AUTHOR=Ouyang Xue , Chen Jialan , Sun Zhimao , Wang Rongbo , Wu Xuan , Li Benjin , Song Congfeng , Liu Peiqing , Zhang Meixiang 

TITLE=Ubiquitin E3 ligase activity of Ralstonia solanacearum effector RipAW is not essential for induction of plant defense in Nicotiana benthamiana

JOURNAL=Frontiers in Microbiology

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2023.1201444

DOI=10.3389/fmicb.2023.1201444

ISSN=1664-302X

ABSTRACT=<p>As one of the most destructive bacterial phytopathogens, <italic>Ralstonia solanacearum</italic> causes substantial annual yield losses of many important crops. Deciphering the functional mechanisms of type III effectors, the crucial factors mediating <italic>R. solanacearum</italic>-plant interactions, will provide a valuable basis for protecting crop plants from <italic>R. solanacearum</italic>. Recently, the NEL (novel E3 ligase) effector RipAW was found to induce cell death on <italic>Nicotiana benthamiana</italic> in a E3 ligase activity-dependent manner. Here, we further deciphered the role of the E3 ligase activity in RipAW-triggered plant immunity. We found that RipAW<sup>C177A</sup>, the E3 ligase mutant of RipAW, could not induce cell death but retained the ability of triggering plant immunity in <italic>N. benthamiana</italic>, indicating that the E3 ligase activity is not essential for RipAW-triggered immunity. By generating truncated mutants of RipAW, we further showed that the N-terminus, NEL domain and C-terminus are all required but not sufficient for RipAW-induced cell death. Furthermore, all truncated mutants of RipAW triggered ETI immune responses in <italic>N. benthamiana</italic>, confirming that the E3 ligase activity is not essential for RipAW-triggered plant immunity. Finally, we demonstrated that RipAW- and RipAW<sup>C177A</sup>-triggered immunity in <italic>N. benthamiana</italic> requires SGT1 (suppressor of G2 allele of <italic>skp1</italic>), but not EDS1 (enhanced disease susceptibility), NRG1 (N requirement gene 1), NRC (NLR required for cell death) proteins or SA (salicylic acid) pathway. Our findings provide a typical case in which the effector-induced cell death can be uncoupled with immune responses, shedding new light on effector-triggered plant immunity. Our data also provide clues for further in-depth study of mechanism underlying RipAW-induced plant immunity.</p>