The barley MLA13-AVR _A13 heterodimer reveals principles for immunoreceptor recognition of RNase-like powdery mildew effectors

Co-evolution between cereals and pathogenic grass powdery mildew fungi is exemplified by sequence diversification of an allelic series of barley resistance genes encoding Mildew Locus A (MLA) nucleotide-binding leucine-rich repeat (NLR) immunoreceptors with a N-terminal coiled-coil domain (CNLs). Each immunoreceptor recognises a matching, strain-specific powdery mildew effector encoded by an avirulence gene ( AVR _{a )} . We present here the cryo-EM structure of barley MLA13 in complex with its cognate effector AVR _A13 -1. The effector adopts an RNase-like fold when bound to MLA13 in planta , similar to crystal structures of other RNase-like AVR _{A e} ffectors purified from E. coli . AVR _A13 -1 interacts via its basal loops with MLA13 C-terminal leucine rich repeats (LRRs) and the central winged helix domain (WHD). Co-expression of structure-guided MLA13 and AVR _A13 -1 substitution variants show that the receptor–effector interface plays an essential role in mediating immunity-associated plant cell death. Furthermore, by combining structural information from the MLA13–AVR _A13 -1 heterocomplex with sequence alignments of other MLA receptors, we designed a single amino acid substitution in MLA7 that enables expanded effector detection of AVR _A13 -1 and the virulent variant AVR _A13 -V2. In contrast to the pentameric conformation of previously reported effector-activated CNL resistosomes, MLA13 was purified and resolved as a stable heterodimer from an in planta expression system. Our study suggests that the MLA13–AVR _A13 -1 heterodimer might represent a CNL output distinct from CNL resistosomes and highlights opportunities for the development of designer gain-of-function NLRs.