Cucumber Root Exudates Research Articles

Recruitment of beneficial cucumber rhizosphere microbes mediated by amino acid secretion induced by biocontrol Bacillus subtilis isolate 1JN2.

At present, the use of beneficial microorganisms to control cucumber Fusarium wilt is a widely used method, and the rhizosphere microecological reset is one of the mechanisms involved. However, how biocontrol strains reshape cucumber rhizosphere microecology remains to be further studied. The composition changes of cucumber root exudates induced by biocontrol strain 1JN2, the microbial ecology of cucumber rhizosphere and the colonization ability of biocontrol strain 1JN2 in cucumber rhizosphere were analyzed through UHPLC-MS/MS analysis, Illumina high-throughput sequencing and SEM, respectively. First, cucumber plants treated with biocontrol Bacillus 1JN2 reduced the disease severity of Fusarium wilt by 60%. Significant changes in cucumber root exudates were found after 1JN2 inoculation and the contents of four amino acids including glutamine, tryptophan, glycine and glutamic acid were significantly increased. Second, It was found that the bacterial diversity in the rhizosphere of cucumber was significantly increased in both the strain treatment group and the amino acid mixture treatment group, The number of Bacillus was the largest in all dominant populations, exceeded 20% in all treatment groups. The bacteria of Hydrogenispora and Vicinamibacteria were significantly increased after treatment. Overall, the results demonstrated that amino acid substances in cucumber root exudates induced by biocontrol strain 1JN2 can shift the cucumber root microenvironment and prevent the occurrence of Fusarium wilt disease.

The combination of Bacillus amyloliquefaciens and Purpureocillium lilacinum in the control of Meloidogyne enterolobii

Nowadays, numerous microorganisms have been identified as effective agents against plant-parasitic nematodes (PPNs) when applied individually. However, under field conditions, the combination of biocontrol agents is a desirable strategy to improve control efficacy against soil-borne pathogens. Since, as different biogents species possess different mechanisms, the combination of strains is an alternative to improve the biocontrol effects. The present study aimed to investigate in planta the efficacy of the association of two commercial bionematicides against M. enterolobii. Here, the combination of Purpureocillium lilacinum strain PL251 (PL) and Bacillus amyloliquefaciens strains D747 (Ba) simultaneously inoculated on cucumber roots, in Florida (USA) soil, significantly reduced (p<0.05) the number of eggs per gram of roots (84%) and gall index compared to negative control. Contrasting with the mixture, the application of each microorganism alone didn’t show any difference to the negative control. In in vitro assays, the combined cell-free supernatants cultures of these microorganisms were also effectiveness on increasing Meloidogyne enterolobii second-stage juvenile (J2) toxicity and inhibit the eggs hatching. Also in in vitro assays, the chemotaxis of M. enterolobii J2 showed to be repellent to both microorganisms. M. enterolobii (J2) repellence was also obtained when root exudates of cucumber were simultaneously inoculated with both microorganisms. On the other side, the exudate of cucumber roots free of those microorganisms was highly attractive to the J2. Despite the notable toxicity of Ba's volatile organic compounds (VOCs) to M. enterolobii J2 compared to the control, the impact of VOCs from Ba alone was significantly more pronounced (P<0.05) when the combined microorganisms were examined. Hence, the enhanced nematode control observed in in vivo experiments is substantiated by the combined impact of the mixture.

Silicon alleviates autotoxicity by regulating membrane lipid peroxidation and improving photosynthetic efficiency in cucumber seedlings (Cucumis sativus L.)

Autotoxicity is one of the main causes of cucumber continuous cropping obstacles. Cinnamic acid (CA), a phenolic acid derivative, acts as an autotoxin found in cucumber root exudates. The autotoxicity mimicked by CA significantly increases membrane lipid peroxidation in root and leaf cells, resulting in damage to the structure and function of root tip cells and leaf cells. It also injures the activity of photosystem PSII, reducing the photosynthetic rate and inhibiting the growth of cucumber plants. Supplementing Si alleviates CA-mimicked autotoxicity by increasing the thickness and smoothness of root tip cell walls, inhibiting the absorption and accumulation of CA, as well as relieving osmotic stress and membrane lipid peroxidation caused by CA in cucumber roots and leaves. Additionally, Si promotes grana lamella (GL) stacking and increases thylakoid numbers in chloroplasts. It also enhances the chlorophyll content in cucumber leaves and improves the energy conversion efficiency of photosystem PSII. These effects lead to an improvement in both photosynthetic efficiency and dry matter accumulation, as well as increased the resistance to CA-induced autotoxicity in cucumber plants. The same concentration of Si had no significant effect on the growth of cucumber seedlings, indicating that Si plays an important role in improving the tolerance to autotoxicity in cucumbers. These findings provide an effective theoretical basis and methods for research regarding the alleviation of continuous cropping obstacles.

Allelochemical p-hydroxybenzoic acid inhibits root growth via regulating ROS accumulation in cucumber (Cucumis sativus L.)

Allelopathy is prevalent in agricultural ecosystems and mediated by plant-derived secondary metabolites (allelochemicals). Allelochemicals are released by donor plants and affect the root growth and development of receptor plants. Allelopathy is responsible for the continuous cropping obstacles in cucumber (Cucumis sativus L.). p-Hydroxybenzoic acid (pHBA), an autotoxin from root exudates of cucumber, has been proposed to be an important allelopathic chemical. However, the molecular mechanism by which pHBA affect root growth and development in cucumber is unknown. Here, we found that pHBA treatment suppressed root growth of cucumber by reducing the meristem activity and cell length. This root growth defect is caused by reduced reactive oxygen species (ROS) accumulation in root tips. After pHBA treatment, the expression levels of several ROS-scavenging-related genes were increased, including peroxidase (POD), catalase (CAT) and metallothionein (MT). Moreover, exogenously application of salicylhydroxamate (SHAM), a peroxidase inhibitor, can partially restore the pHBA treatment induced root growth inhibition. Furthermore, we found that there is natural variation for the inhibitory effect of pHBA on root growth. We also showed that pHBA treatment could maintain higher level of ROS accumulated in the pHBA less sensitive cucumber than that in the pHBA-sensitive cucumber. These results suggest that pHBA inhibits root growth by reducing root tip ROS level in cucumber.

Root-Secreted Spermine Binds to Bacillus amyloliquefaciens SQR9 Histidine Kinase KinD and Modulates Biofilm Formation.

The signal molecules in root exudates that are sensed by plant growth-promoting rhizobacteria (PGPR) are critical to regulate their root colonization. Phosphorylated Spo0A is an important global transcriptional regulator that controls colonization and sporulation in Bacillus species. In this study, we found that deletion of kinD from PGPR strain Bacillus amyloliquefaciens SQR9, encoding an original phosphate donor of Spo0A, resulted in reduced biofilm formation in root exudates compared with the wild-type strain, indicating that KinD is responsible for sensing root exudates. Ligands of B. amyloliquefaciens SQR9 KinD in cucumber root exudates were determined by both the nontargeted ligand fishing method and the targeted surface plasmon resonance detection method. In total, we screened 80 compounds in root exudates for binding to KinD and found that spermine and guanosine could bind to KinD with dissociation constant values of 213 and 51 μΜ, respectively. In addition, calcium l-threonate, N-acetyl-l-aspartic acid, sodium decanoic acid, and parabanic acid could also bind weakly to KinD. The three-dimensional binding models were then constructed to demonstrate the interactions between the root-secreted signals and KinD. It was observed that exogenous spermine reduced the wrinkles of biofilm when kinD was deleted, indicating that KinD might be involved in sensing root-secreted spermine and stabilizing biofilm in response to this negative effector. This study provided a new insight of interaction between a rhizobacterial sensor and root-secreted signals.

Induced root-secreted D-galactose functions as a chemoattractant and enhances the biofilm formation of Bacillus velezensis SQR9 in an McpA-dependent manner.

Chemotaxis towards root exudates and subsequent biofilm formation are very important for root colonization and for providing the beneficial functions of plant growth-promoting rhizobacteria (PGPRs). In this study, in comparison with other root-secreted compounds, D-galactose in the root exudates of cucumber was found to be a strong chemoattractant at the concentration of 1μM for Bacillus velezensis SQR9. Chemotaxis assays with methyl-accepting chemotaxis proteins (MCPs) deletion strains demonstrated that McpA was solely responsible for chemotaxis towards D-galactose. Interestingly, D-galactose significantly enhanced the biofilm formation of SQR9 in an McpA-dependent manner. Further experiment showed that D-galactose also enhanced root colonization by SQR9. In addition, the secretion of D-galactose by cucumber roots could be induced by inoculation with SQR9, indicating that D-galactose may be an important signal in the interaction between plant and SQR9. These findings suggested that the root-secreted D-galactose was a signal, the secretion of which was induced by the beneficial bacteria, and which in turn induced colonization of the bacteria.

Recognition of dominant attractants by key chemoreceptors mediates recruitment of plant growth‐promoting rhizobacteria

Chemotaxis to plant root exudates is supposed to be a prerequisite for efficient root colonization by rhizobacteria. This is a highly multifactorial process since root exudates are complex compound mixtures of which components are recognized by different chemoreceptors. Little information is available as to the key components in root exudates and their receptors that drive colonization related chemotaxis. We present here the first global assessment of this issue using the plant growth-promoting rhizobacterium (PGPR) Bacillus velezensis SQR9 (formerly B. amyloliquefaciens). This strain efficiently colonizes cucumber roots, and here, we show that chemotaxis to cucumber root exudates was essential in this process. We conducted chemotaxis assays using cucumber root exudates at different concentrations, individual exudate components as well as recomposed exudates, taking into account their concentrations detected in root exudates. Results indicated that two key chemoreceptors, McpA and McpC, were essential for root exudate chemotaxis and root colonization. Both receptors possess a broad ligand range and recognize most of the exudate key components identified (malic, fumaric, gluconic and glyceric acids, Lys, Ser, Ala and mannose). The remaining six chemoreceptors did not contribute to exudate chemotaxis. This study provides novel insight into the evolution of the chemotaxis system in rhizobacteria.

Enhanced Control of Plant Wilt Disease by a Xylose-Inducible degQ Gene Engineered into Bacillus velezensis Strain SQR9XYQ.

Bacillus velezensis SQR9 (former B. amyloliquefaciens SQR9) is a plant-growth-promoting rhizobacterium (PGPR) that promotes plant growth and health. The colonization of PGPR strains along plant roots is a prerequisite for them to execute their specific functions. However, one problem of microbial introduction in practice is that the applied PGPR strains do not always successfully colonize the rhizosphere. In Bacillus spp., two-component signal transduction system (TCS) DegS/U regulates flagellar motility, biofilm formation and antibiotic production. Phosphorylation of DegU by DegS is positively affected by DegQ protein. In this study, we constructed a xylose-inducible degQ genetically engineered strain SQR9XYQ to improve the biocontrol activity. The results from in vitro, root in situ, greenhouse experiments and RT-qPCR studies demonstrate that (i) the phosphorylation of DegU in SQR9XYQ can be gradually activated by xylose, which is a component of both cucumber and tomato root exudates, and (ii) biofilm formation, antibiotic expression, colonization activity, and biocontrol efficiency were improved in SQR9XYQ compared with the wild-type strain SQR9. These results suggest that colonization trait is important to biocontrol strains for maintenance of plant health.

Colonization on Cucumber Root and Enhancement of Chlorimuron-ethyl Degradation in the Rhizosphere by Hansschlegelia zhihuaiae S113 and Root Exudates.

The colonization of Hansschlegelia zhihuaiae S113 and its degradation of the herbicide chlorimuron-ethyl in the cucumber rhizosphere was investigated. The results reveal that S113 colonized the cucumber roots (2.14 × 105cells per gram of roots) and were able to survive in the rhizosphere (maintained for 20 d). The root exudates promoted colonization on roots and increased the degradation of chlorimuron-ethyl by S113. Five organic acids in cucumber-root exudates were detected and identified by HPLC. Citric acid and fumaric acid significantly stimulated S113 colonization on cucumber roots, with 18.4 and 15.5% increases, respectively, compared with the control. After irrigation with an S113 solution for 10 days, chlorimuron-ethyl could not be detected in the roots, seedlings, or rhizosphere soil, which allowed for improved cucumber growth. Therefore, the degradation mechanism of chlorimuron-ethyl residues by S113 in the rhizosphere could be applied in situ for the bioremediation of chlorimuron-ethyl contaminated soil to ensure crop safety.

Exogenous Si Alleviation of Autotoxicity in Cucumber (Cucumis sativus L.) Seed Germination is Correlated with Changes in Carbohydrate Metabolism

Benzoic and cinnamic acid derivatives such as 3-phenylpropionic acid (PA) are the main autotoxins identified from cucumber root exudates. In this study, we investigated the effects of silicon (Si) supplemented on cucumber seed germination under PA-induced stress. Cucumber bud seedlings were harvested at 0, 4, 8, 12, 24, 36, and 48 h after treatment and assessed for plant growth, amylase activity and gene expression, and starch metabolism. Results revealed that PA significantly reduced the seed germination rate, seed vigor, radicle length, number of lateral roots, fresh weight, and soluble sugar content of cucumber bud seedlings. However, PA increased the starch content. The supplement of silicon promoted the growth of PA-treated plants and significantly increased the germination rate, seed vigor, and soluble sugar content of cucumber bud seedlings. Compared with those of the control plants, the levels and activities of α- (AMY) and β-amylase (BMY) during germination significantly decreased after PA treatment. The supplement of silicon significantly improved amylase activity, and total amylase and β-amylase activities reached maximum values at 36 h. PA significantly down-regulated the relative expression levels of AMY and BMY, whereas exogenous silicon significantly increased their transcript levels. These results suggest that supplement of Si alleviated cucumber autotoxicity caused by PA during seed germination at both the physiological and molecular levels.

Characterization of extracellular polymeric substances of Bacillus amyloliquefaciens SQR9 induced by root exudates of cucumber.

Bacillus amyloliquefaciens SQR9 is a plant growth-promoting rhizobacterium (PGPRs) that forms biofilm on the roots of plants and protects them from a variety of pathogens. In this study, we reported the effect of root exudates produced by cucumber (Cucumis sativus L.) at different developmental stages on the biochemical composition of the biofilm matrix of SQR9. The results showed that the amino acids present in the root exudates of cucumber were responsible for triggering biofilm formation of SQR9. In addition, when root exudates harvested at different growth phases of cucumber were used as carbon sources for biofilm formation, the resulting biofilm matrixes differed both quantitatively and qualitatively. The biofilm matrix was mostly composed of amino groups observed by confocal laser scanning microscope (CLSM) hence the proteins formed the major component of the resulting extracellular polymeric substances (EPS). The potential use of amino acid-based dietary supplements to control biofilm formation in the plants may be a viable option to improve agricultural productivity by recruiting beneficial association with PGPRs in the manufacture of bio fertilizers or bio controls.

Autotoxicity in cucumber (Cucumis sativus L.) seedlings is alleviated by silicon through an increase in the activity of antioxidant enzymes and by mitigating lipid peroxidation

Autotoxicity in plants limits their growth and that of nearby plants of the same species, which has obvious implications in crop yield and quality. Silicon (Si) has been shown to increase plant tolerance to autotoxic stress. However, the physiological mechanisms underlying the effects of Si in alleviating autotoxicity during germination in cucumber (Cucumis sativus L.) are unknown. Cinnamic acid derivatives, such as 3-phenylpropionic acid (PA), are a class of autotoxins present in cucumber root exudates. Our objective was to investigate Si-induced autotoxic stress tolerance in cucumber seedlings by focusing on the effects of Si on the induction of antioxidant defense pathways. We found that PA treatment significantly reduced seed germination, radicle length, lateral root number, fresh weight, AsA and GSH contents, and the activities of SOD, CAT, and APX in cucumber seedlings, while it increased membrane permeability and levels of MDA, proline, O 2 - , and H2O2. Application of Si enhanced growth of PA-treated plants and significantly increased germination rate, radicle length, lateral root number, fresh weight, AsA and GSH levels, and SOD, CAT, POD, and APX activities. These results suggest that exogenous Si alleviates autotoxicity caused by PA during seed germination by increasing antioxidant enzyme activities and mitigating lipid peroxidation.

Adsorption efficiency of a continuous trapping system and its use for the collection of root exudates from cucumber

Root exudates comprising soluble low-molecular-weight organic compounds (LMWOCs) play a crucial role in the rhizosphere processes. Therefore, accurate determination of the composition and quantity of these compounds is of importance. A continuous trapping system (CTS) with XAD-4 macroporous resin is widely used for collecting root exudates in hydroponics, and ion exchange resins are used in the separation of root exudates into neutral, acid and basic fractions. Here, we studied the adsorption equilibrium isotherms and dynamic adsorption properties of 27 standard LMWOCs on XAD-4 resin. The adsorption isotherms of most LMWOCs matched the Langmuir isotherm equations. Furthermore, we investigated the adsorption efficiency of the CTS for LMWOCs and the recovery ratios during fractionation by ion exchange resins. The adsorption capacities of the CTS for LMWOCs which are non-polar or having large non-polar moieties were higher. The recovery ratios of most LMWOCs were larger than 80% during fractionation. The overall recovery ratios of LMWOCs were ca. 10, 20, and 30% for sugars, organic acids, and amino acids, respectively. Using this collection method, we present the composition and quantity of root exudates of cucumber (Cumumis sativus L.) at four different growth stages. The major components of root exudates were similar to previous works, whereas the quantities were different. The various collection methods may be the main reason for these differences. Therefore, it is necessary to study the overall recovery ratios when the compositions of root exudates via different collection methods are compared.

Quantification and role of organic acids in cucumber root exudates in Trichoderma harzianum T-E5 colonization

The ability to colonize on plant roots is recognized as one of the most important characteristics of the beneficial fungi Trichoderma spp. The aim of this study is to prove that the utilization of organic acids is a major trait of Trichoderma harzianum T-E5 for colonization of cucumber roots. A series experiments in split-root hydroponic system and in vitro were designed to demonstrate the association between the utilization of organic acids and T-E5 colonization on cucumber roots. In the split-root hydroponic system, inoculation with T-E5 (T) significantly increased the biomass of cucumber plants compared with CK (non-inoculation with T-E5). The T-E5 hyphae densely covering the cucumber root surface were observed by scanning electron microscopy (SEM). Three organic acids (oxalic acid, malic acid and citric acid) were identified from both the CK and T treatments by HPLC and LC/ESI-MS procedures. The amounts of oxalic acid and malic acid in T were significantly higher than those in CK. All the organic acids exhibited different and significant stimulation effects on the mycelial growth and conidial germination of T-E5 in vitro. An additional hydroponic experiment demonstrated the positive effects of organic acids on the T-E5 colonization of cucumber roots. In conclusion, the present study revealed that certain organic acids could be used as nutritional sources for Trichoderma harzianum T-E5 to reinforce its population on cucumber roots.

An Improved High-performance Liquid Chromatographic Method for the Determination of Soluble Sugars in Root Exudates of Greenhouse Cucumber Grown under CO2 Enrichment

This study described a simple and quick method to detect trace quantities of a non-reducing sugar (viz. sucrose) in the root exudates of cucumber (Cucumis sativus) under CO2 enrichment. Sucrose was determined by analyzing fructose and glucose before and after invertase digestion using high-performance liquid chromatography. Using this technique, the optimal hydrolysis condition was 5.00 μg·mL−1 invertase for 10 minutes. The detection limit of ultraviolet-visible detector by post-column derivatization with tetrazolium was 0.25, 0.43, 0.48, and 1.95 μg·mL−1 for fructose, glucose, sucrose, and maltose, respectively, and sensitive enough for determination of sugars in root exudates. The dry weight of cucumber at the seedling stage (19 days after transplant) increased by 58.4% when the CO2 level was elevated from 380 to 1200 μmol·mol−1, whereas the differences were not significant at the initial fruiting stage (63 days after transplant). The photosynthesis rate in 1200 μmol·mol−1 CO2 was 58.0% higher than that in 380 μmol·mol−1 CO2 at the seedling stage and 74.2% higher at the initial fruiting stage. Total amount of sugars in cucumber root exudates was significantly increased with increasing CO2 concentration. The total sugars in root exudates increased by 130.4% and 102.3% in 1200 μmol·mol−1 CO2 compared with that in 380 μmol·mol−1 CO2 at seedling and initial fruiting stages, respectively. Elevated CO2 altered sugar composition in root exudates. Sugars in root exudates released per plant were significantly higher at the initial fruiting stage than that at the seedling stage, whereas the differences in sugars released per gram of root tissue between these two growth stages were not significant. Our results suggest that sugars were increased only in as much as root mass increased. This study provides a simple and quick method to detect 1 to 500 μg·mL−1 sugars in root exudates, and the results illustrate the variation in the sugar composition in cucumber root exudates among the CO2 levels and growth stages.

7α- and 7β-Hydroxyorobanchyl acetate as germination stimulants for root parasitic weeds produced by cucumber

Cucumber (Cucumis sativus) plants were found to exude at least 12 germination stimulants for root parasitic weeds including 5 known strigolactones, 7-oxoorobanchol, 7-oxoorobanchyl acetate, orobanchol, orobanchyl acetate, and 4-deoxyorobanchol. Two novel germination stimulants were purified from cucumber root exudates and their structures were determined to be 7αand 7β-hydroxyorobanchyl acetate by 1D and 2D NMR spectroscopy, and ESIand EI-MS spectrometry. The stereochemistry was determined by NOE measurement and by comparing the CD spectra with those of the synthetic standards of four stereoisomers of orobanchol. 7αand 7β-Hydroxyorobanchol were detected by LC-MS/MS, and GC-MS analysis of purified sample confirmed their structures. The germination stimulation activities of 7αand 7β-hydroxyorobanchyl acetate on Orobanche minor were comparable to those of orobanchyl acetate and 7-oxoorobanchyl acetate. By contrast, 7β-hydroxyorobanchyl acetate was a highly potent germination stimulant for Phelipanche ramosa inducing more than 50% germination at 10 pM. © Pesticide Science Society of Japan

Effects of different plant root exudates and their organic acid components on chemotaxis, biofilm formation and colonization by beneficial rhizosphere-associated bacterial strains

It is necessary to understand the roles of root exudates involved in plant-microbe interactions to inform practical application of beneficial rhizosphere microbial strains. Colonization of Bacillus amyloliquefaciens SQR9 (isolated from cucumber rhizosphere) and Bacillus subtilis N11 (isolated from banana rhizosphere) of their original host was found to be more effective as compared to the colonization of the non-host plant. Organic acids in the root exudates of the two plants were identified by High performance liquid chromatography (HPLC). The chemotactic response and effects on biofilm formation were assessed for SQR9 and N11 in response to cucumber and banana root exudates, as well as their organic acids components. Citric acid detected exclusively in cucumber exudates could both attract SQR9 and induce its biofilm formation, whereas only chemotactic response but not biofilm formation was induced in N11. Fumaric acid that was only detected in banana root exudates revealed both significant roles on chemotaxis and biofilm formation of N11, while showing only effects on biofilm formation but not chemotaxis of SQR9. The relationship between PGPR strain and root exudates components of its original host might contribute to preferential colonization. This study advances a clearer understanding of the mechanisms relevant to application of PGPR strains in agricultural production.

Trichoderma harzianum T-E5 significantly affects cucumber root exudates and fungal community in the cucumber rhizosphere

The Fusarium wilt in cucumbers, caused by the pathogenic fungus Fusarium oxysporum f. sp. cucumerinum, is a serious and destructive disease worldwide. An effort was made to explore the role of Trichoderma harzianum T-E5 in reducing the incidence of Fusarium wilt. Three treatments (Control, T1, and T2) were established in the greenhouse experiment. The effects of T-E5 on the composition of root exudates and fungal community in the cucumber rhizosphere were measured. Compared with the control, the application of a bio-organic fertilizer (BIO) enriched with T-E5 was found to decrease the incidence of Fusarium wilt notably and promote the growth of cucumber plants. Based on real-time PCR, the population of F. oxysporum in the control without T-E5 increased from 103 to 104 ITS copiesg−1 soil, whereas the population decreased from 103 to 102 ITS copiesg−1 soil in the T1 and T2 treatment groups when T-E5 was included. Significant difference in fungal community was also found among the treatment groups. HPLC analysis showed that the detected levels of phenolic compounds in control were significantly higher than the levels in the samples subjected to T1 and T2 treatments. The root exudates from the control group significantly increased the numbers of germinating spores of the pathogen compared with those from the samples treated with T1 and T2. In conclusion, the modification of root exudates and the fungal community by the application of BIO might account for the effective suppression of Fusarium wilt disease in cucumbers.

P-Coumaric Acid Influenced Cucumber Rhizosphere Soil Microbial Communities and the Growth of Fusarium oxysporum f.sp. cucumerinum Owen

BackgroundAutotoxicity of cucumber root exudates or decaying residues may be the cause of the soil sickness of cucumber. However, how autotoxins affect soil microbial communities is not yet fully understood.Methodology/Principal FindingsThe aims of this study were to study the effects of an artificially applied autotoxin of cucumber, p-coumaric acid, on cucumber seedling growth, rhizosphere soil microbial communities, and Fusarium oxysporum f.sp. cucumerinum Owen (a soil-borne pathogen of cucumber) growth. Abundance, structure and composition of rhizosphere bacterial and fungal communities were analyzed with real-time PCR, PCR-denaturing gradient gel electrophoresis (DGGE) and clone library methods. Soil dehydrogenase activity and microbial biomass C (MBC) were determined to indicate the activity and size of the soil microflora. Results showed that p-coumaric acid (0.1–1.0 µmol/g soil) decreased cucumber leaf area, and increased soil dehydrogenase activity, MBC and rhizosphere bacterial and fungal community abundances. p-Coumaric acid also changed the structure and composition of rhizosphere bacterial and fungal communities, with increases in the relative abundances of bacterial taxa Firmicutes, Betaproteobacteria, Gammaproteobacteria and fungal taxa Sordariomycete, Zygomycota, and decreases in the relative abundances of bacterial taxa Bacteroidetes, Deltaproteobacteria, Planctomycetes, Verrucomicrobia and fungal taxon Pezizomycete. In addition, p-coumaric acid increased Fusarium oxysporum population densities in soil.Conclusions/SignificanceThese results indicate that p-coumaric acid may play a role in the autotoxicity of cucumber via influencing soil microbial communities.

Selective trans-Cinnamic Acid Uptake Impairs [Ca2+]cyt Homeostasis and Growth in Cucumis sativus L.

To obtain insight into interspecies interactions mediated by allelochemicals, the response of cucumber (Cucumis sativus L. cv Jinyan No.4) and figleaf gourd (Cucurbita ficifolia Bouché) seedlings to trans-cinnamic acid (CA) (1) was investigated. While trans-CA is an autotoxin in cucumber root exudates, figleaf gourd is resistant to it. Cucumber, however, had a high rate of trans-CA uptake by the roots, leading to reduced root growth. The trans-CA treatment also resulted in an intracellular release of Ca(2+) from the vacuole to the cytoplasm, and, thus, an increased [Ca(2+)](cyt) level accompanied by gradual loss of cell viability in cucumber roots. Taken together, these results suggest that [Ca(2+)](cyt) homeostatic disturbance is one of the primary triggers for trans-CA phytotoxicity in cucumber.