A tomato LysM receptor-like kinase promotes immunity and its kinase activity is inhibited by AvrPtoB

Resistance in tomato (Solanum lycopersicum) to infection by Pseudomonas syringae involves both detection of pathogen-associated molecular patterns (PAMPs) and recognition by the host Pto kinase of pathogen effector AvrPtoB which is translocated into the host cell and interferes with PAMP-triggered immunity (PTI). The N-terminal portion of AvrPtoB is sufficient for its virulence activity and for recognition by Pto. An amino acid substitution in AvrPtoB, F173A, abolishes these activities. To investigate the mechanisms of AvrPtoB virulence, we screened for tomato proteins that interact with AvrPtoB and identified Bti9, a LysM receptor-like kinase. Bti9 has the highest amino acid similarity to Arabidopsis CERK1 among the tomato LysM receptor-like kinases (RLKs) and belongs to a clade containing three other tomato proteins, SlLyk11, SlLyk12, and SlLyk13, all of which interact with AvrPtoB. The F173A substitution disrupts the interaction of AvrPtoB with Bti9 and SlLyk13, suggesting that these LysM-RLKs are its virulence targets. Two independent tomato lines with RNAi-mediated reduced expression of Bti9 and SlLyk13 were more susceptible to P. syringae. Bti9 kinase activity was inhibited in vitro by the N-terminal domain of AvrPtoB in an F173-dependent manner. These results indicate Bti9 and/or SlLyk13 play a role in plant immunity and the N-terminal domain of AvrPtoB may have evolved to interfere with their kinase activity. Finally, we found that Bti9 and Pto interact with AvrPtoB in a structurally similar although not identical fashion, suggesting that Pto may have evolved as a molecular mimic of LysM-RLK kinase domains.