A dominant-negative avirulence effector of the barley powdery mildew fungus provides mechanistic insight into barley MLA immune receptor activation

Nucleotide-binding leucine-rich repeat receptors (NLRs) recognize pathogen effectors to mediate plant disease resistance often involving host cell death. Effectors escape NLR recognition through polymorphisms, allowing the pathogen to proliferate on previously resistant host plants. The powdery mildew effector AVR(A13)-1 is recognized by the barley NLR MLA13 and activates host cell death. We demonstrate here that a virulent form of AVR(A13), called AVR(A13)-V2, escapes MLA13 recognition by substituting a serine for a leucine residue at the C-terminus. Counterintuitively, this substitution in AVR(A13)-V2 resulted in an enhanced MLA13 association and prevented the detection of AVR(A13)-1 by MLA13. Therefore, AVR(A13)-V2 is a dominant-negative form of AVR(A13) and has probably contributed to the breakdown of Mla13 resistance. Despite this dominant-negative activity, AVR(A13)-V2 failed to suppress host cell death mediated by the MLA13 autoactive MHD variant. Neither AVR(A13)-1 nor AVR(A13)-V2 interacted with the MLA13 autoactive variant, implying that the binding moiety in MLA13 that mediates association with AVR(A13)-1 is altered after receptor activation. We also show that mutations in the MLA13 coiled-coil domain, which were thought to impair Ca(2+) channel activity and NLR function, instead resulted in MLA13 autoactive cell death. Our results constitute an important step to define intermediate receptor conformations during NLR activation.