A Xanthomonas oryzae type III effector XopL causes cell death through mediating ferredoxin degradation in Nicotiana benthamiana

Plant pathogenic bacteria engage in an ongoing struggle for survival. Gram negative phytopathogenic prokaryotes deploy type III secretion system to deliver effector proteins into plant cells to manipulate the host cellular environment. In many cases, the targets of these effector proteins are unknown. The bacterial blight pathogen, Xanthomonas oryzae pv. oryzae (Xoo), causes a devastating disease in rice and the Xoo-rice interaction is an example of the ongoing evolutionary battle between plants and pathogens. In this study, we show that an effector XopL from Xoo exhibits E3 ubiquitin ligase activity and induces cell death in the nonhost Nicotiana benthamiana (Nb). Through yeast two-hybrid, co-immunoprecipitation, and biomolecular fluorescence complementation assays, it was shown that XopL interacts with ferredoxin (NbFd), an electron transport molecule in plant chloroplasts. XopL induced defense-related response in Nb and specifically targeted NbFd for ubiquitination and ubiquitin-mediated degradation, a process that increased the production of reactive oxygen species (ROS). This study demonstrates the ubiquitination and degradation of NbFd as a result of XopL activity, which further illustrates the tremendous functional diversity in Xanthomonas effector proteins.