Perception of structurally distinct effectors by the integrated WRKY domain of a plant immune receptor

Abstract

Plants use intracellular nucleotide-binding domain (NBD) and leucine-rich repeat (LRR)-containing immune receptors (NLRs) to detect pathogen-derived effector proteins. The <i>Arabidopsis</i> NLR pair RRS1-R/RPS4 confers disease resistance to different bacterial pathogens by perceiving the structurally distinct effectors AvrRps4 from <i>Pseudomonas syringae</i> pv. <i>pisi</i> and PopP2 from <i>Ralstonia solanacearum</i> via an integrated WRKY domain in RRS1-R. How the WRKY domain of RRS1 (RRS1^WRKY) perceives distinct classes of effector to initiate an immune response is unknown. Here, we report the crystal structure of the in planta processed C-terminal domain of AvrRps4 (AvrRps4^C) in complex with RRS1^WRKY Perception of AvrRps4^C by RRS1^WRKY is mediated by the β2-β3 segment of RRS1^WRKY that binds an electronegative patch on the surface of AvrRps4^C Structure-based mutations that disrupt AvrRps4^C-RRS1^WRKY interactions in vitro compromise RRS1/RPS4-dependent immune responses. We also show that AvrRps4^C can associate with the WRKY domain of the related but distinct RRS1B/RPS4B NLR pair, and the DNA-binding domain of <i>At</i>WRKY41, with similar binding affinities and how effector binding interferes with WRKY-W-box DNA interactions. This work demonstrates how integrated domains in plant NLRs can directly bind structurally distinct effectors to initiate immunity.