<b>Cell‐autonomous complementation of</b><i><b>mlo</b></i><b>resistance using a biolistic transient expression system</b>

Abstract

Summary 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.