Quorum sensing is required for the colony establishment and ISR-priming of a plant phyllosphere bacterium Rhodopseudomonas palustris on strain GJ-22

Plant phyllosphere represents a hostile environment to many biocontrol agents but gives no less significance than rhizosphere in terms of plant health. Deploying biocontrol bacteria onto phyllosphere can be efficient to suppress diseases but also challenging due to the lack of knowledge on phyllosphere adaptive traits possessed by biocontrol bacteria. In this study, We demonstrated that a bacterium Rhodopseudomonas palustris strain GJ-22 colonizes plant phylloshere by forming cell aggregates. The formation of cell aggregates required production of exopolysaccharide (EPS) which depended on the function of the rpaI-rpaR quorum sensing (QS) mechanism mediated by the signal molecule p-coumaroyl-HSL. and then,Mutation of the EPS biosynthesis gene Exop1 or the signal molecule biosynthesis gene rpaI compromised the ability of GJ-22 to tolerate reactive oxygen intermediates (ROIs) HOin vitro and to form cell aggregates in vivo. Both mutants failed to prime the plants with induced systemic resistance (ISR) against virus proliferation, but the ISR-priming ability can be restored by exogenous application of p-coumaroyl-HSL onto the mutant-inoculated leaves. This result indicates that QS mechanism also participates in the ISR-priming of GJ-22. Together the result revealed QS mediates the production of EPS and consequently leads to the formation of bacterial cell aggregation which is required for the onset of ISR which provides plant with pathogen resistance.