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.

	Table 8.1 Transgenic tomato modifications
	Trait		Gene	Regulation	Expression
	Fruit ripening
		Ethylene reduction	Bacterial ACC deaminase	Constitutive	Expression
		Ethylene reduction	Phage SAMase	Fruit specific	Expression
		Ethylene reduction	Tomato ACC synthase	Constitutive	Antisense suppression
		Ethylene reduction	Tomato ACC synthase	Constitutive	Sense suppression
		Ethylene reduction	Tomato ACC oxidase	Constitutive	Antisense suppression
		Fruit softening	Tomato fruit PG	Constitutive	Antisense suppression
		Fruit softening	Tomato fruit PG	Constitutive	Sense suppression
		Fruit softening	Tomato fruit PME	Constitutive	Antisense suppression
		Fruit softening	Tomato fruit PG and PE	Constitutive	Antisense suppression
		Fruit softening	Tomato fruit expansin	Constitutive	Sense suppression
		Fruit abscission	Tomato fruit Cel1 and Cel2	Constitutive	Antisense suppression
	Fruit composition
		Sucrose accumulation,	Tomato fruit invertase	Constitutive	Antisense suppression
		Solids content		Constitutive	Expression
		Starch accumulation	Arabidopsis sucrose synthase	Constitutive	Expression
		Fatty acid and flavor content		Constitutive	Expression
		Color	Tomato phytoene synthase	Constitutive	Antisense suppression
		Parthenocarpic	Bacterial tryptophan monoxygenase	Constitutive	Expression
	Seeds
		Increased dormancy	Tomato NCED	Constitutive	Expression
		Decreased dormancy	Arabidopsis abi-1	Constitutive	Expression
	Pathogen and pest resistance and tolerance
		TMV	TMV N	Constitutive	Expression
		CMV	Cucumber mosaic virus CP	Constitutive	Expression
		TSWV	Tomato spotted wilt virus N	Constitutive	Expression
		PhMV	Physalis mottle tymovirus CP	Constitutive	Expression
		Pseudomonas syringae pv tomato	Tomato Pto	Constitutive	Expression
		Xanthomonas campestris pv. Vesicatoria	Pepper Bs2	Constitutive	Expression
		Cladosporium fulvum	Tomato Cf 9	Constitutive	Expression
		Verticillium dahliae	Tomato chitinase	Constitutive	Expression
		Fusarium oxysporum f.sp. lycopersici	Tobacco chitinase and Β 1,3 - glucanase	Constitutive	Expression
		Trichoderma hamatum Xanthomonas campestris	Rubber tree hevein	Constitutive	Expression
		pv. vesicatoria	Tamoto LeETR4	Constitutive	Expression
		Sclerotinia sclerotiorum	Collybia velutipes oxalate decarboxylase	Constitutive	Expression
		Botrytis cinerea	Pear fruit PGIP	Constitutive	Expression
		Phytophthora infestans	Grape reserveratrol	Constitutive	Expression
		Manduca Sexta	Tomato prosystemin	Insect induced	Expression
		Insect resistance	Bt toxins	Constitutive	Expression
		Insect resistance	Bt cry1Ac	Constitutive	Expression
		Nematode resistance	Rice cystatinc-I	Constitutive	Expression
		Root knot nematode, aphid, viral resistance	TomatoMi	Root-specific	Expression
	Plant defense responses
		Extracellular responses	Agrobacterium ipt	Constitutive	Expression
	Environmental stresses
		Salt stress	Yeast HAL2	Constitutive	Expression
		Drought	Arabidopsis ABI-1	Constitutive	Expression
		Chilling and oxidative stress sensitivity	Tomato catalase	Constitutive	Antisense suppression
		Heavy metal tolerance	Bacterial ACC deaminase	Root-specific or stress induced	Expression
	Herbicide resistance
		Thiazopyr resistance	Rabbit liver esterase	Constitutive	Expression
		Quinclorac resistance	Tomato ACC synthase	Constitutive	Antisense suppression
		Fenthion (insecticide) sensitivity	Tamoto Prf	Constitutive	Antisense suppression
	Metabolic modifications
		Increased sucrose unloading	Sucrose phosphate synthase	Root-specific or fruit-specific	Expression
	Foliage coloration
		Increased anthcyanin	Antirrhinum del	Constitutive	Expression
	Floral patterns
		Indeterminate flowering	Tomataogamous	Constitutive	Expression
		Precocious termination	Tomataogamous	Constitutive	Antisense suppression

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.