Quorum sensing is required for the colony establishment and ISR-priming
of a plant phyllosphere bacterium Rhodopseudomonas palustris on strain
GJ-22
Abstract
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)
H2O2 in 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.
