Many bacteria employ quorum sensing (QS) signals to control target gene expression and coordinate communal behaviors. In the notorious plant pathogen Ralstonia solanacearum, which causes a lethal disease known as “bacterial wilt” in many plants worldwide, there are at least two kinds of QS signals, methyl 3-hydroxypalmitate (3-OH PAME) or methyl 3-hydroxymyristate (3-OH MAME) and AHL signals. These signals are synthesized by the phc and sol systems, respectively. In particular, the phc system controls various biological functions and virulence in R. solanacearum. Based on our previous experience, we have raised several questions. (1) Is there any other unknown cell-cell communication signal in R. solanacearum? (2) Is there any interaction between the different QS systems in R. solanacearum?
Our recent findings have answered the questions. We found that anthranilic acid produced by R. solanacearum exhibited an inhibitory activity on sexual mating and hypha formation in Sporisorium scitamineum by interfering with the PKA-cAMP pathway. We continued to discover that this compound also plays an important role in the regulation of biological functions in R. solanacearum. In-frame deletion of trpEG, which is the encoding gene of the synthase of anthranilic acid, resulted in significant impairment of biofilm formation, cellulase production, motility activity, and EPS production. Interestingly, both the in trans expression of trpEG and the addition of exogenous anthranilic acid restored these phenotypes of the trpEG deletion mutant to wild-type strain levels. In addition, we found that the production of C8-AHL, C10-AHL and 3OH-MAME was reduced in the trpEG mutant strain, suggesting that the anthranilic acid might be a new cell-cell communication signal to positively control both the phc and sol QS systems in R. solanacearum.
Furthermore, our results indicated that the trpEG homologues are conserved in all other Ralstonia genomovars with a very high identity, and both anthranilic acid and TrpEG are widely conserved in many other bacterial species. Herein, we demonstrate that the anthranilic acid controls QS signal production, biological functions and virulence in R. solanacearum. Our findings in this study establish the significant roles of the anthranilic acid in intraspecies signalling as a potential cell-cell communication signal as well as in inter-kingdom communication for achieving a competitive advantage in the microbial community, expanding our understanding of bacterial languages for cell-cell communication.