O’Malley M.RChien C.-FPeck S.CNAI-CHUN LINAnderson J.C.Peck, Scott C.Scott C.PeckO'Malley, Megan R.Megan R.O'MalleyAnderson, Jeffrey C.Jeffrey C.AndersonChien, Ching FangChing FangChien2022-12-142022-12-14202014646722https://www.scopus.com/inward/record.uri?eid=2-s2.0-85073441049&doi=10.1111%2fmpp.12876&partnerID=40&md5=f0014951d648231413d8519c057ccafdhttps://scholars.lib.ntu.edu.tw/handle/123456789/626006GacS/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. © 2019 The Authors. Molecular Plant Pathology published by British Society for Plant Pathology and John Wiley & Sons LtdGacS/GacA; plant–microbe interactions; Pseudomonas syringae; two-component regulatory systems; type III secretion; virulence regulation[SDGs]SDG2[SDGs]SDG13bacterial protein; GacA protein, Bacteria; lemA protein, bacterial; transcription factor; Arabidopsis; biological model; gene expression regulation; genetics; metabolism; microbiology; pathogenicity; physiology; Pseudomonas syringae; type III secretion system; virulence; Arabidopsis; Bacterial Proteins; Gene Expression Regulation, Bacterial; Models, Biological; Pseudomonas syringae; Transcription Factors; Type III Secretion Systems; Virulencejournal article10.1111/mpp.12876315886612-s2.0-85073441049