The following points highlight the top nine traits carried by transgenic plants. The traits are: 1. Insect Resistance 2. Herbicide Resistance 3. Virus Resistance 4. Fungal Resistance 5. Stress Resistance 6. Improved Production of Storage Proteins and Lipids 7. Suitable Plants for Food Processing 8. Hybrid Food Crops.
Trait # 1. Insect Resistance:
Pesticide and insecticides are not only toxic to non-target organisms; these also cause many hazards to environment. Transgenic plants are now developed that are able to protect themselves against pests/insects by expressing insecticidal protein or a proteinase inhibitor gene.
The toxic activity of the toxin (Bt toxin) produced by the bacteria species Bacillus thuringiensis is due to insecticidal activity of the protein delta endotoxin. The gene bt-2 encoding this protein was isolated and used for Agrobacterium Ti plasmid mediated transformation of tobacco, tomato and cotton plants.
These transgenic plants were found to be resistant to the pest Manduca sexta. Field tests with other insect pests also showed excellent results. India is now trying to introduce the Bt toxin in cotton for the development of pest resistant variety of this major cash crop.
Trait # 2. Herbicide Resistance:
ADVERTISEMENTS:
Herbicides usually affect processes like photosynthesis or biosynthesis of essential amino acids; thereby they kill weeds as well as crop plants indiscriminately. Scientists have now developed transgenic plants resistant to many powerful herbicides.
The herbicide Basta (L- phosphinothricin, PPT) inhibits glutamine synthase. Scientists isolated a gene from Streptomyces hygroscopicus that inactivates the herbicide Basta and introduced this gene into tobacco and tomato plants.
These transgenic plants have been field tested and proved to be ‘Basta’ resistant. Another class of herbicides induces sulphonylurea compound that inhibits the enzyme acetoacetate synthase (ALS). Transgenic tobacco plants with a mutant ALS gene from tobacco or Arabidopsis were produced that were tolerant to that herbicide.
In this way, many herbicide-resistant transgenic crop plants were developed.
ADVERTISEMENTS:
In another approach, a number of detoxifying enzymes has been isolated from plants as well as microbes. For example (i) nitriles, (coded by the gene bxn in Klebsiella pneumoniae), it detoxifies the herbicide bromoxynil, and (ii) phosphinothricin acetyl transferase or PAT (coded by bar gene in Streptomyces sp), it detoxifies the herbicide PPT (L-phosphinothricin).
Transgenic tomato plants using the bxn gene from Klebsiella and bar gene from Streptomyces and transgenic potato, rapeseed, and sugerbeet plants with the bar gene from Streptomyces have been produced and found to be herbicide resistant. Other herbicide resistant transgenic plants include soya bean, corn, poplar etc.
Trait # 3. Virus Resistance:
The genetic engineering of virus resistance into plants provides a direct and more efficient control of viral disease. Coat Protein-Mediated Protection (CPMP) is a very effective technique in which plants infected by a mild strain of a virus, resist infection by other related strains, a phenomenon referred to as cross protection.
The tobacco mosaic virus (TMV) coat protein (CP) is specifically involved in cross protection. Transgenic plants expressing high levels of coat protein upon injection with TMV, exhibited a decay in disease development. Subsequently, CP genes isolated from different single-stranded sense RNA plant viruses have been cloned into plant expression vectors and then transferred into various crop plants (e.g., tobacco, tomato and potato) using the Agrobacterium tumefaciens mediated plant transformation system.
Trait # 4. Fungal Resistance:
ADVERTISEMENTS:
The epidemic spread of fungal disease in plant is now controlled by various techniques like isolation of resistance genes, transfer of that gene and production of transgenic plant of resistance varieties. For example, Logemonn (1992) developed transgenic plant resistant to fungus Rhizoctonia solani.
Trait # 5. Stress Resistance:
In plants a number of gene has been isolated that provides resistance against stresses like cold, heat, salt, heavy metals, phytohormones etc. The chilling resistance property of Arabidopsis is responsible for the gene encoding glycerol-1 phosphate acyl transferase. This gene was isolated, cloned and then transferred to tobacco plants to make them cold resistant.
Trait # 6. Improved Production of Storage Proteins and Lipids:
The Brazil nut, Berthalletia exceisa, contains two nutritionally important proteins containing sulfur-rich amino acids: methionine and cysteine. Scientists are able to synthesise a segment of DNA that code for a part of these proteins and this DNA probe was used to find and pull out the natural gene from the Brazil nut.
Researchers then transferred the gene into tomato and tobacco plants which are easier to manipulate and also transferred the gene into yeast cells. Early results showed that the genetically engineered yeast do produce the sulfur-rich proteins.
Recombinant DNA technology now engineered the composition of fatty acids in seed oil. Tissue specific antisense RNA expression has recently been used for reducing stearoyl-ACP desaturase activity in seeds to make an alternation in the ratio of saturated to unsaturated fatty acids. Thus, Brassica rapa and B. napus transgenic plants were developed which contained a dramatic increased level of stearate in their seeds.
Trait # 7. Suitable Plants for Food Processing:
A number of plants are now available whose fruits can be stored for a desirable period. For example, a transgenic tomato is developed whose polygalacturonase gene is not expressed. This enzyme attacks the pectin in the cell wall and thus softens the skin. In the transgenic tomato using antisense RNA against that enzyme, ripening was delayed.
This tomato lasts about 10 days longer before rotting and can be left on the vine to ripen and develop more flavour. Therefore, they are described as Flavr Savr. Similarly, long lasting transgenic tomato with elevated sucrose and reduced starch content could also be produced using sucrose phosphate synthase gene.
Trait # 8. Hybrid Food Crops:
ADVERTISEMENTS:
Production of transgenic edible crops tremendously improved their yields, food values as well as disease resistance property. Monsanto, the biotechnology company of USA, produced insect resistant transgenic maize plants, using the Kurstaki gene derived from B thuringiensis.
The gene was attached to CaMV 35s promoter, which enhanced the production of toxin 1000 times. Transgenic rice with increased amount of iron (ferritin protein) and β-carotene has been produced.
The introduced ferritin gene in expressed under the control of a rice seed storage protein glutelin promoter to accumulate iron in the rice grain. Three genes, psy, lyc (cloned from Narcissus pseudo-narcissus) and crt 1 (cloned from Erzvinia uredovora), driven by an endosperm specific promoter (gt 1) have been introduced into rice to synthesise β- carotene. Rice with P-carotene in converted to vitamin A is human body and is released when needed.
The introduction of two bacterial genes day A by Corynebacterium encoding the enzyme di-hydro-dipicolinic acid synthase and lysc by mutant E. coli encoding the enzyme aspartokinase into rice, maize, canola, soya bean etc. has enhanced the lysine content of these seeds by about five folds.
Transgenic rice with several PR-genes (Pathogenesis-related protein) has been developed recently which showed enhanced resistance against the fungi Rhzoctonia solani and Pyricularia oryzae that cause sheath blight and blast disease, respectively. A hybrid commercial Bt rice, “Shan You 63” has now been grown in China in a large scale which showed resistance to four insect pests including leaf folder and stem boarer.
Again Xa21 rice (transgenic IR72) has been developed with enhanced resistance against bacterial blight caused by Xanthomonas oryzae. Thus, bio-engineered food crops including rice may provide new dimension of efficient breeding which would help the farmers to adopt improved varieties and better crop management. It in turn will reduce the use of different agrochemicals and will make a better sustainable environment for all.
Trait # 9. Nitrogen Fixing Ability:
The nitrogen fixing genes (nif) have been isolated from Klebsiella pneumoniae and transferred to E. coli, Salmonella typhimurium, Erzvinia herbicola etc. which are now able to fix atmospheric nitrogen. Besides these prokaryotic organisms, nif genes have been isolated from Rhizobium and transferred to Agrobacterium tumefaciens.
The expression of transferred nif genes has been successful in cereal plants like wheat, paddy etc. through Ti-plasmid of A. tumefaciens. However, there is some concern that new nitrogen-fixing varieties might spread indiscriminantly like weeds or disturb the soil nitrogen cycle.