Tomato

Tomatoes originated in the Andean region of South America under extremely variable climatic conditions. Wild relatives of tomato grow from sea level to subalpine elevations with some ecotypes adapted to flooded conditions and others to extreme drought. Domestication of tomato led to its cultivation as a crop on all continents and traits have been selected to promote abundant production of fruit. Selective breeding from the narrow genetic base of domesticated tomato as well as the introduction of exotic germplasm from the numerous wild relatives of tomato have developed tomato plants producing high-quality fruit for fresh consumption as well as for processed, prepared and stored products valued at approximately US$5 billion annually. Advances in agricultural biotechnology recently have provided the opportunity to expand the genetic resources available for tomato improvement. The goals of tomato genetic engineering have been to protect the tomato crop from environmental and biological assaults, and to improve the quality of tomato fruit in order to deliver greater value in processed tomato products or more healthful and attractive fresh fruit.

Tomato fruit are a significant source of nutrition for substantial portions of the world’s human population because this vegetable crop is widely cultivated and consumed extensively as both a fresh vegetable and concentrated processed products. Tomatoes are rich sources of vitamins, especially ascorbic acid and β-carotene, and antioxidants such as lycopene. A single small tomato is sufficient to supply about a quarter of the vitamins A and C recommended for humans to consume daily (Hamner and Maynard, 1942; Beecher, 1998). Most of the nutritional components in tomato fruit are stabilized by the acid pH of the fruit tissue and many of the human nutrients are conserved during the relatively short and mild processing used in the preparation of most tomato food products. Tomatoes are grown in industrial quantities in many temperate locations, but the stability of the concentrated processed product has made it possible to transport tomato products widely and to prolong the storage of tomato products.

Tomato was one of the first plants to be transformed by Agro bacterium tumefaciens and regenerated into fertile, productive plants (Fillatti et al., 1987). The success of early work to obtain transgenic plants allowed for the first commercial release of a transgenic food product, the Flavr Savr tomato, with extended shelf life of the ripe fruit. The transformation of a large number of tomato varieties has been reported, suggesting that essentially any variety is amenable to genetic transformation. For example, fresh market varieties (Moneymaker, Better Boy), greenhouse varieties (Ailsa Craig), small-fruited fresh varieties (VFNT Cherry) and processing varieties (UC82b) as well as wild tomato relative (L. schilense (Agharbaouiet al., 1995), L. peruvanium (Rudas et al., 1997) and L. hirsutum (Smith et al., 1996)) have all been transformed in academic and commercial research laboratories. Most of the successful transformation protocols for tomato utilize Agro bacteria to deliver transgenes to the hypocotyl sections of newly germinated seedlings, but biolistic approaches also have been utilized. The success of the floral dip methods used in Arabidopsis has not been reported for tomato. Antibiotic resistance of transformed tissues is frequently the preferred method of selection of transgenic tissues, because of its historical success. However, public concerns about the contents of genetically modified food products will undoubtedly lead to the utilization of new selection methods, including positive selection for growth on selective media (Haldrupet al., 1998).

Because fruit are the economically significant crop from tomato plants, many transgenic modifications have targeted the fruit ripening processes to develop products that better withstand harvest, handling, transportation and storage practices utilized in commercial distribution. To reduce processing costs and effectively increase processing yield, transgenic fruit have been developed with increased solids content. To provide novel value-added products, tomato fruit have also been engineered to produce increased components of nutritional value and to produce pharmaceutical compounds. To enhance production efficiency and yield, tomato plants have been engineered for resistance to herbicides, extreme temperatures and pathogens by the transgenic expression of foreign genes not accessible by classical breeding methods. A comprehensive listing of transgenic tomato modifications that have successfully altered aspects of plant growth, morphology and cultivation are summarized in Table 8.1.