The Wheat NLR Protein PM3b Localises to Endoplasmic Reticulum–Plasma Membrane Contact Sites and Interacts With AVRPM3 ^b2/c2 Through Its LRR Domain

Plant nucleotide-binding leucine-rich repeat (NLR) proteins are intracellular immune receptors that directly or indirectly perceive pathogen-derived effector proteins to induce an immune response. NLRs display diverse subcellular localisations, which are associated with the capacity of the immune receptor to confer disease resistance and recognise its corresponding avirulence effector. In wheat, the NLR PM3b recognises the wheat powdery mildew effector AVRPM3^b2/c2 and we examined the molecular mechanism underlying this recognition. We show that PM3b and other PM3 variants localise to endoplasmic reticulum (ER)-plasma membrane (PM) contact sites (EPCS), while AVRPM3^b2/c2 localises to the nucleocytoplasmic space. Additionally, we found that PM3b interacts in planta with AVRPM3^b2/c2 through its LRR domain. We further demonstrate that full-length PM3b interaction with AVRPM3^b2/c2 is considerably weaker than for the isolated PM3b LRR domain or the susceptible PM3 variant PM3CS, indicating that activation of PM3b leads to dissociation of the complex. In line with this, we observed a strong interaction between PM3b and AVRPM3^b2/c2 in a P-loop mutant of PM3b that was unable to initiate a cell death response, or when an inactive variant of AVRPM3^b2/c2 was used. We propose that PM3b transiently interacts with AVRPM3^b2/c2 through residues in the LRR that are conserved among PM3 variants, while the amino acids necessary for full activation and cell death signalling are unique to PM3b. Our data suggests that PM3b localisation and interaction with AVRPM3^b2/c2 differ from other well-studied NLRs and further highlights the mechanistic diversity in NLR-mediated responses against pathogens in plants.