ABSTRACTThe phyllosphere presents a hostile environment for many biocontrol agents; however, it is as significant as is the rhizosphere for plant health. Deploying biocontrol bacteria into the phyllosphere can efficiently suppress diseases; however, the lack of knowledge on the phyllosphere adaptive traits of biocontrol bacteria poses challenges. In this study, we demonstrated thatRhodopseudomonas palustrisGJ-22 colonizes the phyllosphere by forming cell aggregates. The formation of cell aggregates required the production of exopolysaccharides (EPS), which depended on the function of therpaI-rpaRquorum sensing (QS) mechanism, mediated by the signaling moleculep-coumaroyl-HSL (pC-HSL). The mutation of the EPS biosynthesis geneExop1or the signaling molecule biosynthesis generpaIcompromised the ability of GJ-22 to tolerate reactive oxygen intermediates (ROIs), such as H2O2,in vitroand to form cell aggregatesin vivo. Collectively, the results revealed that QS mediates EPS production and consequently leads to bacterial cell aggregation.IMPORTANCEQuorum sensing is used by various bacteria for coordinating the multiplication of bacterial cells in a group and for modulating the behaviors of surrounding microbial species. Host plants can benefit from this interspecies modulation, as it can disrupt the QS circuits of pathogenic bacteria. Some N-acyl homoserine lactone- (AHL-) producing bacteria that were introduced into the phyllosphere as biocontrol agents may establish AHL-based crosstalk with indigenous microbes to steer the nutritional and microecological conditions toward their own and the host plant’s benefit. Here, we showed that biocontrol bacteria introduced into the phyllosphere require a functioning QS circuit to establish colonies and suppress pathogens. Furthermore, our findings provoked a broader investigation into the role of the QS circuit in beneficial microorganism-plant interactions.