Genetic Engineering of Sexual Sterility in Shade Trees

Shade trees unable to produce floral tissues, or that produce only nonreproductive floral organs such as petals, are desirable for a number of reasons. They can reduce the need to clean flower and fruit litter, eliminate hazards from large and fleshy fruits on walks, and lessen allergenic pollen production. Research in herbaceous species has established that introduction of gene constructs created by recombinant DNA technology provides an effective means to manipulate flowers without deleterious effects on vegetative growth. Though not yet demonstrated in trees, this approach will likely be successful in both angiosperms and gymnosperms because genes that control reproductive development are similar in sequence and function among diverse plant species. Key to the practical application of genetically engineered sterility to shade trees, however, is the development of efficient gene transfer and vegetative propagation systems to deliver engineered, sterile trees to the marketplace; these systems are in place for a limited number of species. We discuss the rationale for sexual sterility in arboriculture, methods for genetic engineering of sterility, our progress in engineering sterility in poplars, and the current status of transformation and propagation methods for some common shade tree genera.