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TECHNICAL SESSION: Elucidation of plant immune responses to bacterial plant pathogens

A UBC13-Interacting, RING Type E3 Ligase Acts with BAK1 And BIK1 to Regulate Plant Immunity
Lirong Zeng - Plant Pathology Department, University of Nebraska. Pooja Verma- Plant Pathology Department, University of Nebraska, Yi Zhang- Plant Pathology Department, University of Nebraska, Bangjun Zhou- Plant Pathology Department, University of Nebraska

The tomato UBC13-type ubiquitin E2 Enzyme Fni3 modulates plant immunity through K63-linked ubiquitination. However, the molecular basis underlying the modulation remains elusive. We report here that a RING type E3 ligase, Fti1 regulates plant immunity through acting with BAK1 and BIK1. The tomato Fti1 (Solanum lycopersicum Fni3 interacting protein 1, SlFti1) and its Arabidopsis homolog, AtFti1 were found to interact with Fni3 and AtUBC13, respectively to catalyze K63-linked ubiquitination. The SlFti1 and AtFti1 gene both undergo alternative splicing, generating two transcripts that encode protein isoforms differ solely in the presence of a nuclear localization signal (NLS). The two isoforms of AtFti1 and the isoform that lacks the NLS (AtFti1-2) interacted with BIK1 and BAK1, respectively and the interaction required the N-terminal region of AtFti1. AtFti1-2 ubiquitinated BAK1 but not BIK1 in vitro. Meanwhile, both BIK1 and BAK1 phosphorylated AtFti1-2. The BIK1 phosphorylated AtFti1-2 mainly at amino acid S395, leading to compromise of its E3 ligase activity. The null mutant of AtFti1 displayed reduced sensitivity to flg22 and were more susceptible to infection by Pseudomonas syringae pv tomato (Pst). Consistently, plants overexpressing AtFti1-2 displayed increased accumulation of FLS2 and elevated sensitivity to flg22 and reduced pathogen growth after Pst infection. Our study suggested that AtFti1-2 is a positive regulator of FLS2-mediated immunity and BIK1 targets AtFti1-2 as a feedback. Characterization of the role of BAK1-AtFti1 interaction is undergoing.