RB-TnSeq analysis reveals alcohol and salt tolerance systems in a plant root colonizer Paraburkholderia graminis OAS925

The role of microbial strains in regulating natural stresses and their impact on plant health is well-established. However, the role of microbial tolerance mechanisms in plant response to unnatural or anthropogenic stresses is less understood. Examination of these interactions impact our deeper understanding of plant-microbe interactions and our ability to enhance beneficial functions. In this study we use the model plant Brachypodium distachyon and its prominent root colonizer Paraburkholderia graminis OAS925 to investigate mechanisms of tolerance to alcohol and salt stress. We examined the ability of OAS925 to reduce root growth inhibition during exposure to short chain alcohols and salt. We also examined the tolerance mechanism for OAS925 towards these stresses using RB-TnSeq fitness assays. The most prominent tolerance systems in OAS925 are genes specifically involved in membrane transport (such as the Mla operon), efflux systems (e.g., RND efflux systems), signaling and regulation (PrtR/PrtI, NtrY/NtrX, and EnvZ/OmpR), and oxidative stress response (GshB). Our findings provide a model where bacterial membrane integrity, active solvent efflux, and stress signaling are crucial not only for bacterial survival but also for maintaining the root colonization and biofilm formation that confer protection to the host plant.