Direct biochemical evidence for type III secretion-dependent translocation of the AvrBs2 effector protein into plant cells

The calmodulin-dependent adenylate cyclase domain (Cya) of the Bordetella pertussis cyclolysin was used as a reporter protein to study the direct translocation of the Xanthomonas effector protein, AvrBs2, into the plant host cell. Adenylate cyclase activity (production of cAMP) depends on the presence of eukaryotic plant calmodulin and is only active after translocation from the prokaryotic cell into the eukaryotic plant cell. Here, we show that infection of pepper plants by Xanthomonas campestris pv. vesicatoria strains expressing the AvrBs2:Cya fusion protein results in detectable increases of cAMP levels in plant cells as early as 3 h after inoculation. Adenylate cyclase activity was shown to be type III secretion-dependent as the Xanthomonas hrp mutations, hrcV or hrpF, failed to produce detectable levels of cAMP in infected pepper plants. Furthermore, the N-terminal secretion and translocation signals of AvrBs2 were shown to be required for activity of the fusion protein in the plant. A single genomic copy of the avrBs2:cya fusion gene expressed under the control of the wild-type avrBs2 promoter was used to compare the effect of a susceptible and resistant plant interaction on the kinetics of effector protein delivery. Implications of these results and additional applications of this reporter construct are discussed.