Chemoperception of specific amino acids controls phytopathogenicity in Pseudomonas syringae pv. tomato

In this work, we have examined the chemoreceptor repertoire of Pseudomonas syringae pv. tomato DC3000, a foliar pathogen that causes bacterial speck in tomato. This strain has 49 chemoreceptors and four copies of the core chemosensory signaling proteins. We have focused our attention on the PscA chemoreceptor and we have determined its ligand profile. We show that PscA recognizes specifically and with high affinity the amino acids L-aspartic, L-glutamic and D-aspartic. Interestingly, aspartic and glutamic acids are the most abundant amino acids in tomato plant apoplast. Moreover, we found that the perception of these amino acids through PscA chemoreceptor exerts a double function, namely, in mediating chemotaxis and in modulating the levels of the secondary bacterial messenger cyclic di-GMP, causing alterations in biofilm development and in bacterial motility. This receptor was found to play a key role in the infection process. Moreover, we demonstrated that a saturating concentration of D-aspartic acid reduces P. syringae virulence on tomato, suggesting that ligand-mediated interference of key chemoreceptors may be an alternative strategy to control bacterial virulence in plants.