Transgenic Plants for Crop Improvement
Insect-resistant transgenic plants The Bt gene of a bacterium, Baccilus thuringiensis has been found to encode the toxins called endotoxin which pose cidal effect on certain insect pests. These toxins are of different types such as beta-endotoxin and delta-endotoxin. For detailed discussion of physical and chemical nature of these toxins see Biological Control of Plant Pathogens, Pests and Weeds (section bacterial pesticides). Preparations of Bt gene in powder form have been made available in market for commercial use.
Similarly, transgenic tomato plants has also been produced through cell/tissue culture and transformation techniques. Outline of introduction of Bt gene in a crop plant is shown in Fig. 9.7.
Table 9.8. Transgenic plants which have been produced by using recombinant DNA technology by inserting valuable traits.
Traits | Plants |
Herbicide resistance | Corn, cotton, oilseed rape, potato, tobacco, tomato |
Insect resistance | Corn, cotton, oilseed rape, potato, tobacco, tomato |
Virus resistance | Corn, cucumber, melon, papaya, potato, tobacco, tomato |
Modified seed storage protein | Sunflower, rice, soybean |
Modified ripening | Tomato |
Modified seed oil | Oilseed rape |
Agglutinin (wheat germ) | Corn |
Herbicide-resistant transgenic plants
Herbicides are used in agriculture for killing the weeds (the unwanted plants). However, the herbicides disturb the metabolic activity of photosynthesis or synthesis of amino acids. In addition, due to their use in free hand, environmental pollution occurs and, therefore, biodegradable new herbicides are being developed which will be ecofriendly and environmentally safe. For the development of herbicide resistant plants, two strategies are being applied: (i) modification of target molecules that may be insensitive to herbicides, and (ii) degradation of herbicides.
The mechanism of action of different herbicides differ. Therefore, attempt must be made to develop resistance against at least three herbicides e.g. glyphosate, sulphonylurea and imidazolinones. A herbicide resistant gene for RPSPS (5-enolpyruvate-shikimate-3-phosphate -synthase) was isolated from plants resistant to glyphosate (active ingredient of Roundup herbicide). The resistant gene for EPSPS was transferred to petunia plants and transgenic petunia was developed which was resistant to glyphosate. The other transgenic plants of tomato was developed by introducing a mutant als gene (for the enzyme ALS, acetolactate synthase) of tobacco or arbidopsis. The enzyme ALS was inhibited by the herbicides sulphonylurea compounds (active ingredient of Gleen & Qust herbicide) and imidazolinones. A gene resistant to PPT (L-phosphinothricin), an active ingredient of herbicide 'Basta’, was isolated from Medicago sativa. It inhibits the enzyme GS (glutamine synthase) which is involved in ammonia assimilation. This gene resistant to PPT was incorporated into tobacco, as a result of which transgenic tobacco was produced which was resistant to PPT. Similar enzyme has been isolated from Streptomyces hygroscopicus by the scientists of Hoechst (Germany) and Plant Genetic System. This enzyme also inactivates the herbicide 'Basta'. Transgenic plants resistant to 'Basta' has been produced by introducing the bacterial gene.