Identification, isolation and characterization of mildew resistance locus O in rosaceae
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The economically important plant family Rosaceae consists of over 100 genera and 3,000 species that include many important fruit, nut, ornamental, and wood crops. Its members represent diverse biological systems which provide high-value nutritional foods and desirable aesthetic and industrial products. Powdery mildew is a major disease of Rosaceae, requiring control by costly and environmentally harmful chemicals. In this study we identify a Mildew resistance locus O (Mlo) gene in sequenced peach (GU189298), plum (GU189300) and apricot (GU189299) bacterial artificial chromosomes (BACs). Comparative analysis of the BACs with Arabidopsis, Populus and Vitis genome sequences identified high degree of synteny with Populus and Vitis but not with Arabidopsis. Inclusion of Prunoideae Mlo genes expanded the clades in phylogenetic analysis with Arabidopsis and Vitis Mlo genes. Peach Mlo (PpMlo1) was further characterized. To test function of PpMlo1, the Fragaria x ananassa LF9 line was transformed with pDAJ3 (Mlo expressed in sense orientation) and pDAJ4 (Mlo expressed in antisense orientation) using Agrobacterium-mediated transformation. Transgenic plants were challenged with Podosphaera macularis conidia, and spore germination and mycelia growth were determined after 72h. Results obtained from the pathogen challenge experiment lead us to infer that antisense repression of the Fragaria Mlo homolog results in conferring resistance to the Fragaria-specific pathogen. Our results indicate that silencing of Mlo is potentially a viable strategy in imparting resistance to powdery mildew within Rosaceae, and that Fragaria can be used as a host to test the function of genes derived from related tree species. To the best of our knowledge, this work is the first attempt to test the potential of using a Mlo-based resistance strategy to combat powdery mildew in Rosaceae.