Mutation of barley improves resistance to yellow mosaic virus disease without growth or yield penalties.

The soil-borne yellow mosaic virus disease, which is caused by the bymoviruses barley yellow mosaic virus (BaYMV) and/or barley mild mosaic virus (BaMMV), seriously threatens winter barley production in Europe and East Asia. Both viruses are transmitted by the soil-borne plasmodiophorid and are difficult to eliminate through chemical or physical measures in the field, making breeding for resistant cultivars the optimal strategy for disease control. The resistance locus was cloned encoding the host factor gene (), whose loss-of-function variants confer broad-spectrum resistance to multiple strains of BaMMV/BaYMV. Most resistance-conferring variants have been identified in six-rowed barley landraces/historic cultivars, and their introgression into modern two-rowed malting cultivars is difficult because is located in a peri-centromeric region with suppressed recombination. In this study, we used CRISPR/Cas9 genome editing to modify in the BaYMV/BaMMV-susceptible elite malting barley cv. 'Golden Promise' and obtained the mutants and . in the mutant encodes a protein lacking a cysteine residue, and contains a protein-coding frameshift. Both mutants were completely resistant to BaYMV. The knockout mutant showed complete BaMMV resistance, while showed decreased viral accumulation but no disease symptoms if compared to 'Golden Promise'. Both edited lines, as well as the previously produced EMS-induced mutant '10253-1-5' in the elite malting barley cv. 'Barke' background, displayed no growth or yield penalties in garden experiments or bymovirus-free field trials. Line '10253-1-5' showed improved resistance and yield performance compared to the wild-type and its sibling line when grown in infectious fields. Therefore, genome editing of the host factor gene could facilitate the breeding of barley varieties with resistance to bymoviruses.