Chelator sensing and lipopeptide interplay mediates molecular interspecies interactions between soil bacilli and pseudomonads

Some bacterial species are important members of the rhizosphere microbiome and confer protection to the host plant against pathogens. However, our knowledge is still limited about the multitrophic interactions determining the ecological fitness of these biocontrol bacteria in their highly competitive natural niche. In this work, we have investigated the molecular mechanisms underlying interactions between B. velezensis, considered as model plant-associated and beneficial species in the Bacillus genus, and Pseudomonas as a rhizosphere-dwelling competitor. Our data show that B. velezensis boosts its arsenal of specialized antibacterials upon the perception of the secondary siderophore pyochelin produced by phylogenetically distinct pseudomonads and some other genera. We postulate that B. velezensis has developed some chelator sensing systems to learn about the identity of its surrounding competitors. Illustrating the multifaceted molecular response of Bacillus, surfactin is another crucial component of the secondary metabolome mobilized in interbacteria competition. Its accumulation not only enhances motility capability but, unexpectedly, this lipopeptide also acts as a chemical trap that reduce the toxicity of other lipopeptides released by Pseudomonas challengers. This favors the persistence of Bacillus populations upon competitive root colonization. Our work thus highlights new ecological roles for bacterial secondary metabolites acting as key drivers of social interactions.