P-519 - Unraveling the code: BAK1 phosphorylation in eNAD(P) signaling and plant immunity

Plants rely on receptor-like kinases (RLKs) and receptor-like proteins (RLPs) to perceive extracellular signals and mount immune responses. During pathogen infection, NAD(P) is released into the extracellular space, acting as an important immune signal. Recent studies have shown that extracellular NAD(P) [eNAD(P)] binds to the L-type lectin receptor-like kinase VI.2 (LecRK-VI.2) which associates with the co-receptor BAK1 to trigger eNAD(P)-induced immunity and systemic acquired resistance (SAR). LecRK-VI.2/BAK1 phosphorylation is essential for eNAD(P) signaling and tightly regulated, given BAK1’s broad role in plant immunity and development. Our study aims to identify eNAD(P)-specific phosphorylation sites in BAK1 and their roles in immune signaling, uncovering phosphocodes that dictate BAK1’s function across diverse signaling pathways. Using LC-MS, we identified novel and conserved BAK1 phosphorylation sites specifically induced by eNAD(P) in Arabidopsis, distinct from those triggered by PTI or BR signaling. Functional analysis of phospho-deficient mutants revealed significantly impaired eNAD(P)-induced local and systemic resistance, as well as reduced biological SAR. Our findings reveal unique phosphorylation codes within BAK1, highlighting its phosphorylation-dependent specificity in eNAD(P) signaling and advancing our understanding of broad RLK/RLP activation mechanisms.