Database Product Description
- Host Organism
- Solanum lycopersicum (Tomato)
- Delayed softening through suppression of polygalacturonase (PG) enzyme activity.
- Trait Introduction
- Agrobacterium tumefaciens-mediated plant transformation.
- Proposed Use
Production for human consumption.
- Product Developer
- Zeneca Seeds
Summary of Regulatory Approvals
Summary of Introduced Genetic Elements Expand
Characteristics of Solanum lycopersicum (Tomato) Expand
Modification Method Expand
Characteristics of the Modification Expand
Environmental Safety Considerations Expand
Food and/or Feed Safety Considerations Expand
The tomato is a vine-like herb of the nightshade family (Solanaceae) that also includes potatoes, peppers and eggplants. Botanically this vegetable is a fruit (a berry), which although being a perennial plant in the tropics, is grown as an annual plant in northern climates. The tomato is a native of the Peru, Bolivia, and Ecuador area of the Andes Mountains. Its antiquity is uncertain in regard to cultivation but it was being cultivated when Europeans discovered America. However, it was not generally cultivated in the United States until 1835 because, until then, it was widely believed to be poisonous.
Tomatoes are propagated from seeds. In temperate regions seeds are generally started in greenhouses, hotbeds, or cold frames; the plants are set out in the fields when danger of frost is past. Fresh tomatoes are harvested by hand, while those destined for canning or for processing into soups, sauces and ketchups are harvested by machine. The numerous varieties differ greatly in plant form and fruit type, the latter ranging from a small currant size through cherry, plum, and pear forms to the large, nearly round fruits, 10 cm (4 in) or more in diameter, which are the most widely grown. All forms include red- and yellow-fruited varieties.
Tomatoes are a valuable source of food minerals and vitamins, and are low in calories. One medium-sized tomato provides 57% of the recommended daily allotment (RDA) of vitamin C, 25% RDA vitamin A, and 8% RDA iron, yet it has only 35 calories. Tomatoes are also rich in an anti-oxidant called lycopene, a carotenoid that has been found to protect cells from oxidants that have been linked to cancer. In laboratory tests, lycopene was found to be twice as powerful as beta-carotene in neutralizing free radicals. Lycopene has been linked to risk reduction for a number of types of cancers, including prostate, lung and stomach, pancreatic, cervical, colorectal, oral and esophageal cancers.
In the fresh market industry the tomato fruit is often picked at the mature green or breaker stages for long-distance shipping, and is then subsequently ripened by treatment with the gas, ethylene (12 to 18 h at 20ºC). For processing tomatoes the ethylene-producing compound, ethephon or Ethrel, is applied prior to harvest when only 10% of the fruit is ripe; this accelerates and concentrates fruit ripening and facilitates once-over machine harvest.
Pectin is a building block in plant cell walls and is what gives tomatoes their firmness. Fruit softening during ripening is due to the breakdown of cell wall pectin by an enzyme called polygalacturonase (PG). Tomato lines B, Da, and F were genetically engineered to express delayed softening by inserting a truncated version of the PG encoding gene in either the sense (lines Da and F) or the “antisense” (line B) orientation. The presence of the partial PG gene, in either sense or antisense orientation, suppresses the expression of endogenous PG enzyme at the onset of fruit ripening.
In the case of line B, the mechanism of action is likely linked to the hybridization of antisense and sense messenger-RNA (mRNA) transcripts, resulting in a decreased amount of free positive sense mRNA available for protein translation. For lines Da and F, which contain the truncated PG gene in the sense orientation, reduced PG expression may be due to coordinate suppression of transcription of both the endogenous gene and the introduced truncated gene.
In any event, reduced PG expression decreases the breakdown of pectin and leads to fruit with slowed cell wall breakdown, better viscosity characteristics and delayed softening. Tomato lines B, Da and F have improved harvest and processing properties that allow the transgenic tomatoes to remain longer on the vine to develop their natural flavour, maintain their firmness for shipping and produce a thicker consistency in processing.
Tomato lines B, Da and F were field tested in the United States from 1991 to 1994. The agronomic characteristics of lines B, Da and F were evaluated extensively in laboratory, greenhouse, and field experiments. It was determined that tomato lines B, Da and F did not exhibit weedy characteristics, nor did they have any effect on non-target organisms or the general environment. The transformed tomato lines were not expected to impact on threatened or endangered species.
Cultivated tomatoes are self-fertile, and almost exclusively self-pollinating. Their unique flower and anther morphology makes tomato an essentially cleistogamous plant, in which self-pollination and fertilization occur within an unopened flower. A low crossing rate between tomato varieties was demonstrated and attributed to the limited availability of pollen and poor foraging activity of insect pollinators.
Several related species are found as weeds in tomato fields, however, commercial tomato is generally sexually incompatible with these weedy relatives. Two Solanum species, S. lycopersicoides and S. rickii, neither of which is a weed pest in the United States, can be crossed with commercial tomato only under specific, controlled conditions requiring human intervention. The cherry tomato, L. esculentum var. cerasiforme can be crossed with tomato, L. esculentum var. esculentum. However, it would be very unlikely for these transgenic tomato lines to hybridize with cherry tomatoes in the United States since the rate of outcrossing in tomatoes is low and cherry tomatoes are not common in areas devoted to the large-scale cultivation of tomatoes. It was concluded that the chance of genetic exchange among tomato crops was small and outcrossing to other species, even more remote. In the event that an outcrossing event involving pollen from the transgenic tomato lines did occur, it was unlikely that the delayed-ripening trait would increase the plant’s weediness or probability of survival.
The transgenic line F was used to develop the hybrid lines 1401F, H282F, 11013F and 7913F, which, along with the parental transgenic line, were evaluated in comparison with the non-transgenic parental lines during the course of the food safety assessment.
The analysis of nutrients from the novel 1401F hybrid line and the non-transgenic 1401 line did not reveal any significant differences in the levels of macro- and micronutrients in either fresh tomatoes or processed tomato paste. An extensive analysis of fresh fruit and tomato paste samples derived from tomato line F was made and results compared to the range of values produced by non-modified, commercial tomato varieties. Results for the bulk components (soluble sugars, structural carbohydrate, moisture, ash, fat, dietary fibre, protein and oil) and for mineral (Na, K, Ca, Mg, P, Fe) and vitamin (A, E, B1, B2, niacin, B6, folate, C) content fell within the range expected of commercial tomato fruit and fruit purees. Analysis of known anti-nutritional factors and potential toxins, including lectins, glycoalkaloids (alpha-tomatine, solanine, chaconine) and biogenic amines (tyramine, nicotine and serotonin), showed that similar levels were present in both modified and unmodified fruit and tomato paste.
The reduced synthesis of native PG was not judged to have any potential for human toxicity or allergenicity. Polygalacturonase is a natural component of all food plants. The partial PG gene present in the transformed tomato lines was identical to the corresponding endogenous tomato gene and did not encode for any new protein products, and therefore was not expected to have any toxic properties.
Links to Further Information Expand
This record was last modified on Friday, March 26, 2010