An In-depth Analysis of Iron and Pathogenicity Regulatory Pathways in Pseudomonas syringae pv. syringae B728a

Abstract

Pseudomonas syringae pv. syringae strain B728a (P.s.s. B728a) is an economically significant plant pathogen that is capable of successful epiphytic colonization of leaf surfaces. Although the virulence factors associated with this pathogen’s ability to cause disease have been well studied, the transition from epiphyte to pathogen is not well understood. The research described in this dissertation utilizes high throughput sequencing transcriptome analyses to define an iron regulatory network that is predicted to be utilized during the epiphytic portion of the P.s.s. B728a lifecycle. This dissertation also describes a collaborative microarray analysis that analyzes the P.s.s. B728a transcriptome at a global level.

An iron associated sigma factor, AcsS, encoded within a peptide synthesis rich region of the P.s.s. B728a genome is shown to regulate the citrate siderophore achromobactin. RNA-seq transcriptome analysis reveals that this sigma factor regulates expression of genes predicted to be involved in functions that are important during the epiphytic stage of P.s.s. B728a, including genes involved in iron response, secretion, extracellular polysaccharide production, and cell motility.

As part of a collaboration, the transcriptomes of the P.s.s. B728a genome and nine deletion mutants in regulatory genes were analyzed by microarray analayses using seven treatment conditions, including epiphytic and in planta conditions. As part of these microarray analyses, results are described for the global regulator, GacS, and a downstream transcription factor, SalA. This study confirms the role of GacS and SalA in the regulation of major virulence components of P.s.s. B728a such as phytotoxin production and Type III secretion. This study also elucidates a role for GacS and SalA regulation of genes important for epiphytic survival and function, including the Type VI secretion system, iron acquisition, and EPS production.