Genetic transformation provides the means for modifying single horticultural traits in perennial plant cultivars without altering their phenotype. This capability is particularly valuable for perennial plants and tree species in which development of new cultivars is often hampered by their long generation time, high levels of heterozygosity, polyembryony etc. Recombinant DNA technology offers opportunities for widening the available gene pool for fruit improvement; it also allows the introduction of several desirable genes in a single event, and can reduce the time to introgress novel genes into elite backgrounds. Although genome transfer via protoplast fusion can circumvent such problems such as sexual incompatibility, polyembryony, and male or female sterility encountered in conventional sexual crossing, transferring only the desirable gene into one species is difficult and nearly impossible. During protoplast fusion, desirable and undesirable traits in the whole genome of the donor parent-introduced into the recipient partner at the same time. Comparatively, genetic transformation is more targets oriented than the genome transfer.

Transgenic walnut was the first transformed fruit tree that was reported by McGranahan and co-workers in 1988. Since then, transgenic technology has been successfully introduced in other fruit trees such as apple, citrus, pear, grapevine, kiwifruit, peach, plum, avocado, papaya, mango, etc. Various genes have been introduced into fruit crops to improve agronomical traits including enhancing bacterial, viral, pest and environmental stress resistance, increasing storage life span, shortening juvenile stage and improving fruit qualities etc. The most successful case came from papaya (Bau et al. 2004), the transgenic papaya with the papaya ring-spot virus coat protein gene (PRSV-CP) performed well and was permitted to grow in commercial scale in the open field (see Chap. 9). Besides this, the U.S. has allowed approximately 124 confined trials of transgenic trees including fruits; transgenic plum clone C-5 with plum pox virus coat protein gene (PPV-CP) has been

The National Key Laboratory of Crop Genetic Improvement and College of Horticulture and Forestry, Huazhong Agricultural University, Wuhan, Hubei, P. R. China

M.Fladung and D.Ewald (Eds.)

Tree Transgenesis: Recent Developments

© Springer-Verlag Berlin Heidelberg 2006

released into field in Poland (Malinowski et al. 1998) and Romania (Ravelonandro et al. 2002) since 1998.

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