Transgenic Plants for Crop Improvement
For crop improvement several genetic traits have been introduced in plants and thereby plant's efficiency has been increased several times as compared to normal plants. In this regards some of the plants have been described.
Insect-resistant transgenic plants
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.
The other approach has been isolation of the toxin gene (Bt
2from B. thuringiensis)
and its introduction into Ti-DNA plasmid of Agrobacterium tumifaciens.
The genetically modified A tumifaciens
was allowed to infect the desired plant. Thus Ti-plasmid mediated transformation of several plants has been done, for example tobacco, cotton, tomato, corn etc. (Table 9.8). Field experiments has also been done with Manducta sexta
which is a serious pest of tobacco. It has been found that 75-100 per cent larvae of M. sexta
died when chewed the leaves of transgenic tobacco, whereas the control plants (that were not transgenic) were severely damaged by the insect. Besides, when tobacco plant was crossed with a normal (control) plant, the resistance gene was inherited as per Mendelian principle.
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.
||Corn, cotton, oilseed rape, potato, tobacco, tomato
||Corn, cotton, oilseed rape, potato, tobacco, tomato
||Corn, cucumber, melon, papaya, potato, tobacco, tomato
|Modified seed storage protein
||Sunflower, rice, soybean
|Modified seed oil
|Agglutinin (wheat germ)
In 1996, the first two companies 'Mycogen' and 'Ciba Seeds' commercialized Bt
insect resistant seeds of corn which has shown very effective protection from the European corn borer.
The second insect resistant gene is cowpea tripsin inhibitor (CpTI) gene. In cowpea (Vigna unguiculata)
the level of CpTI is high and, therefore, it is resistant to the attack of major storage pest of seeds called bruchid beetle (Callosobruchus maculatus).
CpTI has been found toxic in nature to many insect pests. Therefore, the cpti
gene was isolated and joined to CaMV 35S promoter with one marker gene and incorporated into Agrobacterium.
The genetically engineered Agrobacterium
infected leaf discs of tobacco and delivered cpti
gene. Finally from the infected leaf discs transgenic tobacco plants were produced that contained high level of CpTI.
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.
Fig. 9.7. Construction of a transgenic plant and expression of Bt gene against insect larvae.
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.
A number of microorganisms are associated with the degradation of herbicides (see Environmental Biotechnology).
Obviously, degradation is accomplished by genes encoding specific enzymes such as PAT (phosphinothricin acetyl transferase encoded by bar
gene of Streptomyces
spp. that degrades herbicide PPT), nitrilase (encoded by bxn
gene of Klebsieila bromoxynil),
GST (glutathione-S transferase that degrades the herbicide Atrazine), etc. By using these genes several transgenic crop plants have been produced, for example, transgenic potato, oilseed rape, and sugarbeet (all containing bar
gene) and transgenic tomato (containing bxn
gene). The other herbicide resistant transgenic crop plants are corn, cotton, soybean, etc. (Table 9.8). These have been released for commercial cultivation after field trials.