Transgenic virus resistant trees: current status and future prospects

Domestication of fruit trees involves a change in the reproductive biology of the plants by shifting from sexual reproduction to vegetative propagation (Zohary and Spiegel Roy 1975). As a rule, cultivated varieties of fruit trees are maintained vegetatively by cuttings, rootings of twigs, or suckers. This is in sharp contrast with the life cycle of their wild relatives, which reproduce from seed. Wild populations maintain themselves through sexual reproduction, and have high levels of heterozygosity. Fruit trees and grapevines have undergone thousands of years of domestication and centuries of breeding (Zohary and Hopf 1993). From the standpoint of evolution under domestication this means a severe restriction on selection. Man made selection led to some improved cultivars, sometimes at the cost of genetic variability (Zohary and Hopf 1993, Maghuly et al. 2005,2007.2009). Most important fruit crops, in contrast to forest tree species, have been vegetatively propagated as cultivar clones, a factor which has also contributed to the spread of latent viruses and phytoplasmas (Laimer 2002). Given their nutritional and dietetic value, fruit crops contribute considerably to an improved world food production and human nutrition. Current fruit production faces problems with biotic and abiotic stress factors during production, harvest and storage.