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

The barley geneMloencodes a prototype of a novel class of plant proteins. Inmlomutants, absence of the 60 kDa wild-type Mlo protein results in broad-spectrum resistance to the powdery mildew fungus,Erysiphe graminisf. sp.hordei. To directly assess its function,Mlowas transiently expressed with a marker gene encoding a modified green fluorescent protein (GFP) in leaf epidermal cells ofmloresistant barley lines. Fungal inoculation of epidermal cells transfected with wild-typeMloled to haustorium formation and abundant sporulation. Therefore, expression of the wild-typeMlogene, inmloresistant genotypes, is both necessary and sufficient to restore susceptibility to fungal attack. Complementation ofmloresistance 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