Xanthomonas axonopodis pv. punicae (Xap) causing bacterial blight is an important pathogen that incurs significant losses to the exportability of pomegranate. Xap uses the Xop TTSS-effector, via the type three secretion system, to suppress pomegranate immunity. Here, we investigate the role of XopL during blight pathogenesis. We observed that XopL is essential for its in planta growth and full virulence. Leaves inoculated with Xap ΔxopL produced restricted water-soaked lesions compared to those inoculated with wild-type Xap. XopL supports Xap for its sustained multiplication in pomegranate by suppressing the plant cell death (PCD) event. We further demonstrated that XopL suppresses immune responses, such as callose deposition and production of reactive oxygen species (ROS). RT-qPCR analysis revealed that immune responsive genes were upregulated when challenged with Xap ΔxopL, whereas upregulation of such genes was compromised in the complemented strain containing the xopL gene. The transiently expressed XopL::EYFP fusion protein was localized to the plasma membrane, indicating the possible site of its action. Altogether, this study highlights that XopL is an important TTSS-effector of Xap that suppresses plant immune responses, including PCD, presumably to support the multiplication of Xap for a sufficient time-period during blight disease development.
© 2017 Institute of Botany, Chinese Academy of Sciences.