Progress in Tree Fruit Improvement Through Molecular Genetics

Abstract

can be significant, and, coupled with the need for multiple-year evaluation in different growing areas, make evaluation of transgenic tree fruits expensive and time-consuming. The requirements for evaluating the performance of new transgenic cultivars of tree fruits are the same as those required for conventional cultivar development; thus, the incorporation of molecular genetics within existing breeding programs is perhaps the most efficient approach to the development of improved transgenic tree fruit cultivars. Difficulties notwithstanding, there are several inherent advantages in the use of gene transfer for tree fruit improvement. Once a useful transformant is isolated, assuming stability of transgene expression (and this assumption has yet to be adequately tested for tree fruits), vegetative propagation—the normal route of multiplying tree fruits— provides for virtually unlimited production of the desired transgenic line. Fixation through the sexual cycle is not required. The dominance of a few major cultivars in many tree fruit crops such as pear, apple, sour cherry (Prunus cerasus L.), and citrus maximizes the impact that an improved transgenic cultivar can have. For example, over 90% of U.S. pear production can be accounted for by three cultivars, ‘Bartlett’, ‘Beurre Bosc’, and ‘Anjou’. An improvement in any of these three, particularly ‘Bartlett’, which accounts for ≈50% of the North American crop (O’Rourke, 1999), can have a significant impact on production. Currently, almost all of the sour cherry production in the United States is based on ‘Montmorency’. Over 50% of the world and U.S. apple crop is based on ‘Red Delicious’, ‘Golden Delicious’, ‘Granny Smith’, ‘Gala’, and ‘Fuji’ (O’Rourke, 1998). While this heavy reliance on a few major cultivars is not favorable in terms of genetic vulnerability, it is, in fact, the current situation, although breeding programs are releasing new cultivars that will broaden the genetic base. The current major cultivars, though, can potentially be improved in fruit quality and made less vulnerable to insects and diseases through transformation. Most work in transformation of perennial tree fruits is concentrated in apple, citrus, pear and Prunus. Research focuses on disease and insect resistance, manipulation of fruit ripening/softening, and alteration of tree architecture.