Systemic disease protection elicited by plant growth promoting rhizobacteria strains: relationship between metabolic responses, systemic disease protection, and biotic elicitors

A study of plant defensive systemic responses induced by three plant growth promoting rhizobacteria (PGPR) on Arabidopsis thaliana Col 0 against Pseudomonas syringae pv. tomato DC3000 at the biochemical and transcriptional levels is reported in this paper. All three strains decreased disease severity when applied to A. thaliana prior to pathogen inoculation. At the biochemical level, each of the three strains induced ethylene (ET) when incubated with 1-amino-cyclopropane-1-carboxylic acid, and salicylic acid (SA) production in the plant. Plants treated with each of the three strains were also reduced in salicylic acid production after pathogen challenge compared to untreated controls. This effect was more marked in plants treated with Chryseobacterium balustinum AUR9, the strain most effective in decreasing disease severity. The expression level of PR1, a transcriptional marker of the SA-dependent pathway in C. balustinum AUR9-treated plants, is fourfold that of controls while the expression of PDF1.2, a transcriptional marker for the SA-independent pathway, is not induced. C. balustinum cell wall lipopolysaccharides, being putative bacterial elicitor molecules, are able to reproduce this systemic induction effect at low doses. From these observations, we hypothesize that certain PGPR strains are capable of stimulating different systemic responses in host plants. With C. balustinum AUR9, the SA-dependent pathway is stimulated first, as indicated by increases in SA levels and PR1 expression, followed by induction of the SA-independent pathway, as indicated by the increases in ET concentrations. The effects of both pathways combined with respect to disease suppression appear to be additive.