NIK receptor kinases propagate phytocytokine-triggered immunity by driving early phosphorylation events.

The perception of damage-associated molecular patterns and phytocytokines by pattern recognition receptors activates danger-triggered immunity, thereby amplifying plant immune responses. However, how phytocytokine receptor complexes are assembled, activated and regulated remains poorly understood. Here we show that all six members of the nuclear-shuttle-protein-interacting kinase (NIK) family, a clade within LRRII-RLK subfamily, contribute redundantly but distinctively to immune responses triggered by Peps and SCOOP12 in Arabidopsis. NIKs associate with the phytocytokine receptors PEPR1 and MIK2 and the co-receptor BAK1, and ligand perception transiently enhances these associations. NIK1, NIK3 and NIK6 undergo robust ligand-dependent phosphorylation that requires PEPR signalling, whereas PEPR1 and BAK1 can each phosphorylate NIKs. NIKs are dispensable for Pep2-induced PEPR-SERK association but are required for full phosphorylation output from the receptor complex, for efficient recruitment of BIK1 and PBL1 and for the activation of BIK1. Genetic complementation further identifies Ser615 and Tyr618 as essential for NIK1-mediated immune signalling, with Tyr618 preferentially controlling BIK1 recruitment and phosphorylation. These findings define NIKs as non-canonical, signal-propagating components of multiple phytocytokine receptor complexes and reveal how early phosphorylation events mediate danger-triggered immunity.