Study of bacterial determinants involved in the induction of systemic resistance in bean by Pseudomonas putida BTP1

The ability of Pseudomonas putida BTP1 to induce resistance in bean to Botrytis cinerea was demonstrated in soil experiments on plants pre-inoculated at the root level with the bacteria before challenge with the leaf pathogen. As a first step to characterize the molecules from BTP1 responsible for induction of systemic resistance in bean, heat-killed cells and supernatant from culture in an iron-limited medium were tested for their protective effect. Most of the resistance-eliciting activity of the strain was retained in the crude cell-free culture fluid. In vivo assays with samples from successive fractionation steps of the BTP1 supernatant led, (i) to the conclusion that salicylic acid, pyochelin and pyoverdin, previously identified as Pseudomonas determinants for induced systemic resistance (ISR), were not involved in systemic resistance triggered by BTP1, and (ii) to the isolation of fractions containing one main metabolite that retained most of the resistance-inducing activity in bean. Although this molecule remains to be structurally characterized, its isolation is an addition to the range of determinants from plant growth-promoting rhizobacteria (PGPR) known to stimulate plant defences.