A revised model for the role of GacS/GacA in regulating type III secretion by Pseudomonas syringae pv. tomato DC3000.

Megan R O’Malley,Ching‐Fang Chien,Jeffrey C Anderson,Nai‐Chun Lin,Scott C Peck

doi:10.1111/mpp.12876

Abstract

SummaryGacS/GacA is a conserved two‐component system that functions as a master regulator of virulence‐associated traits in many bacterial pathogens, including Pseudomonas spp., that collectively infect both plant and animal hosts. Among many GacS/GacA‐regulated traits, type III secretion of effector proteins into host cells plays a critical role in bacterial virulence. In the opportunistic plant and animal pathogen Pseudomonas aeruginosa, GacS/GacA negatively regulates the expression of type III secretion system (T3SS)‐encoding genes. However, in the plant pathogenic bacterium Pseudomonas syringae, strain‐to‐strain variation exists in the requirement of GacS/GacA for T3SS deployment, and this variability has limited the development of predictive models of how GacS/GacA functions in this species. In this work we re‐evaluated the function of GacA in P. syringae pv. tomato DC3000. Contrary to previous reports, we discovered that GacA negatively regulates the expression of T3SS genes in DC3000, and that GacA is not required for DC3000 virulence inside Arabidopsis leaf tissue. However, our results show that GacA is required for full virulence of leaf surface‐inoculated bacteria. These data significantly revise current understanding of GacS/GacA in regulating P. syringae virulence.

Highlights

In P. syringae, GacS/GacA functions as a master regulator of multiple virulence traits, including T3SS deployment, toxin production and motility (Chatterjee et al, 2003; Heeb and Haas, 2001; Mole et al, 2007)
We further show that GacA is dispensable for DC3000 virulence in the leaf interior but is required for virulence of leaf surface-inoculated bacteria, most likely due to motility defects caused by loss of gacA
We introduced a transcriptional reporter consisting of the promoter of T3SS effector gene avrPto fused to green fluorescent protein into DC3000 and AC811

Summary

Introduction

In P. syringae, GacS/GacA functions as a master regulator of multiple virulence traits, including T3SS deployment, toxin production and motility (Chatterjee et al, 2003; Heeb and Haas, 2001; Mole et al, 2007). Syringae B728a had decreased field fitness yet had sufficient levels of type III secretion to trigger a host defence response and was fully virulent in laboratory infections of host plants (Hirano et al, 1997; Willis et al, 1990).

Results

Conclusion