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

Powdery mildew (PM) is disease caused by Podosphaera leucotricha in apple and Erysiphe necator in grapevine. Frequent applications of fungicides are required to control the disease, a practice that has negative effects on the environment. To reduce the amount of chemicals required to control these pathogens, the development of resistant apple and grapevine varieties should become a priority. PM pathogenesis is associated with pathogen-dependent activation of specific MLO genes during early stages of infection. These activated genes, which function is the down-regulation of defense, are responsible for PM susceptibility (S-genes) and their knock-out results in 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 were activated during PM infection, whereas VvMLO6 was not. The knock-down through RNA interference (RNAi) of VvMLO6, 11 and 13, alone or combined, did not decrease PM severity, whereas the knock-down of VvMLO7 in combination with VvMLO6 and VvMLO11 reduced severity up to 77%. In apple, clade V genes MdMLO11 and 19 were up-regulated upon PM inoculation. The knock-down of the two genes through RNAi showed that the knock-down of MdMLO19 reduced PM disease severity up to 75%, whereas the knock-down of MdMLO11, alone or combined with MdMLO19, did not result in resistance. The presence of non-functional alleles of MdMLO19 was investigated in apple germplasm. An insertion that caused the formation of an early stop codon was found in MdMLO19. The resulting truncated protein lacks 185 amino-acids and the calmodulin-binding domain. The frequency of this insertion was estimated around 27%, a result higher than anticipated. The presence of this loss-of-function mutation in homozygosity was expected to correlate with PM-resistance, but it did not. However, the absence of the insertion correlated with PM-susceptibility, suggesting that the loss-of-function of MdMLO19 is required for resistance, but not sufficient on its own