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. Overall design: 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

植物病原细菌在植物免疫的早期阶段遭遇细胞外pH值急剧上升。对碱性化对病原体-宿主相互作用的影响尚未进行全面表征。本研究采用全局转录组学方法，评估了环境碱性化对番茄黄萎病菌Pseudomonas syringae pv. tomato DC3000体外培养的影响。除了Ⅲ型分泌系统（T3SS）外，我们还发现编码其他致病因子如铁摄取和冠毒素生物合成的基因表达受到环境碱性化的强烈影响。此外，我们还发现细胞外功能σ因子AlgU在pH 5.5时被诱导，在pH 7.8时被抑制，这两个pH值分别是病原体在触发免疫反应前后的可能经历的pH水平。这种pH依赖性的控制需要周质蛋白酶AlgW和MucP的存在，它们作为环境感应系统的一部分，在特定条件下激活AlgU。这是AlgU活性pH依赖性的第一个例子，暗示了一种调控途径模型，其中pH值影响依赖蛋白质水解的AlgU激活。这些结果有助于加深对细胞外pH值在宿主-病原体相互作用中作用的理解。总体设计：对Pst DC3000野生型和algU敲除菌株在pH值分别为5.5、6.5和7.8的KB培养基生长条件下的RNA测序数据进行比较基因表达谱分析。