Reduction of MLO Expression in Petunia Increases Resistance to Powdery Mildew

Susceptibility genes are plant genes used by pathogens to establish infection. We examined whether resistance to powdery mildew (Podosphaera xanthii) could be obtained in Petunia hybrida by knocking out the susceptibility gene MLO.  A petunia gene, PhMLO, was identified that had 85% amino acid similarity to SlMLO1 from tomato. Based on the PhMLO sequence, an RNAi construct was developed. The PhMLO RNAi construct was introduced into leaf discs of P. hybrida ‘Mitchell’ by Agrobacterium-mediated transformation.  Ten independently-transformed shoots were rooted and the regenerated T0 plants were propagated vegetatively. RT-PCR analysis found that four RNAi lines (2, 3, 8, 11) had reduced levels of PhMLO expression compared to transgenic controls. The RNAi line with the lowest PhMLO expression (line 11) did not survive. In two experiments with whole plants, leaves of RNAi lines and controls were inoculated with a 10-μl suspension of P. xanthii conidia (1×10⁵ conidia/ml). The time course of infection and the infected leaf area were examined over a 16-day incubation in an growth chamber.  RNAi lines 2, 3, and 8 showed delayed infection and a reduced area of infection compared to transgenic controls.  The transgenic lines were self-pollinated and T1 progeny were produced that will be examined for powdery mildew resistance and pleiotropic effects on growth.