In this article we will discuss about the applications of plant transformation.
Plant transformations/transgenics have immense utility in improving plant productivity and improving plant product quality. Transformations may be used to introduce new or novel genes into the existing cultivars to create altogether new market or to displace the old product.
The improvements may relate to the improvement in nutritional value of the plant or the functional properties in processing or consumption. Plant transgenic may be used to block the production of certain metabolites by controlling the over-expression or inhibition (antisense expression) of any of the critical enzymes as illustrated.
In the present given case, the level of metabolite D may be controlled by the over-expression or inhibition (antisense expression) of any of the three enzymes. However, enzyme 3 may be the best target. This example illustrates, as to how one can change the chemical constituents of the products through introduction of novel genes from altogether unrelated sources.
Historically, the first genetically engineered whole food to be produced commercially, was FLAVR SAVR tomato in 1994 in USA. This product is a tomato with an antisense gene blocking the production of polygalacturonase during fruit ripening. Polygalacturonase is a key enzyme involved in the degradation of pectic components of the cell walls in plants.
Fruit softening during ripening of tomatoes has been inhibited by the expression of an antisense RNA to tomato polygalacturonase. Fruit ripening could also be regulated by controlling production of ripening hormone ethylene.
Ethylene is produced from S-adenosylmethionine by conversion of l-amino-cyclopropane-l- carboxylic acid (ACC) under the control of ACC synthase followed by the production of ethylene by an ACC oxidase or ethylene forming enzyme (EFE).
This is shown as follows:
Inhibition of this pathway by transformation allows control of fruit ripening and senescence in plants. Control of insects through application of insecticides is a common feature in our agricultural system. Genetic engineering of insect resistance offers an option for reduction in use of chemicals in agriculture. Further, in-built plant resistance against insects is a major component of any IPM strategy.
Transgenes options available for insect resistance include:
1. Protease inhibitors
2. α-amylase inhibitors
3. Lectins
4. Bacterial toxins
Protease inhibitors influence the digestion of protein adversely and thereby retard the growth rate of insects. A cowpea protease inhibitor (CpTI) was the first plant gene used successfully for genetic engineering of insect resistance. Lectins are carbohydrate binding proteins that may bind to the epithelia of gut and be toxic to insects. Alpha-amylase inhibitors may also be effective in the control of insects.
Bacillus thuringiensis (Bt) produces crystalline proteins with insecticidal action against many Lepidopteran, Dipteran and Coleopteran insects. During sporulation, the bacteria produce intracellular crystals of Bt. These crystals when ingested by the insects, are degraded in the insect gut by specific proteases. Toxic fragments are then released which disrupt the lining of the insect’s gut and the insect is killed. This technology of using Bt transgenics has now been commercialized.
Options are available to have genetically engineered resistance against plant diseases caused by fungi, bacteria, viruses and nematodes. Proteins effective against bacteria may be isolated from many sources including plants. Candidate genes may be identified by screening likely antimicrobial agents against cultures of the bacteria.
Similarly, genes conferring resistance against fungi may also be identified. Chitinase perhaps acts by degrading fungal cell walls. Proteins such as thionins and per-matins including osmotin and zeamatin have also been shown to be effective against bacteria or fungi. Production of transgenics with resistance to viruses has been one of the first few successful applications of genetic engineering.
The introduction and expression of coat protein gene has been found of widespread success. Introduction and expression of viral replicase gene, and that of both sense and antisense expression of parts of the viral genome may be protective against viral diseases.
The use of herbicide resistance genes in plant transformation and creation of commercially, acceptable transgenics has revolutionised agriculture from the view point that the target cultivar with herbicide resistance gene tolerates the application of herbicides and the weeds are selectively killed.
This has resulted into saving a lot on manual/mechanical eradication of weeds. Glyphosate is most-commonly used broad spectrum herbicide. This chemical prevents the synthesis of aromatic amino acids in plants by inhibiting 5-enolpyruvyl shikimate-3-phosphate (EPSP).
Several transgenics using mutant ESPS genes recovered from Salmonella typhimurium have been engineered. Overproduction of normal ESPS leads to glyphosate resistance in the target plant. Structure of glyphosate is as given here.
Other potential broad spectrum herbicides against which transgenics are useful options include suphonylureas, imidazollinones, triazalopyrimidines, 2-dichlorophenoxyacetic acid (2,4-D), phosphinothricin, atrazine and bromoxynil.
There are immense potentials of regenerating novel plant transformations conferring resistances against parasitic plants, abiotic stresses, etc. Some of these possibilities have been put to commercial scale in huge acreage in several countries.
Release of transgenic plants into the environment is regulated in many countries to prevent dangerous products entering the food chain or causing harm to the environment. Release of plants with a herbicide resistance gene may be considered unsafe if the transgenic plant is able to cross freely, with closely related weed species which may allow development of herbicide resistant weeds.
Likewise plants producing foods with potentially toxic, allergenic or anti-nutritional properties may require careful evaluation by biosafety regulatory agencies and these are in place to have checks and balances, so that, only safe products are released for commercial cultivation.
It is obvious that from the stand point of research one can expect that the properties of the plants will change considerably, over the coming years, given the new opportunities to re-programme them genetically, using the powerful tools of plant biotechnology. However, which products will occupy commercial space will depend on which products are economically and environmentally, sound and socially, acceptable.
However, let us be clear that by now the debate over commercial acceptance of transgenic cultivars has gone into oblivion by the simple fact that by now about 134 million ha area is under transgenic crops worldwide without any harmful effect.