Molecular signals required for type III secretion and translocation of theXanthomonas campestris AvrBs2 protein to pepper plants

Strains of Xanthomonas campestris pv. vesicatoria (Xcv) carrying avrBs2 are specifically recognized by Bs2 pepper plants, resulting in localized cell death and plant resistance. Agrobacterium-mediated transient expression of the Xcv avrBs2 gene in plant cells results in Bs2-dependent cell death, ind icating that the AvrBs2 protein alone is sufficient for the activation of d isease resistance-mediated cell death in planta. We now provide evidence th at AvrBs2 is secreted from Xcv and that secretion is type III (hrp) depende nt. N- and C-terminal deletion analysis of AvrBs2 has identified the effect or domain of AvrBs2 recognized by Bs2 pepper plants. By using a truncated P seudomonas syringae AvrRpt2 effector reporter devoid of type III signal seq uences, we have localized the minimal region of AvrBs2 required for type II I secretion in Xcv. Furthermore, we have identified the region of AvrBs2 re quired for both type III secretion and translocation to host plants. The ma pping of AvrBs2 sequences sufficient for type III delivery also revealed th e presence of a potential mRNA secretion signal.