Cell-autonomous complementation of mlo resistance using a biolistic transient expression system

The barley gene Mlo encodes a prototype of a novel class of plant proteins. In mlo mutants, absence of the 60 kDa wild-type Mlo protein results in broad-spectrum resistance to the powdery mildew fungus, Erysiphe graminis f, sp. hordei. To directly assess its function, Mlo was transiently expressed with a marker gene encoding a modified green fluorescent protein (GFP) in leaf epidermal cells of mlo resistant barley lines. Fungal inoculation of epidermal cells transfected with wild-type Mlo led to haustorium formation and abundant sporulation. Therefore, expression of the wild-type Mlo gene, in mlo resistant genotypes, is both necessary and sufficient to restore susceptibility to fungal attack. Complementation of mlo resistance alleles was restricted to single host cells, indicating a cell-autonomous function for the wild-type Mlo protein. We discuss our findings with respect to source-sink relationships of plants and biotrophic fungi and the potentially wide-ranging use of the transient complementation assay to analyse host compatibility and defence in response to powdery mildew attack.