Bilstein-Schloemer, M. | bioRxiv Science

bioRxiv2026

Targeting of the barley cell-surface receptor SRF3 by the Blumeria hordei effector AVRA13 overlaps with AVRA13 recognition by MLA and the induction of NLR-mediated cell death.

Pathogens secrete effector proteins to promote virulence. Despite their recognition by barley Mla resistance genes, the structurally-related Blumeria hordei (Bh) AVRA effectors are maintained in the Bh genome, suggesting virulence functions critical for fungal pathogenicity. Using proximity-dependent protein labelling in transgenic barley, we detected distinct host protein interactomes for five AVRAs despite their structural homology and convergence on MLAs. We report the specific interaction of the highly conserved AVRA13 effector with the barley cell-surface receptor SRF3. AVRA13 disrupts HvSRF3-HvBAK1 interaction and alters HvSRF3 plasma membrane levels. AVRa13-expression desensitizes iron-induced restriction of Bh growth, suggesting that AVRA13 facilitates fungal proliferation by manipulating SRF3-mediated iron homeostasis. Our results suggest that MLAs have diversified to specifically detect the residues that underly Bh effector neo-functionalization and intrinsic AVRA virulence functions. Together, these findings identify SRF3 as molecular link between pathogen virulence, immune recognition, and iron homeostasis.

plant biology↗

bioRxiv2023

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.

plant biology↗