Xanthomonas campestris utilizes IAA to regulate its viability and virulence by altering the production of BCAAs and ROS.

Indole-3-acetic acid (IAA) is an important plant hormone that regulates a variety of physiological processes in plants, and it is also produced by some microbes. However, the biosynthesis and roles of IAA in microorganisms, particularly in plant pathogens, remain to be determined. In this study, the plant pathogen Xanthomonas campestris pv. campestris (Xcc) strain XC1 was shown to produce IAA via an  l-tryptophan ( l-Trp)-dependent pathway. The intermediate metabolite indole-3-ethanol and Xcc1569 encoding aromatic amino acid aminotransferase were shown to be partially involved in the uncharacterized sub-pathway in an  l-Trp-dependent IAA biosynthetic pathway. IAA positively regulated the viability of XC1, as indicated by its colony-forming units (CFUs), extracellular polysaccharide production, protease activity, and virulence on cabbage. IAA also negatively regulated reactive oxygen species (ROS) production in XC1. Furthermore, RNA-Seq revealed a gene cluster, ilvCGM-leuA, encoding the products responsible for branched-chain amino acid (BCAA) biosynthesis, which was negatively regulated by IAA. High-performance liquid chromatography (HPLC) analysis showed that IAA negatively regulated valine and leucine production. Deletion of ilvC significantly increased the CFUs and reduced the ROS levels of XC1. Exogenous BCAA addition to mutant strain ΔilvC restored the CFU and ROS levels to those of wild-type strain XC1. These results revealed an IAA signaling cascade in XC1 that involved ilvCGM-leuA, BCAA production, ROS production, and colony formation. These IAA-regulated phenotypes contributed to the virulence of Xcc in host plants. Overall, these results explain IAA-mediated plant-Xcc interactions and underscore the potentially significant role of IAA in microbial physiology.