MOLECULAR CHARACTERIZATION OF THE Dm3 FAMILY OF RESISTANCE GENES

¹ Department of Vegetable Crops, University of California, Davis, California 95616

² Dpto de Genetica Molecular de Plantas. CNB-CSIC. Campus UAM. Cantoblanco 28049 Madrid, Spain

Genes conferring resistance to downy mildew (Dm genes) map to three major clusters in lettuce along with genes for resistance to other pathogens. The largest cluster contains at least ten Dm genes, including Dm3. Dm3 has been cloned using a combination of map-based cloning and candidate gene approaches. Analysis of the organization of the Dm3 region has revealed the presence of a large multigene family (RGC2 family) of approximately 25 members including Dm3. Analyses of mutants and marker data suggested that RGC2B was required for Dm3 specificity. Transgenic complementation demonstrated that RGC2B was sufficient for Dm3 specificity in two genotypes lacking Dm3 specificity, Cobham Green, which contains no known Dm genes and an EMS-induced Dm3 mutant of Diana. RGC2B transgenic plants were resistant against a range of Avr3-expressing isolates from diverse geographical origins. RT-PCR of a microsatellite marker within the open reading frame of RGC2 homologs indicated that the majority of RGC2 paralogs are transcribed throughout the development of the plant.