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

Nucleotide-binding leucine-rich repeat receptors (NLRs) recognize pathogen effectors to mediate plant disease resistance, which is often accompanied by a localized host cell death response. Effectors can escape NLR recognition through various polymorphisms, allowing the pathogen to proliferate on previously resistant host plants. The powdery mildew effector AVRA13-1 is recognized by the barley NLR MLA13 and activates host cell death. We demonstrate here that a virulent form of AVRA13, called AVRA13-V2, escapes MLA13 recognition by substituting a serine for a leucine residue at the C-terminus. Counterintuitively, this substitution in AVRA13-V2 resulted in an enhanced MLA13 association and prevented the detection of AVRA13-1 by MLA13. Therefore, AVRA13-V2 is a dominant-negative form of AVRA13 and has likely contributed to the breakdown of Mla13 resistance. Despite this dominant-negative activity, AVRA13-V2 failed to suppress host cell death mediated by the MLA13 auto-active "MHD" variant. Neither AVRA13-1 nor AVRA13-V2 interacted with the MLA13 auto-active variant, implying that the binding moiety in MLA13 that mediates association with AVRA13-1 is altered after receptor activation. We also show that mutations in the MLA13 coiled-coil signalling domain, which were thought to impair Ca2+-channel activity and NLR function, instead resulted in MLA13 auto-active cell death. The data constitute an important step to define intermediate receptor conformations during NLR activation.