RK3, a G-Type LecRLK, Interacts with FLS2 and BAK1 to Promote flg22-Triggered Immunity

Lectin receptor-like kinases (LecRLKs) are a large subfamily of receptor-like kinases (RLKs), and their N-terminal lectin domain is predicted to reversibly bind to carbohydrates. Within this family, G-type LecRLKs represent a distinct subclass defined by an extracellular S-locus glycoprotein (SLG) domain, which was originally identified for its role in governing self-incompatibility in Brassica species. Emerging evidence suggests that G-type LecRLKs are involved in plant immunity; however, only a small fraction have been functionally characterized, leaving the roles of most family members largely unknown. In this study, we identified RK3 (Receptor Kinase 3) as the most strongly induced gene within the G-type LecRLK clade VI upon infection with Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). Through both gain- and loss-of-function analyses, we demonstrated that RK3 positively regulates flg22-induced immune signaling events, including oxidative burst and mitogen-activated protein kinase (MAPK) activation, as well as downstream responses such as defense gene expression and ethylene production. Remarkably, the immune-enhancing activity of RK3 does not require its kinase domain. Critically, both full-length RK3 and a kinase-deleted variant (RK3-ΔK) constitutively interact with FLS2 (Flagellin-Sensing 2) and BAK1 (BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1). This provides direct evidence that RK3 functions primarily as a co-regulatory component within the PRR complex, independent of its kinase activity. Moreover, ectopic expression of RK3 in tomato enhanced resistance to Pst DC3000, highlighting its potential utility in engineering disease resistance in crops. Thus, RK3 reveals a non-canonical, kinase-independent mechanism by which a G-type LecRLK potentiates plant immunity, expanding our understanding of RLK signaling complexity.