A systematic screen of conserved R. solanacearum effectors reveals the role of RipAB, a nuclear-localized effector that suppresses immune responses in potato.

Both Solanum tuberosum and Ralstonia solanacearum phylotype IIB originated in South America and share a long-term coevolutionary history. However, our knowledge of potato bacterial wilt pathogenesis is scarce due to the technical difficulties of potato plant manipulation. Thus, we established a multiple screening system (virulence screening of effector mutants in potato, growth inhibition of yeast, and transient expression in Nicotiana benthamiana) of core type III effectors (T3Es) of a major potato pathovar of phylotype IIB, to provide more research perspectives and biological tools. Using this system, we identified four effectors contributing to virulence during potato infection and two of them exhibited multiple phenotypes in two other systems, including RipAB. Further study showed that RipAB is an unknown protein with a nuclear localization signal (NLS). Furthermore, we generated a RipAB complementation strain and transgenic RipAB-expressing potato plants, and subsequent virulence assays confirmed that R. solanacearum requires RipAB for full virulence. Compared with wild type potato, transcriptomic analysis of transgenic RipAB-expressing potato plants showed a significant down-regulation of Ca2+ signaling related genes in enriched Pathogen and Plant Interaction (PPI) gene ontology (GO) term. We further verified, during infection, RipAB is required for the downregulation of four Ca2+ sensors, StCML5, StCML23, StCML-Cast, and StCDPK2, and a Ca2+ transporter, StCNGC1. Further evidence showed that immune-associated ROS burst is attenuated in RipAB transgenic potato plants. In conclusion, a systematic screen of conserved R. solanacearum effectors revealed an important role for RipAB, which interferes with Ca2+ -dependent gene expression to promote disease development in potato. This article is protected by copyright. All rights reserved.