Plant immunity is tightly controlled by a complex and dynamic regulatory network, which ensures optimal activation upon detection of potential pathogens. Accordingly, each component of this network is a potential target for manipulation by pathogens. Here, we report that RipAC, a type III-secreted effector from the bacterial pathogen Ralstonia solanacearum, targets the plant E3 ubiquitin ligase PUB4 to inhibit pattern-triggered immunity (PTI). PUB4 plays a positive role in PTI by regulating the homeostasis of the central immune kinase BIK1. Before PAMP perception, PUB4 promotes the degradation of non-activated BIK1, while, after PAMP perception, PUB4 contributes to the accumulation of activated BIK1. RipAC leads to BIK1 degradation, which correlates with its PTI-inhibitory activity. RipAC causes a reduction in pathogen-associated molecular pattern (PAMP)-induced PUB4 accumulation and phosphorylation. Our results shed light on the role played by PUB4 in immune regulation, and illustrate an indirect targeting of the immune signalling hub BIK1 by a bacterial effector.

PUB4 was immunoprecipitated from PUB4-FLAG or PUB4-FLAG RipAC-GFP plants after treatment with PAMPs (1 mM flg22Paer, 1 mM elf18Ecol) or water (as mock treatment). The abundance of phosphorylated peptide was calculated as the ratio of intensities of phosphorylated vs PUB4 control peptide sum. Values represent the percentage of PUB4 phosphorylated peptide in PAMP-treated vs mock-treated seedlings, with 100 % indicating the same level of PUB4 phosphorylation in PAMP- and mock-treated seedlings.

PUB4 was immunoprecipitated from PUB4-FLAG or PUB4-FLAG RipAC-GFP plants. The samples were digested with trypsin and analysed by parallel reaction monitoring (PRM) LCMS/MS. Values represent the sum of intensities of PUB4 corresponding peptides. Data are means ± SE of two biological replicates, each of which contains three technical replicates (two-way ANOVA, Tukey’s multiple comparison test).