Interactions of Wheat Powdery Mildew Effectors Involved in Recognition by the Wheat NLR PM3.

To successfully colonize the living tissue of its host, the fungal wheat powdery mildew pathogen produces diverse effector proteins that are suggested to reprogram host defense responses and physiology. When recognized by host immune receptors, these proteins become avirulence (AVR) effectors. Several sequence-diverse AVRPM3 effectors and the suppressor of AVRPM3-PM3 recognition (SVRPM3^a1/f1) are involved in triggering allele-specific, Pm3-mediated resistance, but the molecular mechanisms controlling their function in the host cell remain unknown. Here, we describe that AVRPM3^b2/c2, AVRPM3^a2/f2 and SVRPM3^a1/f1 form homo- and heteromeric complexes with each other, suggesting they are present as dimers in the host cell. Alphafold2 modelling substantiated previous predictions that AVRPM3^b2/c2, AVRPM3^a2/f2 and SVRPM3^a1/f1 all adopt a core RNase-like fold. We found that a single amino acid mutation in a predicted surface exposed region of AVRPM3^a2/f2 enables it to trigger the PM3b immune receptor, which does not recognize wildtype AVRPM3^a2/f2. This indicates that differential AVRPM3 recognition by variants of the highly related PM3 immune receptors is due to subtle differences in similar protein surfaces of sequence-diverse AVRs. Our study reveals complex molecular interactions between powdery mildew effectors. These findings suggest that structural similarity, rather than sequence conservation, underlies both the promiscuous dimerization of these effectors and their recognition by specific PM3 immune receptors.