Two cucumber genotypes were chosen that differ in Fusarium wilt resistance and inoculated with Fusarium oxysporum (Schlechtend, Fr) f. sp. cucumerinum (Owen) Snyder & Hansen (FO) in a hydroponic nutrient solution system to determine whether differences in plant genotype affected rhizosphere microbial community and its involvement in resistance to root-borne pathogens. Response of the FO population and microbial communities were compared in the nutrient solution and on root surfaces. Results demonstrated that FO inoculation resulted in a higher FO population on root surfaces and a lower population in the nutrient solution for the susceptible genotype JinYan NO. 4 (JYan); however, an inverse pattern was observed in the resistant JinYou NO. 1 (JYou). Similarly, promotive effects on FO spore germination and germ-tube elongation were only observed in root exudates from FO-inoculated susceptible JYan. FO inoculation resulted in overall increases in cultivable fungi and oomycetes, and actinomycete populations on root surfaces of the resistant JYou compared to the JYan counterpart. PCR-DGGE analysis of 16S rDNA fragments indicated that following FO inoculation, significant changes in the bacterial structure isolated from the root surfaces of susceptible plants were observed, such as an increased diversity index, a decreased evenness index, and the occurrence of several types of bacteria that decomposed organic substances. These results suggest that a complex interaction between plant genotypes and pathogen affected the rhizosphere microbial community.

• Brassinosteroids (BRs) are a new class of plant hormones that are essential for plant growth and development. Here, the involvement of BRs in plant systemic tolerance to biotic and abiotic stresses was studied. • The effects of 24-epibrassinolide (EBR) on plant stress tolerance were studied through the assessment of symptoms of photooxidative stress by chlorophyll fluorescence imaging pulse amplitude modulation, the analysis of gene expression using quantitative real-time PCR and the measurement of hydrogen peroxide (H₂O₂) production using a spectrophotometric assay or confocal laser scanning microscopy. • Treatment of primary leaves with EBR induced systemic tolerance to photooxidative stress in untreated upper and lower leaves. This was accompanied by the systemic accumulation of H₂O₂ and the systemic induction of genes associated with stress responses. Foliar treatment of EBR also enhanced root resistance to Fusarium wilt pathogen. Pharmacological study showed that EBR-induced systemic tolerance was dependent on local and systemic H₂O₂ accumulation. The expression of BR biosynthetic genes was repressed in EBR-treated leaves, but elevated significantly in untreated systemic leaves. Further analysis indicated that EBR-induced systemic induction of BR biosynthetic genes was mediated by systemically elevated H₂O₂. • These results strongly argue that local EBR treatment can activate the continuous production of H₂O₂, and the autopropagative nature of the reactive oxygen species signal, in turn, mediates EBR-induced systemic tolerance.

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.

Fusarium wilt, caused by Fusarium oxysporum f. sp. cucumerinum (FO), is one of the major diseases in cucumber (Cucumis sativus) production. Root and foliar applications of 24-epibrassinolide (EBL), an immobile phytohormone with antistress activity, were evaluated for their effects on the incidence of Fusarium wilt and changes in the microbial population and community in roots of cucumber plants. EBL pre-treatment to either roots or shoots significantly reduced disease severity followed by an improved plant growth regardless of the treatment methods applied. EBL applications decreased the Fusarium population on root surfaces and in nutrient solution, but increased the population of fungi and actinobacteria on root surfaces. PCR-DGGE analysis showed that FO-inoculation had significant effects on the bacterial community on root surfaces as expressed by a decreased diversity index and evenness index, but EBL applications alleviated these changes. Moreover, several kinds of decomposing bacteria and growth-promoting bacteria were identified from root surfaces of FO-inoculated plants and EBL-pre-treated plants, respectively. Overall, these results show that the microbial community on root surfaces was affected by a complex interaction between phytohormone-induced resistance and plant pathogens.

Root and foliar applications of 24-epibrassinolide (EBL), an immobile phytohormone with antistress activity, were evaluated for their effects on reducing fusarium wilt and their influence on antioxidant and phenolic metabolism in roots of cucumber plants (Cucumis sativus L. cv. Jinyan No. 4). EBL pretreatment significantly reduce disease severity together with improved plant growth and reduced losses in biomass regardless of application methods. EBL treatments significantly reduced pathogen-induced accumulation of reactive oxygen species (ROS), flavonoids, and phenolic compounds, activities of defense-related and ROS-scavenging enzymes. The enzymes included superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, catalase as we as phenylalanine ammonia-lyase and polyphenoloxidase. There was no apparent difference between two application methods used. EBL applications triggered a slight increase in H2O2 concentration followed by increases in the transcript levels of WRKY transcription factor and defense-related genes. This study demonstrated that EBL enhanced resistance to fusarium wilt by a novel mechanism that was not related to its active transport or increase in antioxidant system.

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

To provide an insight into the mechanism of interspecific interactions mediated by allelochemicals, cucumber and figleaf gourd seedlings were compared on their response to cinnamic acid, an autotoxin from root exudates of cucumber. Reactive oxygen species metabolism and plasma membrane H(+)-ATPase activity were examined in roots upon exposure to cinnamic acid. This exposure resulted in significant increases in activities of NADPH oxidase, superoxide dismutase, guaiacol peroxidase, and catalase, as well as in O(2)(.-) production and H(2)O(2) content, in cucumber roots but not in figleaf gourd roots. Notably, the cucumber roots produced significant amount of reactive oxygen species (ROS) immediately after cinnamic acid treatment, consequently increasing membrane peroxidation, decreasing membrane H(+)-ATPase activity, and losing root viability. By contrast, no such changes were observed in figleaf gourd roots. All these results indicated that there was an interspecies difference in the recognition of allelochemicals, which induced oxidative stress accompanied by root cell death in cucumber, an autotoxic plant, but not in figleaf gourd, a cucumber relative.