The GacA global regulator is required for the appropriate expression of Erwinia chrysanthemi 3937 pathogenicity genes during plant infection

Pathogenicity of the phytopathogenic enterobacterium Erwinia chrysanthemi, the causal agent of soft rot disease on many plants, is a complex process involving several factors whose production is regulated by a complex, intertwined regulatory network. In this work we characterized the GacA regulator, member of the GacS-GacA two-component system, as a global regulator which is required for disease expression but not for bacterial multiplication in planta during the first stages of the plant infection. GacA was shown to control the expression of plant cell wall-degrading enzymes and hrp genes in vitro. Analysis of virulence gene expression during infection of Arabidopsis thaliana revealed a coordinated expression of these virulence genes at 12 h post infection and showed that GacA is required for the appropriate production of virulence factors in planta. GacA might partly act by negatively controlling the expression of the pecT gene encoding the global repressor PecT, indicating a hierarchy in the pathways involved in the E. chrysanthemi regulatory network.