Using Early Flowering Transgenic Apple to Accelerate the Breeding of Donor Parents with Multiple Loci for Disease Resistance (Malus x domestica)

One of the goals of the USDA-NIFA-SCRI RosBREED project is to develop donor parents with multiple loci for disease resistance. Due to the long generation time of tree fruit crops the accumulation of pyramided resistance loci for multiple diseases by conventional breeding methods could require decades. The use of early flowering, BpMADS4-transgenic apple can decrease generation time to 12-14 months and accelerate breeding. Molecular markers are used to select for the accumulation of resistance loci and retention of the BpMADS4-transgene in the initial breeding cycles and then in the final breeding cycle to select for retention of resistance loci and loss of the BpMADS-transgene which segregates 1:1. Current rapid cycle breeding methods for apple typically use greenhouse grown, early-flowering transgenics as female parents. However, simultaneously selecting for multiple disease resistance and fruit quality loci will require large segregating populations (1,500-3,000 seed) which are not easily obtained under greenhouse conditions due to the economic limitations of greenhouse production and the limited number of fruit produced on greenhouse grown trees. Larger segregating apple populations can be easily produced by using early flowering transgenics as the male (pollen) parent and field grown non-transgenic trees as the female parent. This approach does require containment of transgenic pollen and seed in the orchard but this can be accomplished at relatively low cost. Rapid cycling breeding using early flowering transgenics as pollen parents is currently being used in the RosBREED project to pyramid multiple fire blight (Erwinia amylovora) and scab (Venturia inaequalis) resistance loci in apple.