Barley Ror1 encodes a class XI myosin required for mlo‐based broad‐spectrum resistance to the fungal powdery mildew pathogen

Abstract

Loss-of-function alleles of plant MLO genes confer broad-spectrum resistance to powdery mildews in many eudicot and monocot species. Although barley (Hordeum vulgare) mlo mutants have been used in agriculture for more than 40 years, understanding of the molecular principles underlying this type of disease resistance remains fragmentary. Forward genetic screens in barley have revealed mutations in two Required for mlo resistance (Ror) genes that partially impair immunity conferred by mlo mutants. While Ror2 encodes a soluble N-ethylmaleimide-sensitive factor-attached protein receptor (SNARE), the identity of Ror1, located at the pericentromeric region of barley chromosome 1H, remained elusive. We report the identification of Ror1 based on combined barley genomic sequence information and transcriptomic data from ror1 mutant plants. Ror1 encodes the barley class XI myosin Myo11A (HORVU.MOREX.r3.1HG0046420). Single amino acid substitutions of this myosin, deduced from non-functional ror1 mutant alleles, map to the nucleotidebinding region and the interface between the relay-helix and the converter domain of the motor protein.Ror1 myosin accumulates transiently in the course of powdery mildew infection. Functional fluorophorelabeled Ror1 variants associate with mobile intracellular compartments that partially colocalize with peroxisomes. Single-cell expression of the Ror1 tail region causes a dominant-negative effect that phenocopies ror1 loss-of-function mutants. We define a myosin motor for the establishment of mlo-mediated resistance, suggesting that motor protein-driven intracellular transport processes are critical for extracellular immunity, possibly through the targeted transfer of antifungal and/or cell wall cargoes to pathogen contact sites.

The Calmodulin-like proteins, CML13 and CML14 Function as Myosin Light Chains for the Class XI Myosins inArabidopsis

“…Unlike all other myosins studied to date, XI-G is reported to function in the active movement of F-actin filaments during sperm nuclear migration, rather than the canonical myosin role of cargo transporters (Ali et al, 2020). In barely (Hordeum vulgare), the ortholog of myosin XI-A was identified as the causative mutation in Ror1 (Required for mlo powdery mildew resistance) and was speculated to be involved in delivering antifungal and/or cell wall cargoes to the sites of pathogen recognition (Acevedo-Garcia et al, 2022). Ror1 could also be forming a signalosome or nanodomain complex with immunity signalling components, similar to Arabidopsis myosin XI-K forming a BIK1 (Botrytis-induced kinase 1) nanodomain to facilitate innate immunity (Wang et al, 2024).…”

A fungal plant pathogen overcomesmlo-mediated broad-spectrum disease resistance by rapid gene loss

“…Genetic suppressors of mlo -based resistance cause partial susceptibility to powdery mildew. These include the Required for mlo-specified resistance ( Ror ) genes in barley [28, 16, 1], as well as PENETRATION ( PEN ) genes in Arabidopsis thaliana , which are major components of pre-penetration resistance to powdery mildew [16, 55, 87, 38]. In addition, abiotic stress conditions can result in a temporary break-down of mlo -based resistance [81, 65].…”