Genetic modification of agronomic traits in fruit crops

The genetic improvement of fruit crops has a range of objectives, including:

• selecting cultivars or rootstocks which tolerate biotic and abiotic stresses, allowing reduced pesticide use and controlling damage from, for example, plant diseases and pests, frost or drought
• reducing the size and altering the shape (apical dominance) of the plant in order to increase orchard plant density, lower harvesting and pruning costs, shorten the unproductive period and improve radiation of the canopy
• selecting self-fertile genotypes, both to eliminate pollinators in the orchards which, in some cases, do not produce marketable fruit, and to maintain a more consistent yield over time
• achieving simultaneous fruit ripening for mechanical harvesting, supplying cultivars with a different ripening season
• selecting genotypes with higher nutritional value of the fruit (sugar, oil, vitamins, functional components such as flavonoids)
• improving the organoleptic qualities and shelf-life of fruits.

In meeting these and other objectives, conventional genetic improvement of most species of fruit crops faces a range of obstacles. These include the long juvenility period of some species, seedlessness, frequent inter- and intra-species incompatibility, high heterozygosity, sterility and the presence of specific traits only in wild species. These characteristics make conventional breeding techniques difficult, expensive and time consuming (Mehlenbacher 1995). Common techniques used to reduce juvenility, for example, such as grafting scions on adult plants, are not always effective in all species. This explains why some fruit crops have been improved almost exclusively with clonal selection, using variability from spontaneous mutations or selecting plants derived from natural hybrydisation. Recent developments, such as induced mutations by ionising irradiation, have given few promising results both for cultivars and rootstocks, as in the example of olive, almond and cherry. However, few of these mutations have been commercialised, partly because stable mutations and significant improvement are rare.

Recent molecular and biotechnological approaches such as somaclonal variation or gene transformation, which are the main subject of this section, offer an attractive alternative to conventional genetic improvement, since they make possible a greater range of improvements to commercial varieties in a relatively short period of time with minimum or no change to other characteristics. Protoplast technology in fruit crops, for example, provides the potential for making significant changes to varieties, since it can be used for:

1. somatic hybridisation: fusion of cells belonging to different species or genera not sexually compatible, both in making symmetric and asymmetric hybrids (cybrids) to create stable new variations;
2. transferring alien genes by the technique of recombinant DNA:
   (a) co-cultivation protoplasts with Agrobacterium
   (b) direct DNA uptake with fusogen agents or by electroporation
   (c) fusion of bacterial spheroplasts with protoplasts, and
   (d) uptake of liposome carrying DNA into protoplasts;
3. selection by selective agents (toxin, culture filtrate of pathogens, and others).

In particular, cybrids may make a good impact on genetic improvement since some important characteristics are governed by organelle genome. Among fruit crops, cybrids are reported almost exclusively in Citrus spp by several authors (Vardi and Galum 1988; Grosser et al. 1996; Saito et al. 1993). Studies on inheritance of organelle genomes in citrus somatic hybrids have been carried out by Moreira et al. (2000). Somatic hybrids have been obtained between species of Citrus (Moriguchi et al. 1997; Gou and Deng 2001) and from different genera (Motumura et al. 1995). Hybrids have been used to improve rootstocks to control tree size (Gmitter et al. 1992; Moriguchi et al. 1997; Deng et al. 1992); improve resistance to diseases (Deng et al. 1995); and to improve the scion (Grosser et al. 1998) to strengthen resistance to viruses, nematodes and Phytophthora, as well as confer cold hardiness, drought and salt resistance (Louzada et al. 1992; Guo and Deng 2001). Using direct gene transfer to protoplasts, transgenic plants have already been recovered from Citrus sinensis (Kobayashi and Uchimiya 1989; Vardi et al. 1990) and strawberry (Nyman and Wallin 1992).