(27e) An Immunosuppressive Subtilase from Commensal Rhizobacteria Modulates Plant Host Immune Activation

Plants grow in association with a complex microbiome that contains beneficial, pathogenic, and commensal microbes. Many of these microbes produce immune eliciting microbial associated molecular patterns (MAMPs), yet not all microbes activate the plant immune system. The mechanisms that microbes, particularly commensals, utilize to suppress or evade immune detection are not well understood. A common MAMP in plant systems is flg22, a 22 amino acid portion of bacterial flagellin, which is sensed by innate immunity pattern recognition receptor FLS2. By individually screening 165 root-associated commensal bacteria in a root growth inhibition assay with flg22 and Arabidopsis we identify 40% (66/165) of these strains suppress plant immune activation. Many of the strongest suppressors are from the order Xanthomonadales. Using bacterial genetics, we demonstrate a type II-secreted subtilase, which we call Immunosuppressive Subtilase A (IssA), is a major contributor to deactivating flg22 and preventing perception by the plant immune system in these strains. Further, we show recombinant IssA has strong proteolytic activity against flg22 peptide. Taken together these results elucidate a novel immunomodulatory protease and mechanism of immune evasion by plant-associated microbes. This fundamental genetic and biochemical understanding will enable us to engineer plant microbiome systems to improve plant host growth and health.