Functional conservation of wheat and rice Mlo orthologs in defense modulation to the powdery mildew fungus

Homologs of barley Mlo are found in syntenic positions in all three genomes of hexaploid bread wheat, Triticum aestivum, and in rice, Oryza sativa. Candidate wheat orthologs, designated TaMlo-A1, TaMlo-B1, and TaMlo-D1, encode three distinct but highly related proteins that are 88% identical to barley MLO and appear to originate from the three diploid ancestral genomes of wheat. TaMlo-B1 and the rice ortholog, OsMlo2, are able to complement powdery mildew-resistant barley mlo mutants at the single-cell level. Overexpression of TaMlo-B1 or barley Mlo leads to super-susceptibility to the appropriate powdery mildew formae speciales in both wild-type barley and wheat. Surprisingly, overexpression of either Mlo or TaMlo-B1 also mediates enhanced fungal development to tested inappropriate formae speciales. These results underline a regulatory role for MLO and its wheat and rice orthologs in a basal defense mechanism that can interfere with forma specialis resistance to powdery mildews.