MEDEA-interacting protein LONG-CHAIN BASE KINASE 1 promotes pattern-triggered immunity in Arabidopsis thaliana.

KEY MESSAGE: Arabidopsis LONG-CHAIN BASE KINASE 1 (LCBK1) interacts with MEDEA, a component of PCR2 complex that negatively regulates immunity. LCBK1 phosphorylates phytosphingosine and thereby promotes stomatal immunity against bacterial pathogens. Arabidopsis polycomb-group repressor complex2 (PRC2) protein MEDEA (MEA) suppresses both pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). MEA represses the expression of RPS2 and thereby attenuates AvrRpt2 effector-mediated ETI. However, the mechanism of MEA-mediated PTI diminution was not known. By screening the Arabidopsis cDNA library using yeast-2-hybrid interaction, we identified LONG-CHAIN BASE KINASE1 (LCBK1) as an MEA-interacting protein. We found that lcbk1 mutants are susceptible to virulent bacterial pathogens, such as Pseudomonas syringae pv maculicola (Psm) and P. syringae pv tomato (Pst) but not the avirulent strain of Pst that carries AvrRpt2 effector. Pathogen inoculation induces LCBK1 expression, especially in guard cells. We found that LCBK1 has a positive regulatory role in stomatal closure after pathogen inoculation. WT plants close stomata within an hour of Pst inoculation or flg22 (a 22 amino acid peptide from bacterial flagellin protein that activates PTI) treatment, but not lcbk1 mutants. LCBK1 phosphorylates phytosphingosine (PHS). Exogenous application of phosphorylated PHS (PHS-P) induces stomatal closure and rescues loss-of-PTI phenotype of lcbk1 mutant plants. MEA overexpressing (MEA-Oex) plants are defective, whereas loss-of-function mea-6 mutants are hyperactive in PTI-induced stomatal closure. Exogenous application of PHS-P rescues loss-of-PTI in MEA-Oex plants. Results altogether demonstrate that LCBK1 is an interactor of MEA that positively regulates PTI-induced stomatal closure in Arabidopsis.