Many bacteria that are pathogens for mammals, insects or plants use a specialized apparatus called the type III secretion system to inject a diverse set of effector proteins into the cytoplasm of their eukaryotic host cells in order to alter cellular processes in favour of the pathogen’s lifestyle. However, direct cellular targets have been identified for only a few effector proteins and the elucidation of their mode of action is of fundamental interest for the understanding of bacterial virulence strategies. The effector XopJ from the phytopathogen Xanthomonas campestris pv. vesicatoria (Xcv), the causal agent of bacterial spot disease on tomato and pepper plants, belongs to the YopJ-superfamily of effector proteins. Members of this family are found among plant and animal pathogens, as well as plant symbionts. We show here that within plant cells XopJ targets the proteasomal subunit RPT6 to suppress host proteasome activity and thus protein turnover. In pepper leaves, this leads to reduced accumulation of the defence hormone salicylic acid (SA) and also attenuates SA-mediated defence responses such as tissue degeneration and defence gene expression. XopJ from Xcv is the first example of a bacterial effector protein targeting the host proteasome and our results also suggest a central role of the proteasome in plant immunity.
The picture above represents an in planta visualization of the interaction between the bacterial type III effector XopJ and the proteasomal subunit RPT6 using bi-molecular fluorescence complementation.