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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.

 
     
 
 
     



     
 
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