Mechanisms of plant protection against two oxalate-producing fungal pathogens by oxalotrophic strains of Stenotrophomonas spp.

KEY MESSAGE: Oxalotrophic Stenotrophomonas isolated from tomato rhizosphere are able to protect plants against oxalate-producing pathogens by a combination of actions including induction of plant defence signalling callose deposition and the strengthening of plant cell walls and probably the degradation of oxalic acid. Oxalic acid plays a pivotal role in the virulence of the necrotrophic fungi Botrytis cinerea and Sclerotinia sclerotiorum. In this work, we isolated two oxalotrophic strains (OxA and OxB) belonging to the bacterial genus Stenotrophomonas from the rhizosphere of tomato plants. Both strains were capable to colonise endophytically Arabidopsis plants and protect them from the damage caused by high doses of oxalic acid. Furthermore, OxA and OxB protected Arabidopsis from S. sclerotiorum and B. cinerea infections. Bacterial inoculation induced the production of phenolic compounds and the expression of PR-1. Besides, both isolates exerted a protective effect against fungal pathogens in Arabidopsis mutants affected in the synthesis pathway of salicylic acid (sid2-2) and jasmonate perception (coi1). Callose deposition induced by OxA and OxB was required for protection against phytopathogens. Moreover, B. cinerea and S. sclerotiorum mycelial growth was reduced in culture media containing cell wall polysaccharides from leaves inoculated with each bacterial strain. These findings suggest that cell walls from Arabidopsis leaves colonised by these bacteria would be less susceptible to pathogen attack. Our results indicate that these oxalotrophic bacteria can protect plants against oxalate-producing pathogens by a combination of actions and show their potential for use as biological control agents against fungal diseases.