Strategies for obtaining fire blight resistance in apple by rDNA technology

The fire blight disease of apple (Malus x domestica), caused by the bacterium Erwinia amylovora, is difficult to manage on most cultivars because of their susceptibility and the paucity of effective control materials. Because of apple's heterozygosity, self-incompatibility and long generation time, rDNA technology is more attractive than conventional breeding for development of resistant cultivars with fruit of high quality. Previously we showed the effectiveness of certain anti-microbial genes (attacin, lysozyme) at increasing fire blight resistance, but their animal origin put their acceptability to consumers and growers into serious question. Genes from E. amylovora (hrpN) and phages (lysozyme, depolymerase) also appear to be effective at increasing resistance. They are probably more acceptable than animal genes. Most acceptable to consumers and growers are likely to be alterations in the expression of native apple genes resulting in enhanced resistance. We were able to add a copy of the apple transcription facilitator gene, MpNPR1, to susceptible apple cultivars and obtain increased fire blight resistance. Also we obtained some apple lines in which certain pathogen-protein receptor genes of apple, DIPM1-4, are partially silenced; these appear to have increased resistance to E. amylovora. The preferred strategy is to alter the expression of native apple genes, using plant (preferably apple) promoters, without use of antibiotic or herbicide resistance selectable marker genes, in order to facilitate approval by regulatory agencies, and acceptance by consumers and apple growers.