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Genetic engineering offers numerous potentially useful genetic manipulations for the improvement of horticultural crops. Nevertheless, there are several impediments to the efficient integration of genetic engineering into plant improvement programs. The ability to regenerate plants from tissue cultures and, therefore, to genetically engineer most plant species is limited to specific genotypes. This may constrain introduction of genetically engineered traits into crops that have complex genetics, are highly heterozygous, or are propagated by asexual reproduction. Even in crops that are self-compatible, diploids, genotypic specificity of the transformation process often necessitates backcrossing for transfer of genetically engineered traits into elite lines and to reduce problems associated with tissue culture-induced genetic variation. This limits progress in improving other traits compared to other breeding strategies. Other challenges to applying genetic engineering exist. The major genomics initiatives currently underway for gene discovery from a broad range of organisms will provide plant improvers access to most genes. Yet there remains a dearth in tissue-specific, developmentally timed, and environmentally responsive promoters for appropriate expression of introduced genes (transgenes). Furthermore, transgene expression is not as controllable as desired due to transgene silencing. Transgene integration is a random process and further refinements of targeted DNA integration will likely enhance the stability of expression of transgenic traits. The availability of other tools, such as selectable marker genes, will become limiting as multiple transgenic traits are combined within a species. In addition to technical problems, there likely will be problems of access to proprietary technology and testing to meet federal regulations and public acceptance. While these various challenges and limitations currently may constrain progress in application of genetic engineering to horticultural crop improvement, it is foreseeable that with further improvements of genetic engineering technology and development of the appropriate molecular tools, genetic engineering will become a component of most plant improvement programs.

652 Challenges and Opportunities for Application of Genetic Engineering to Horticultural Crop Improvement