Role of MLO genes in susceptibility to powdery mildew in apple and grapevine

Powdery mildew (PM) is a major fungal disease that threatens thousands of plant species. PM is caused by Podosphaera leucotricha in apple and Erysiphe necator in grapevine. Powdery mildew is controlled by frequent applications of fungicides, having negative effects on the environment, and leading to additional costs for growers. To reduce the amount of chemicals required to control this pathogen, the development of resistant apple and grapevine varieties should become a priority. PM pathogenesis is associated with up-regulation of specific MLO genes during early stages of infection, causing down-regulation of plant defense pathways. These up-regulated genes are responsible for PM susceptibility (S-genes) and their knock-out causes durable and broad-spectrum resistance. All MLO S-genes of dicots belong to the phylogenetic clade V. In grapevine, four genes belong to clade V. VvMLO7, 11 and 13 are up-regulated during PM infection, while VvMLO6 is not. Chapter 2 reports the genome-wide characterization and sequence analysis of the MLO gene family in apple, peach and woodland strawberry, and the isolation of apricot MLO homologs. Twenty-one homologues were found in apple, 19 in peach and 17 in woodland strawberry. Evolutionary relationships between MLO homologs were studied and syntenic blocks constructed. Candidate genes for causing PM susceptibility were inferred by phylogenetic relationships with functionally characterized MLO genes and, in apple, by monitoring their expression following inoculation with the PM causal pathogen P. leucotricha. In apple, clade V genes MdMLO11 and 19 were up-regulated, whereas the two other members of clade V, MdMLO5 and 7, were not up-regulated. The clade VII gene MdMLO18 was also up-regulated upon P. leucotricha infection. Chapter 3 reports the knock-down, through RNA interference, of MdMLO11 and 19, as well as complementation of the mutant phenotype by expression of the MdMLO18 gene in the Arabidopsis thaliana triple mlo mutant Atmlo2