Phosphocode-dependent functional dichotomy of a common co-receptor in plant signaling

Multicellular organisms employ cell-surface receptor kinases (RKs) to sense and process extracellular signals. Many plant RKs form ligand-induced complexes with shape-complementary co-receptors for their activation(1). The best-characterized co-receptor is BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1 (BAK1), which associates with numerous leucine-rich repeat (LRR)-RKs to control immunity, growth, and development(2). Here, we report key regulatory events controlling the functionality of BAK1 and, more generally, LRR-RKs. Through a combination of phospho-proteomics and targeted mutagenesis, we identified conserved phosphosites that are required for BAK1 immune function in Arabidopsis thaliana (hereafter Arabidopsis). Strikingly, these phosphosites are not required for BAK1-dependent brassinosteroid (BR)-regulated growth. In addition to revealing a critical role for BAK1 C-terminal tail phosphorylation, we identified a conserved tyrosine phosphosite that may be required for functionality of the majority of Arabidopsis LRR-RKs, and separates them into two distinct functional classes. Our results suggest a phosphocode-based dichotomy of BAK1 functionality in plant signaling, and provide novel insights into receptor kinase activation, which have broad implications for our understanding of how plants respond to their changing environment.