Database Product Description
- Host Organism
- Beta vulgaris (Sugar Beet)
- Phosphinothricin (PPT) herbicide tolerance, specifically glufosinate ammonium.
- Trait Introduction
- Agrobacterium tumefaciens-mediated plant transformation.
- Proposed Use
Production for human consumption.
- Product Developer
- Bayer CropScience (Aventis CropScience(AgrEvo))
Summary of Regulatory Approvals
Summary of Introduced Genetic Elements Expand
Characteristics of Beta vulgaris (Sugar Beet) Expand
Donor Organism Characteristics Expand
Modification Method Expand
Characteristics of the Modification Expand
Environmental Safety Considerations Expand
Food and/or Feed Safety Considerations Expand
Sugar beet (Beta vulgaris L. ssp. vulgaris var. altissima) is a botanical variety of B. vulgaris ssp. vulgaris, as are other comestible and fodder beets, and is a member of the goosefoot (Chenopodiaceae) family. It is an important source of sugar (sucrose), accounting for approximately 40 percent of global production.
Sucrose is present in limited quantities in many plants, including various palms and the sugar maple, but sugar beet and sugarcane are the only commercially important sources. More than half of the world sugar supply is obtained from sugarcane, which is grown in tropical and subtropical climates. The rest is supplied by the sugar beet, which is grown in temperate countries. Sugar is manufactured from the roots of the sugar beet; the leaves and tops are removed after harvesting and used as livestock feed. The roots are cut into cossettes, or chips, from which the juice is extracted. The juice is processed to yield sugar and beet molasses, and the remaining pulp is used for domestic animal feed. Beet molasses is also fed to livestock; table molasses is not made from beets because of difficulties in purification. The sugar that is produced from the sugar beet is identical to the sugar that is derived from sugarcane. It is widely used as a sweetener for foods, in preserves, and in the manufacture of candies, baked goods, and soft and alcoholic beverages. Sugar is also used as the raw material for fermentation products such as ethyl alcohol, butyl alcohol, glycerin, citric acid, and levulinic acid. Sugar is an ingredient in some transparent soaps, and it can be converted to esters and ethers, some of which yield tough, insoluble, and infusible resins.
Effective weed management is critical to sugar beet production. Weeds cause significant losses in sugar beet yield and crop quality, and also cause harvesting problems. The low growth habit of the sugar beet plant renders it more susceptible to competition from weeds compared to other crops, such as corn or soybean. Many weed species grow taller than sugar beet and thus aggressively compete for light, water and nutrients. The economic threshold for weed control in sugar beet production is therefore quite low, given the high value of the crop, and the significant losses caused by the presence of weeds. Weeds control strategies include the use of herbicides alone, and herbicides in combination with hand weeding. Sugar beet herbicides can be applied prior to crop emergence (e.g., triallate, EPTC, ethofumesate), and post-emergence (e.g., ethofumesate, triflusulfuron, desmedipham, phenimedipham, sethoxydim).
The sugar beet line T120-7 was genetically engineered to express tolerance to glufosinate ammonium, the active ingredient in phosphinothricin herbicides (Basta®, Rely®, Finale®, and Liberty®). Glufosinate ammonium acts by inhibiting the plant enzyme glutamine synthetase, a key enzyme that detoxifies ammonia by incorporating it into glutamine. Inhibition of this enzyme leads to an accumulation of ammonia in the plant tissues, which kills the plant within hours of application. Phosphinothricin herbicides applied at rates recommended for effective weed control are toxic to conventional sugar beet varieties. The modified T120-7 line permits growers to use phosphinothricin-containing herbicides for weed control in the cultivation of sugar beet.
Glufosinate tolerance in T120-7 sugar beet is the result of the introduction of the pat gene into the beet genome via Agrobacterium-mediated transformation. The pat gene was isolated from the common soil fungus, Streptomyces viridochromogenes, and encodes the enzyme phosphinothricin-N-acetyltransferase (PAT). PAT catalyses the acetylation of phosphinothricin which detoxifies it into an inactive compound.
The transgenic sugar beet line T120-7 was field tested in the United States from 1994 to 1997, as well as in Canada, Western and Eastern Europe, and in the former Soviet Union. T120-7 was evaluated extensively and no differences were found in agronomic characteristics, plant emergence and seedling vigour when compared to standard commercial sugar beet varieties growing in nearby fields. It was demonstrated that the transformed sugar beet line did not negatively affect beneficial or non-target organisms, and it was not expected to impact on threatened or endangered species.
Sugar beet is an outcrossing species, and is mainly wind pollinated. It is a biennial, developing a rosette of leaves and a large taproot in the first year, and bolting (i.e., forming an inflorescence bearing stem) in the second year. Bolting is induced only after the plant has undergone vernalization at the end of the first season. Certain environmental conditions, such as low temperatures and longer daylength, may induce bolting in the first year. Sugar beet is also very sensitive to frost, and is a poor competitor with other plants. Large amounts of pollen are produced during the reproductive phase, which is carried by wind over long distances.
The genus Beta is comprised of four sections: i) Beta, to which sugar beet belongs; ii) Corollinae; iii) Nanae; and iv) Procumbentes. All species within the Beta section (e.g., B. vulgaris ssp. maritima, B. macrocarpa) are cross-compatible and produce fertile progeny. Hybrids between sugar beet and members of the other three sections do not occur naturally, and cross-breeding with other members of the Chenopodiaceae is unlikely. As with conventional sugar beet cultivars, T120-7 plants are harvested prior to the initiation of the bolting phase. Since sugar beets are usually not allowed to bolt, except in fields or plots grown for seed production, there is little opportunity for uncontrolled pollen flow. Sugar beets grown for seed production are also isolated from other sugar beet fields to minimize pollen flow. This is usually acheived by the use of isolation distances enforced by seed certification agencies.
The development of glufosinate-tolerant hybrids is possible due to the ability of sugar beet to outcross with related plants. However, the glufosinate-tolerance trait is not expected to provide a competitive advantage to hybrid plants unless grown in managed environments routinely subjected to glufosinate applications. In the event that a glufosinate-tolerant hybrid survived, the herbicide-tolerant individual could be controlled using mechanical and/or other available chemical means. T120-7 is considered unlikely to increase the weediness potential of any other cultivated plant or native wild species with which it may interbreed. T120-7 also does not possess growth and reproductive characteristics that would render it weedier than conventional sugar beet.
Sugar beet intended for human consumption is generally converted directly to refined white sugar through extensive purification processes. The PAT protein in T120-7 sugar beet was expressed at low levels in the sugar beet roots and tops, and was not detected in refined sugar or molasses derived from T120-7. Consequently there will be no human exposure to PAT protein as a result of consumption of refined sugar derived from T120-7 sugar beet.
Sugar beet pulp is a by-product of the refining process and is normally dried and pelleted for use as a livestock feed. In recent years, it has been purified and used as an additive, at levels of less than 1%, in some specific foods, such as food fibre for breakfast cereals. The PAT protein was not detected in the pulp produced from T120-7 sugar beet.
T120-7 line was compared with non-transgenic sugar beets in analyses of numerous compositional components, including crude fat, crude protein, fibre, ash, carbohydrate, calories, calcium, magnesium, phosphorus, potassium, and sodium. With the exception of fibre content, no statistically significant differences were observed. The differences in fibre content were ascribed to environmental factors associated with different growing locations and conditions. The process fractions, refined sugar, molasses and dried pulp produced from T120-7 sugar beet were assessed by proximate analysis (fatty acids, amino acids, minerals and sugar profiles). The dried pulp fraction revealed differences in ash content and some amino acids. It was determined that these differences in nutrient content would not impact on animal nutrition, as dried pulp comprises less than 25% of the content of livestock feed. The molasses fraction revealed differences in fat, protein, calcium, and some amino acids. These differences were small and not expected to impact the nutritional content of animal feed. Analysis of the refined sugar fraction showed a difference in tryptophan levels. Importantly, there were no significant differences in carbohydrate content, which is the major nutrient in refined sugar (98%).
Potential toxicity and allergenicity of the transformed sugar beet line was investigated through examination of the amino acid sequence and physiochemical characteristics of the introduced PAT protein. It was determined that the PAT enzyme has a very low potential for toxic or allergenic effects. No significant homologies with the amino acid sequences of known protein toxins or allergens were detected, the PAT enzyme did not display the heat stability characteristic of allergenic compounds, and was rapidly degraded under conditions simulating mammalian digestion. The low concentrations of the PAT enzyme found in T120-7 tissues, and so in the food or livestock feed products derived from them, further supported the conclusion that the PAT enzyme possessed little or no potential for allergenicity or toxicity.
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This record was last modified on Friday, March 26, 2010