Interactions of sequence diverse effector proteins of wheat powdery mildew control recognition specificity by the corresponding immune receptor

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 (SVRPM3a1/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 AVRPM3b2/c2, AVRPM3a2/f2 and SVRPM3a1/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 AVRPM3b2/c2, AVRPM3a2/f2 and SVRPM3a1/f1 all adopt a core RNase-like fold. We found that a single amino acid mutation in a predicted surface exposed region of AVRPM3a2/f2 resulted in recognition by the PM3b immune receptor, which does not recognize wildtype AVRPM3a2/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. Based on our findings, we propose a model in which homodimers of AVRPM3s are recognized by their corresponding PM3 variants and that heterodimer formation with SVRPM3a1/f1 allows for evasion of recognition.