biofilm formation only partially predicts beneficial protection against rhizosphere pathogens.

Plant roots form associations with both beneficial and pathogenic soil microorganisms. While members of the rhizosphere microbiome can protect against pathogens, the mechanisms are poorly understood. We hypothesized that the ability to form a robust biofilm on the root surface is necessary for the exclusion of pathogens; however, it is not known if the same biofilm formation components required are necessary WCS365 is a beneficial strain that is phylogenetically closely related to an opportunistic pathogen N2C3 and confers robust protection against N2C3 in the rhizosphere. We used this plant-mutualist-pathogen model to screen collections of WCS365 increased attachment mutants () and surface attachment defective () transposon insertion mutants that form increased or decreased levels of biofilm on an abiotic surface, respectively. We found that while the WCS365 mutants had altered biofilm formation , only a subset of these mutants, including those involved in large adhesion protein (Lap) biosynthesis, flagellin biosynthesis and O-antigen biosynthesis, lost protection against N2C3. We found that the inability of WCS365 mutants to grow , and the inability to suppress pathogen growth, both partially contributed to loss of plant protection. We did not find a correlation between the extent of biofilm formed and pathogen protection indicating that biofilm formation on abiotic surfaces may not fully predict pathogen exclusion . Collectively, our work provides insights into mechanisms of biofilm formation and host colonization that shape the outcomes of host-microbe-pathogen interactions.