Environmental alkalization suppresses deployment of virulence strategies in Pseudomonas syringae pv. tomato DC3000

Plant pathogenic bacteria encounter a drastic increase in apoplastic pH during the early stages of plant immunity. The effects of alkalization on pathogen-host interactions have not been comprehensively characterized. Here we used a global transcriptomic approach to assess the impact of environmental alkalization on Pseudomonas syringae pv. tomato DC3000 in vitro. In addition to the Type 3 Secretion System (T3SS), we found expression of genes encoding other virulence factors such as iron uptake, and coronatine biosynthesis to be strongly affected by environmental alkalization. We also found activity of extracytoplasmic function sigma factor, AlgU, was induced at pH 5.5 and suppressed at pH 7.8, which are pH levels that this pathogen would likely experience before and during pattern triggered immunity, respectively. This pH-dependent control requires the presence of periplasmic proteases, AlgW and MucP, that function as part of the environmental sensing system that activates AlgU in specific conditions. This is the first example of pH-dependency of AlgU activity, suggesting a regulatory pathway model where pH affects the proteolysis-dependent activation of AlgU. These results contribute to deeper understanding of the role apoplastic pH has on host-pathogen interactions. Comparative gene expression profiling analysis of RNA-seq data for Pst DC3000 WT and algU deletion strains across pH-modified KB media growth condition at pH 5.5, 6.5 and 7.8