real-time monitoring. Future work should distinguish active exudate-mediated recruitment from drought-driven environmental filtering and integrate multi-omics, plant genetics, functional validation, and multi-location field trials to determine whether rhizosphere dialogue can become a predictive framework for climate-resilient grass production.

drought stress↗

PubMed2004

[Effects of lanthanum on root membrane permeability and nutrient contents in root exudate of rice].

This study showed that when lanthanum was in lower concentration (< or = 50 microg x ml(-1)), the membrane permeability of rice root was stabilized by decreasing the electrolyte exosmosis, and the contents of K+, Ca2+, P and H+ in root exudate were lower than the control; while the concentration of La3+ was in the range of 100-400 microg x ml(-1), the electrolyte exosmosis was reduced in the first 6 hours, and then significantly increased. When the concentration of La3+ was over 500 microg x ml(-1), the electrolyte exosmosis was continuously enlarged because of the destruction of the membrane. Under this condition, K+, Ca2+, P and H+ in root exudate were higher than the control. It could be concluded that the concentration of 50 microg x ml(-1) La3+ was safe to rice growing.

Calcium↗

PubMed1999

Tomato seed and root exudate sugars: composition, utilization by Pseudomonas biocontrol strains and role in rhizosphere colonization.

The role of tomato seed and root exudate sugars as nutrients for Pseudomonas biocontrol bacteria was studied. To this end, the major exudate sugars of tomato seeds, seedlings and roots were identified and quantified using high-performance liquid chromatographic (HPLC) analysis. Glucose, fructose and maltose were present in all studied growth stages of the plant, but the ratios of these sugars were strongly dependent on the developmental stage. In order to study the putative role of exudate sugar utilization in rhizosphere colonization, two approaches were adopted. First, after co-inoculation on germinated tomato seeds, the root-colonizing ability of the efficient root-colonizing P. fluorescens strain WCS365 in a gnotobiotic quartz sand-plant nutrient solution system was compared with that of other Pseudomonas biocontrol strains. No correlation was observed between the colonizing ability of a strain and its ability to use the major exudate sugars as the only carbon and energy source. Secondly, a Tn5lacZ mutant of P. fluorescens strain WCS365, strain PCL1083, was isolated, which is impaired in its ability to grow on simple sugars, including those found in exudate. The mutation appeared to reside in zwf, which encodes glucose-6-phosphate dehydrogenase. The mutant grows as well as the parental strain on other media, including tomato root exudate. After inoculation of germinated sterile tomato seeds, the mutant cells reached the same population levels at the root tip as the wild-type strain, both alone and in competition, indicating that the ability to use exudate sugars does not play a major role in tomato root colonization, despite the fact that sugars have often been reported to represent the major exudate carbon source. This conclusion is supported by the observation that the growth of mutant PCL1083 in vitro is inhibited by glucose, a major exudate sugar, at a concentration of 0.001%, which indicates that the glucose concentration in the tomato rhizosphere is very low.

Carbohydrate Metabolism↗

PubMed2002

Biological activity of soil organic matter mobilized by root exudates.

In order to study the biological activity of soil organic matter mobilized by agrarian (Zea mays: cultivars Mytos and Samantha) and forest (Picea abies Karst. and Pinus sylvestris L.) root exudates, two different soils, an Eutric Cambisol (EC) and a Rendzic Leptosol (RL), were considered. Soil organic matter extracts were obtained by treating the soils with water (control) or plant root exudates. The extracts were characterized by hormone-like activities and gas chromatographic/mass spectrometric (GC/MS) measurements. Their effects on the nitrogen metabolism in maize seedlings were evaluated. The nitrogen organification in the maize seedlings has been greatly stimulated by all the organic acid extracts from the agrarian soil, while the extracts from the forest soil had no influence upon the metabolism; this indicated a probable link between the plant and the environment. The different biological activities of the extracts are discussed.

Gas Chromatography-Mass Spectrometry↗

PubMed2004

Identification and quantitation of compounds in a series of allelopathic and non-allelopathic rice root exudates.

An investigation of the chemical basis for rice allelopathy to the rice weed arrowhead (Sagittaria montevidensis) was undertaken using GC/MS and GC/MS/MS techniques. Twenty-five compounds were isolated and identified from the root exudates of both allelopathic and non-allelopathic rice varieties. Phenolics, phenylalkanoic acids, and indoles were among the chemical classes identified. Two indoles previously unreported in rice were detected in the exudates, 5-hydroxy-2-indolecarboxylic acid and 5-hydroxyindole-3-acetic acid. Several other compounds identified in this study have not previously been reported in rice root exudates, namely mercaptoacetic acid, 4-hydroxyphenylacetic acid, and 4-vinylphenol. The levels of 15 compounds present in the exudates were quantified using GC/MS/MS. Six of the seven most abundant compounds were phenolic acids. Significant differences exist between the allelopathic and non-allelopathic cultivars in their production of three of these six compounds. Greater amounts of trans-ferulic acid, p-hydroxybenzoic acid, and caffeic acid were detected in the exudates of allelopathic cultivars. The seventh compound, abietic acid, was significantly higher in the non-allelopathic cultivars.

Abietanes↗

PubMed2004

Flavonoids from shoots, roots and roots exudates of Brassica alba.

Analysis of extracts obtained from shoots, roots and exudates of Brassica alba revealed the presence of 3,5,6,7,8-pentahydroxy-4'-methoxy flavone in shoots, as well as 2',3',4',5',6'-pentahydroxy chalcone and 3,5,6,7,8-pentahydroxy flavone in roots and exudates. Apigenin was also found in the shoots and roots, but not in the root exudates.

Chalcones↗

PubMed1978

Influence of light on pectic enzymes in root exudates of Trifolium alexandrinum inoculated with Rhizobium trifolii.

An in vitro experiment was conduced under bacteriologically controlled conditions to examine the effect of light on the production of pectin methyl esterase (PME) and pectin polygalacturonase (PG) in the root exudates of Trifolium alexandrinum inoculated with an efficient strain of Rhizobium trifolii. The results revealed that PME and PG increased with an increase in the duration of light to which plants were exposed. However, both the enzymes were detected in the root exudates of nonphotosynthesizing plants.

Esterases↗

PubMed2001

The role of root exudates in aluminium resistance and silicon-induced amelioration of aluminium toxicity in three varieties of maize (Zea mays L.).

Aluminium (Al) toxicity is widely considered to be the most important growth-limiting factor for plants in strongly acid soils (pH<5.0). The inhibition of root elongation in three varieties of maize (Zea mays L. vars Clavito, HS701b and Sikuani) was followed over the first 48 h of Al treatment, and during the initial 10 h elongation was determined on an hourly basis. The silicon (Si)-induced amelioration of Al toxicity was investigated by pre-treating seedlings for 72 h in nutrient solutions with 1000 microM Si before transfer into solutions with 0, 20 or 50 microM Al (without Si). Plants were either grown in complete low ionic strength nutrient solutions (CNS) or in low salt solutions of 0.4 mM CaCl2 (LSS). In addition, the role of root exudation of organic compounds as a mechanism of Si-induced alleviation of Al toxicity was investigated. Aluminium-induced inhibition of root elongation in the maize var. HS701b was observed within 1 h of Al exposure. After a lag time of at least 8 h, Si-induced alleviation of Al toxicity was observed in this variety when grown in LSS. In the Al-resistant var. Sikuani, Al-resistance was only observed after exposure to 50 microM Al, and not after exposure to 20 microM Al, suggesting that there exists a threshold Al concentration before the mechanisms of Al resistance are activated. Aluminium stimulated root exudation of oxalic acid in all three varieties, but exudate concentrations did not increase with either Al resistance or with Si pretreatment. Aluminium and Si triggered release of catechol and of the flavonoid-type phenolics: catechin, and quercetin. In the Al-resistant variety, Sikuani, Al-exposed plants pretreated with Si exuded up to 15 times more phenolics than those plants not pretreated with Si. The flavonoid-type phenolics, to date unconsidered, appear to play a role in the mechanism(s) of Si-induced amelioration of Al toxicity.

Aluminum↗

PubMed1982

Legume-Rhizobium interactions: cowpea root exudate elicits faster nodulation response by Rhizobium species.

Preinfection events in legume-Rhizobium symbiosis were analyzed by studying the different nodulation behaviors of two rhizobial strains in cowpeas (Vigna sinensis). Log-phase cultures of Rhizobium sp. strain 1001, an isolate from the plant nodule, initiated host responses leading to infection within 2 h after inoculation, whereas log-phase cultures of Rhizobium sp. strain 32H1 took at least 7 h to trigger a discernible response. The delay observed with strain 32H1 could be eliminated by incubating the rhizobial suspension, before inoculation, for 4.5 h either in the cowpea rhizosphere/rhizoplane condition or in the root exudate of cowpea plants, grown without NH(4) in the rooting medium. The delay could not be eliminated by incubating the rhizobial suspension in the rooting medium of plants grown in the presence of 5 mM NH(4), indicating that there is a regulatory role of combined nitrogen in triggering preinfection events by the legume. The substance(s) in the root exudate which elicited the faster nodulation response by Rhizobium sp. strain 32H1 could be separated into a high-molecular-weight fraction by Sephadex G-100 gel filtration. The data support the notion that legume roots release substances that favor the development of rhizobial features essential for infection and nodulation.

Journal Article↗

PubMed2006

Identification of phytotoxic substances from early growth of barnyard grass (Echinochloa crusgalli) root exudates.

Barnyard grass is a problematic weed worldwide. It competes with crops and causes reduction in crop yields. In this study, barnyard grass suppressed rice emergence, and the degree of rice inhibition was proportional to the density of barnyard grass. Root exudates of barnyard grass reduced germination and growth of lettuce, rice, and monochoria. Fifteen compounds potentially involved in the phytotoxic activities of barnyard grass were isolated and identified, including phenolics, long-chain fatty acids, lactones, diethyl phthalate, acenaphthene, and derivatives of phthalic acids, benzoic acid, and decane. Quantities of diethyl phthalate, decanoic acid, myristic acid, stearic acid, 7,8-dihydro-5,6-dehydrokavain, and 7,8-dihydrokavain were 2.7, 11.1, 19.6, 35.5, 10.3, and 15.5 microg/ml of barnyard grass root exudates, respectively. The two lactones exhibited the greatest inhibition, followed by the phenolics and the derivatives of phthalic acids. Fatty acids had stronger suppression than diethyl phthalate and ethyl ester-4-ethoxy-benzoic acid. The acenaphthene and decane derivatives were the least phytotoxic. The phytotoxins released by barnyard grass roots showed strong inhibition on growth of broadleaf indicator plants and paddy weeds, but were less effective on barnyard grass itself and rice. Our study revealed that in addition to competition, barnyard grass also interferes with rice and other plants in its surroundings by chemical means.

Echinochloa↗