Mechanisms of PGPR-mediated induced systemic resistance in cucumber

Plant growth-promoting rhizobacteria (PGPR) mediate induced systemic resistance (ISR) to multiple pathogens of cucumber but the mechanisms employed by PGPR to mediate ISR are largely unknown. The effect of SA production by strain 90-166 on ISR or mini-Tn5phoA mutants that lacked SA production was investigated. The SA{dollar}sp-{dollar} mutants mediated ISR in cucumber against Colletotrichum orbiculare. Strain 90-166 mediated ISR to Pseudomonas syringae pv. tabaci in transgenic tobacco expressing salicylate hydroxylase indicating that SA produced by 90-166 is not the primary bacterial determinant of ISR.; ISR mediated by 90-166 was improved by addition of an EDDHA in vivo. A mini-TnphoA mutant, 90-166-2882, which did not produce detectable siderophores lost ISR-activity in cucumber against C. orbiculare. Purified siderophore fractions did not induce resistance in cucumber indicating that there was not a direct role for siderophores produced by 90-166 in ISR. However, mutations in other phenotypes were observed with the Sid{dollar}sp-{dollar} mutant indicating that further characterization of the mutant is necessary.; Non-living PGPR components (culture filtrate) from 90-166 or P. fluorescens 89B-61 (culture filtrate, heat-killed cells) also induced resistance to C. orbiculare in cucumber. Purified cell wall components from both strains and purified cell membrane components from strain 89B-61 significantly reduced disease severity caused by C. orbiculare (P = 0.05) indicating that bacterial cell surface components from two PGPR strains are involved in ISR.; The PGPR strains 90-166 and B. pumilus INR7 were inoculated onto cucumber seed and plants were monitored for changes in community structure of bacterial endophytes. Endophyte community structure of plants treated with 90-166 differed significantly (P = 0.05) from the noninduced and pathogen-induced treatments in richness (N0) and diversity indices (N1 and N2). These indices were not significantly greater for INR7 treatment however, cluster analysis using BIOSIM indicated that community membership from PGPR-treated plants was more similar to those from control treatments. The bacterial endophyte community associated with PGPR-treated cucumbers was structurally distinct from the community associated with either pathogen-induced or nontreated cucumbers. The increase in community diversity appears to be associated specifically with PGPR treatment and not specifically with an increase in disease resistance in the host plant.