Expression of Xanthomonas campestris pv. vesicatoria type III effectors in yeast affects cell growth and viability

The gram-negative bacterium Xanthomonas campestris pv. vesicatoria is the causal agent of spot disease in tomato and pepper. X. campestris pv. vesicatoria pathogenicity depends on a type III secretion system delivering effector proteins into the host cells. We hypothesized that some X. campestris pv. vesicatoria effectors target conserved eukaryotic cellular processes and examined phenotypes induced by their expression in yeast. Out of 21 effectors tested, 14 inhibited yeast growth in normal or stress conditions. Viability assay revealed that XopB and XopF2 attenuated cell proliferation, while AvrRxo1, XopX, and XopE1 were cytotoxic. Inspection of morphological features and DNA content of yeast cells indicated that cytotoxicity caused by XopX and AvrRxo1 was associated with cell-cycle arrest at G0/1. Interestingly, XopB, XopE1, XopF2, XopX, and AvrRxo1 that inhibited growth in yeast also caused phenotypes, such as chlorosis and cell death, when expressed in either host or nonhost plants. Finally, the ability of several effectors to cause phenotypes in yeast and plants was dependent on their putative catalytic residues or localization motifs. This study supports the use of yeast as a heterologous system for functional analysis of X. campestris pv. vesicatoria type III effectors, and sets the stage for identification of their eukaryotic molecular targets and modes of action.