Type III secretion and in planta recognition of the Xanthomonas avirulence proteins AvrBs1 and AvrBsT

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Abstract
The hrp gene cluster of the plant pathogen Xanthomonas campestris pv. vesicatoria (Xcv) encodes a type III secretion system required for the delivery of virulence and avirulence proteins into the plant. Some of these effector proteins, e.g. AvrBs1 and AvrBsT, are recognized by pepper plants carrying corresponding resistance genes, triggering the hypersensitive reaction (HR). In this study, epitope tagged AvrBs1 and AvrBsT proteins were detected in culture supernatants only in the presence of a functional type III apparatus and not in a hrcV mutant, showing that both proteins are secreted by Xcv in an hrp-dependent manner. Expression of both avirulence genes is constitutive and independent of the hrp gene regulators, hrpG and hrpX. Transient expression of avrBs1 and avrBsT in resistant host plants using Agrobacterium tumefaciens-mediated gene transfer resulted in the induction of a specific HR. This indicates that recognition occurs intracellularly, and suggests that during the Xcv infection, AvrBs1 and AvrBsT are translocated from Xcv into the plant cell. We describe a conserved protein motif which is present in the N-terminal region of all known Xcv avirulence proteins and discuss its potential role in translocation into plant cells.