Type III effector proteins from the plant pathogen Xanthomonas and their role in the interaction with the host plant.

Pathogenicity of Xanthomonas campestris pathovar (pv.) vesicatoria and most other Gram-negative bacterial plant pathogens largely depends on a type III secretion (TTS) system which is encoded by hypersensitive response and pathogenicity (hrp) genes. These genes are induced in the plant and are essential for the bacterium to be virulent in susceptible hosts and for the induction of the hypersensitive response (HR) in resistant host and non-host plants. The TTS machinery secretes proteins into the extracellular milieu and effector proteins into the plant cell cytosol. In the plant, the effectors presumably interfere with cellular processes to the benefit of the pathogen or have an avirulence activity that betrays the bacterium to the plant surveillance system. Type III effectors were identified by their avirulence activity, co-regulation with the TTS system and homology to known effectors. A number of effector proteins are members of families, e.g., the AvrBs3 family in Xanthomonas. AvrBs3 localizes to the nucleus of the plant cell where it modulates plant gene expression. Another family that is also present in Xanthomonas is the YopJ/AvrRxv family. The latter proteins appear to act as SUMO cysteine proteases in the host. Here, we will present an overview about the regulation of the TTS system and its substrates and discuss the function of the AvrRxv and AvrBs3 family members in more detail.