A gacA- mutant of Pseudomonas syringae hyper-responds to type III secretion-inducing plant signals yet is less virulent
M. O'MALLEY (1), S. Peck (2), J. Anderson (1) (1) Department of Botany & Plant Pathology, Oregon State University, U.S.A.; (2) Division of Biochemistry, University of Missouri, U.S.A.

The type III secretion system (T3SS) of phytopathogenic bacteria is an important virulence determinant that must be produced and assembled at the start of infection. We previously showed that T3SS-associated genes in Pseudomonas syringae pv. tomato DC3000 are induced by specific organic acids exuded by plants, and that the abundance of these metabolites influences the outcome of infection. However, how these plant signals are perceived by DC3000 and how they regulate T3SS deployment is unknown. To investigate the underlying mechanism(s) involved, we tested a DC3000 mutant that lacks the two-component response regulator gacA for its ability to respond to the bioactive metabolites. This mutant strain was previously reported to be less virulent and have lower expression of T3SS-associated genes. Surprisingly, we found that T3SS genes are hyper-expressed in gacA- in response to the bioactive organic acids. To clarify these results, we confirmed the decreased virulence of gacA- on Arabidopsis. Furthermore, we measured the levels of T3SS-associated transcripts in gacA- during infection of Arabidopsis and found enhanced T3SS gene expression also occurs in planta. Based on these data, we are currently testing if loss of gacA- virulence is due a) decreased effector delivery, b) mis-regulation of T3SS-independent virulence determinants, and/or c) a stronger host immune response. Our progress towards elucidating the molecular mechanisms of observed gacA- phenotypes will be presented.

Abstract Number: C22-4, P8-231
Session Type: Concurrent