Cisgenic approach for improved disease resistance in apple

Swiss and more generally European apple (Malus × domestica) production is hampered by several diseases, the most destructive being fire blight, caused by Erwinia amylovora. On the other hand, there are apple scab, caused by Venturia inaequalis and powdery mildew, caused by Podosphaera leucotricha, which represent the major phytosanitary problems. Classical breeding has produced many scab and mildew resistant cultivars and efforts to breed also fire blight resistant cultivars are currently undertaken. Marker assisted selection (MAS) increases efficiency by allowing early non-destructive screening of seedlings and identifying genotypes showing pyramids of resistance genes. If the development of markers for MAS was the primary goal of genetic analysis in the 1990s, identification and cloning of resistance genes is now the goal. The first and until now the sole resistance gene which has been isolated and transformed into a susceptible apple cultivar is the gene HcrVf2 (Rvi6), responsible for the Vf scab resistance present in most classically bred scab resistant cultivars. Much effort is currently spent in the identification and positional cloning of other apple genes conferring resistance to apple scab and fire blight. In our labs, we identified the putative scab resistance gene Rvi15 and two fire blight resistance genes namely from 'Evereste' and Malus × robusta 5. The functionality of these candidate genes is currently under scrutiny by complementation experiments. However, the final goal is the creation of a product, e.g., an improved apple cultivar that is resistant to scab and fire blight. The ideal product would have advantages to the environment and producer, and should raise as little concern as possible with consumers. To accomplish this 'ideal product', we opted for the cisgenic approach by introducing the scab resistance gene HcrVf2 with its own regulatory sequences into the highly susceptible apple cultivar, 'Gala', through Agrobacterium transformation. All marker genes were eliminated after transformation. Similarly, we are currently introducing into both the readily developed cisgenic 'Gala' and in the untransformed 'Gala' the putative Malus own fire blight resistance gene candidates, aiming at both proof of functionality of the identified candidates and possibly at rapid development of a fire blight and scab resistant cisgenic apple.