Differential phosphorylation of receptor kinase SlLYK4 mediates immune responses to bacterial and fungal pathogens in tomato.

Bacterial wilt caused by Ralstonia solanacearum is a devastating plant disease. Exopolysaccharide (EPS), a major virulence factor of R. solanacearum, elicits pattern-triggered immunity (PTI) in tomato, but the means by which EPS is recognized in the plant remain poorly understood. We found that tomato non-arginine-aspartate (non-RD) receptor kinase SlLYK4 mediates the perception of R. solanacearum EPS and positively regulates resistance to bacterial wilt. The RD receptor kinases SlLYK1 and SlLYK13 are required for EPS-triggered immune responses and form complexes with SlLYK4. These receptor kinase complexes have dual functions in recognizing bacterial EPS and fungal chitin. Phosphorylation of serine-320 in the juxtamembrane domain of SlLYK4 is essential in EPS- and chitin-mediated signaling, whereas phosphorylation of serine-334 or serine-634 in the C-terminal domain is required for chitin or EPS signaling, respectively. Our results reveal the mechanism underlying EPS recognition in tomato and provide insight into how differential phosphorylation of receptor kinase regulates antibacterial and antifungal immunity.