Novel CRISPR/Cas9-Derived mlo Alleles in Barley: Resistance to Powdery Mildew and Microbiome Implications

Barley grown in temperate regions is often challenged by powdery mildew disease. An effective solution is mildew resistance locus o (mlo)-based resistance, which is monogenic, durable, and broad-spectrum. While the pleiotropic effects of mlo mutations on above-ground tissues are well documented, their impact on the root-associated microbiome remains underexplored. We utilized CRISPR/Cas9 to generate novel mlo mutant lines and evaluated their resistance to causal fungus Blumeria hordei. We further examined if mlo knockout has any impact on the overall root microbiome diversity and composition under field-like conditions and applied DESeq2 to compare the abundance of microbial taxa between mutants and wild type. We created five novel resistant mlo lines, including the first mutants with amino acid alterations in the protein's extracellular region. Mutant lines showed significantly reduced B. hordei colony formation (0.5-5%). While microbial alpha and beta diversity were not significantly altered, a few microbial taxa displayed time-dependent shifts in abundance. Overall, our study demonstrates the effectiveness of CRISPR/Cas9 in generating mlo-based resistance. Moreover, the study revealed functionally important residues in the protein's extracellular region. Finally, we present the first evidence of limited mlo-associated effects on root microbiome diversity and relative abundance of microbial taxa.