Stefano Pessina, Dario Angeli, Stefan Martens, Richard G.F. Visser, Yuling Bai, Francesco Salamini, Riccardo Velasco, Henk J. Schouten and Mickael Malnoy

Plant Biotechnology Journal, 11 MAY 2016

Summary

Varieties resistant to powdery mildew (PM; caused by Podosphaera leucotricha) are a major component of sustainable apple production. Resistance can be achieved by knocking-out susceptibility S-genes to be singled out among members of the MLO (Mildew Locus O) gene family. Candidates are MLO S-genes of phylogenetic clade V up-regulated upon PM inoculation, such as MdMLO11 and 19 (clade V) and MdMLO18 (clade VII). We report the knock-down through RNA interference of MdMLO11 and 19, as well as the complementation of resistance with MdMLO18 in the Arabidopsis thaliana triple mlo mutant Atmlo2/6/12. The knock-down of MdMLO19 reduced PM disease severity by 75%, whereas the knock-down of MdMLO11, alone or in combination with MdMLO19, did not result in any reduction or additional reduction of susceptibility compared with MdMLO19 alone. The test in A. thaliana excluded a role for MdMLO18 in PM susceptibility. Cell wall appositions (papillae) were present in both PM-resistant and PM-susceptible plants, but were larger in resistant lines. No obvious negative phenotype was observed in plants with mlo genes knocked down. Apparently, MdMLO19 plays the pivotal role in apple PM susceptibility and its knock-down induces a very significant level of resistance.

See http://onlinelibrary.wiley.com/doi/10.1111/pbi.12562/full

Figure 1: Disease severity recorded after 7 days from the inoculation with O. neolycopersici of A. thaliana Col-0, A. thaliana Atmlo2/6/12 mutant, A. thaliana Atmlo2/6/12 mutants expressing MdMLO18 line A and A. thaliana Atmlo2/6/12 mutants expressing MdMLO18 line B. Histograms, representing average PM severity, show data of 7 to 24 biological replicates. Error bars show the standard errors of the mean. The asterisks indicate statistically significant differences compared with Gala, according to the Kruskal–Wallis test (P = 0.01).