MLO genes silencing reduces susceptibility to powdery mildew in grapevine

Erysiphe necator is the causal agent of powdery mildew (PM), one of the most destructive diseases of grapevine. Powdery mildew is controlled by sulphur-based and synthetic fungicides, which every year are dispersed in the environment. This is why PM resistant varieties should become a priority for sustainable grapevine and wine production. Resistance can be achieved by knocking-out susceptibility S-genes, such as those residing at genetic loci known as MLO (Mildew Locus O). All MLO S-genes of dicots belong to the phylogenetic clade V, including grapevine genes VvMLO7, 11, 13, which are up-regulated during PM infection, and VvMLO6, which is not up-regulated. Before adopting a gene editing approach to knockout candidate S-genes, the evidence that loss-of-function of MLO genes can reduce PM susceptibility is necessary. We show that the knock-down through RNA interference of VvMLO6, 7, 11 and 13. Knock-down of VvMLO6, 11 and 13, alone or combined, did not decrease PM severity, whereas the knock-down of VvMLO7in combination with VvMLO6 and VvMLO11, reduces severity up to 77%. Cell wall appositions (papillae), a response to PM attack, were present in both resistant and susceptible lines, but were larger in resistant lines