Characterization of the Pseudomonas syringae pv. tomato AvrRpt2 protein: demonstration of secretion and processing during bacterial pathogenesis

Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000) expressing avrRp t2 is specifically recognized by plant cells expressing RPS2 activity, resu lting in localized cell death and plant resistance. Furthermore, transient expression of this bacterial avrRpt2 gene in plant cells results in RPS2-de pendent cell death. This indicates that the AvrRpt2 protein is recognized i nside RPS2 plant cells and is sufficient for the activation of disease resi stance-mediated cell death in plants. We explored the possibility that Pst DC3000 delivers AvrRpt2 protein to plant cells via the hrp (type III) secre tion pathway. We now provide direct evidence that mature AvrRpt2 protein is secreted from Pst DC3000 and that secretion is hrp dependent. We also show that AvrRpt2 is N-terminally processed when Arabidopsis thaliana plants ar e infected with Pst DC3000 expressing avrRpt2. Similar N-terminal processin g of AvrRpt2 occurred when avrRpt2 was stably expressed in A. thaliana. No cleavage of AvrRpt2 was detected in bacteria expressing avrRpt2 in culture or in the plant extracellular fluids. The N-terminus of AvrRpt2 was not req uired for RPS2 recognition in planta. However, this region of AvrRpt2 was e ssential for Pst DC3000-mediated elicitation of RPS-dependent cell death in A. thaliana leaves.