The AvrB_AvrC Domain of AvrXccCof Xanthomonas campestris pv. campestris Is Requiredto Elicit Plant Defense Responsesand Manipulate ABA Homeostasis

Plant disease induced by Xanthomonas campestris pv. campestrisdepends on type III effectors but the molecular basisis poorly understood. Here, AvrXccC8004 was characterized,and it was found that the AvrB_AvrC domain was essentialand sufficient to elicit defense responses in an Arabidopsisresistantecotype (Col-0). An upregulation of genes in respondingto the AvrB_AvrC domain of AvrXccC8004 wasshown in a profile of host gene expression. The molecularchanges were correlated with morphological changes observedin phenotypic and ultrastructural characterizations.Interestingly, the abscisic acid (ABA)-signaling pathway wasalso a prominent target for the AvrB_AvrC domain ofAvrXccC8004. The highly elicited NCED5, encoding a key enzymeof ABA biosynthesis, was increased in parallel withABA levels in AvrXccC8004 transgenic plants. Consistently,the X. campestris pv. campestris 8004 ΔavrXccC mutant wasseverely impaired in the ability to manipulate the accumulationof ABA and induction of ABA-related genes in challengedleaves. Moreover, exogenous application of ABA alsoenhanced the susceptibility of Arabidopsis to the X. campestrispv. campestris strains. These results indicate that theAvrB_AvrC domain of AvrXccC8004 alone has the activity tomanipulate ABA homeostasis, which plays an important rolein regulating the interactions of X. campestris pv. campestrisand Arabidopsis