Fine-tuning of IPA1 transactivation activity by E3 ligase IPI7-mediated non-proteolytic K29-ubiquitination during Magnaporthe oryzae infection.

The Ideal Plant Architecture 1 (IPA1) transcription factor promotes rice yield and immunity through phosphorylation at its amino acid residue Ser163 as a switch. Although phosphorylated IPA1 mimic, IPA1(S163D), directly targets the promoter of immune response gene WRKY45, it cannot activate its expression. Here, we identified a co-activator of IPA1(S163D), a RING-finger E3 ligase IPA1 interactor 7 (IPI7), which fine-tunes the transcriptional activity of IPA1 to timely promote plant immunity and simultaneously maintain growth for yield. IPI7 interacts with IPA1 and promotes K29-polyubiquitination of IPA1 in vitro and in vivo. However, the stability of IPA1 protein is not affected by IPI7-mediated ubiquitination. The IPI7-promoted K29-polyubiquitination of IPA1 is induced by Magnaporthe oryzae infection and required for phosphorylated IPA1 to transactivate WRKY45 expression for immune response but not for plain IPA1 to transactivate DENSE AND ERECT PANICLES 1 (DEP1) expression for panicle development. IPI7 knockout impairs IPA1-mediated immunity but not yield. Our study reveals that plants utilize non-proteolytic K29-ubiquitination as a response to pathogen infection to fine-tune IPA1 transactivation activity for promoting immunity.