NLR surveillance of pathogen interference with hormone receptors induces immunity.

Phytohormone signalling pathways have an important role in defence against pathogens mediated by cell-surface pattern recognition receptors and intracellular nucleotide-binding leucine-rich repeat class immune receptors (NLR). Pathogens have evolved counter-defence strategies to manipulate phytohormone signalling pathways to dampen immunity and promote virulence. However, little is known about the surveillance of pathogen interference of phytohormone signalling by the plant innate immune system. The pepper (Capsicum chinense) NLR Tsw, which recognizes the effector nonstructural protein NSs encoded by tomato spotted wilt orthotospovirus (TSWV), contains an unusually large leucine-rich repeat (LRR) domain. Structural modelling predicts similarity between the LRR domain of Tsw and those of the jasmonic acid receptor COI1, the auxin receptor TIR1 and the strigolactone receptor partner MAX2. This suggested that NSs could directly target hormone receptor signalling to promote infection, and that Tsw has evolved a LRR resembling those of phytohormone receptors LRR to induce immunity. Here we show that NSs associates with COI1, TIR1 and MAX2 through a common repressor-TCP21-which interacts directly with these phytohormone receptors. NSs enhances the interaction of COI1, TIR1 or MAX2 with TCP21 and blocks the degradation of corresponding transcriptional repressors to disable phytohormone-mediated host immunity to the virus. Tsw also interacts directly with TCP21 and this interaction is enhanced by viral NSs. Downregulation of TCP21 compromised Tsw-mediated defence against TSWV. Together, our findings reveal that a pathogen effector targets TCP21 to inhibit phytohormone receptor function, promoting virulence, and a plant NLR protein has evolved to recognize this interference as a counter-virulence strategy, thereby activating immunity.