CHARACTERIZATION OF 3 LOCI CONTROLLING RESISTANCE OF ARABIDOPSIS-THALIANA ACCESSION MS-O TO 2 POWDERY MILDEW DISEASES

Arabidopsis thaliana accession La-er was susceptible, and accession Ms -0 was resistant, to powdery mildew diseases caused by Erysiphe crucif erarum UEA1 and E. cichoracearum UCSC1. The resistance reaction phenot ype of A. thaliana Ms-0 to both pathogens was characterized, and the r esistance loci were genetically mapped. Growth of E. cruciferarum UEA1 on Ms-0 leaves was arrested after formation of the first appressorium : the underlying host epidermal cell collapsed, and occasionally there was necrosis of one or two host mesophyll cells. Growth of E. cichora cearum UCSC1 on Ms-0 leaves was arrested after emergence of several ge rm tubes from the conidium, and there was necrosis of host mesophyll c ells at the sites of infection. Examination of F-2 progeny of a cross La-er x Ms-0 indicated that two independently-segregating dominant loc i were required for resistance to E. cruciferarum UEA1. One locus, nam ed RPW6, was genetically mapped to chromosome 5, in a 5.6 cM interval flanked by pCITf16 and PI. The other locus, named RPW7, mapped to chro mosome 3 in a 8.5 cM interval flanked by CDC2A and AFC1. Independent e ffects of RPW6 and RPW7 on E. cruciferarum UEA1 could be detected by q uantitative measurements of growth of mycelium and production of conid ia. Resistance to E. cichoracearum UCSC1 mapped to a single locus, nam ed RPW8, at a location on chromosome 3 which we could not distinguish from RPW7. Evidently, RPW7 and RPW8 define either a complex resistance locus, or a common resistance gene with dual specificity.