Biotechnological Applications in Agriculture

Biotechnological Applications in Agriculture

To increase food production, three broad approaches may be considered:

• agro-chemical based agriculture;
• organic agriculture; and
• genetically engineered crop-based agriculture.

Chemicals in Agriculture

The Green Revolution achieved large increases in cereal production and contributed to a substantial rise in food supply. These yield increases were achieved partly by using improved crop varieties and largely by better agronomic management and use of agrochemicals (fertilisers and pesticides).

However, agrochemicals are often expensive for small farmers in developing countries, and further improvement in yield using conventional breeding alone may be limited. Biotechnology offers alternative approaches to obtain higher productivity while reducing reliance on chemical inputs and minimising adverse environmental impacts. One such approach is the development and use of genetically modified crops.

Genetically Modified Organisms (GMO) are plants, bacteria, fungi or animals whose genetic material has been altered by deliberate manipulation. GM plants have been developed for a range of agricultural and industrial purposes and have several practical benefits.

Benefits and Objectives of Genetic Modification in Crops

• Make crops more tolerant to abiotic stresses such as drought, salinity, cold and heat.
• Reduce reliance on chemical pesticides by creating pest-resistant crops.
• Reduce post-harvest losses by improving resistance to storage pests and pathogens.
• Increase efficiency of mineral and nutrient use by plants to conserve soil fertility.
• Enhance nutritional value of food, for example, rice enriched with provitamin A (commonly referred to as Golden Rice), by introducing biosynthetic genes that produce beta-carotene.

Genetic Modification in Tomato

Beyond food crops, biotechnology has been used to create tailor-made plants that supply industrial raw materials such as specialised starches, renewable fuels and pharmaceuticals (plant-made pharmaceuticals). Several specific applications in crop protection and improvement are described below.

Bacillus thuringiensis (Bt) and Bt Toxin

Bacillus thuringiensis (abbreviated Bt) is a bacterium that produces protein crystals during a particular growth phase. These crystals contain insecticidal proteins known as Bt toxins.

Bt toxin proteins are synthesised as inactive protoxins. When an insect susceptible to a particular Bt toxin ingests the crystal protoxin, the alkaline pH of the insect midgut solubilises the crystal and proteolytic enzymes activate the protoxin. The activated toxin binds to specific receptors on the surface of the midgut epithelial cells and creates pores in the cell membrane. Pore formation disrupts ion balance, causes cell swelling and lysis, and eventually results in insect death.

Because most Bt toxins are specific to particular groups of insects, genes encoding Bt toxins have been isolated (cloned) and introduced into crop plants to confer resistance to those insect pests. Such transgenic crops thereby act as a built-in bio-pesticide.

Bt Cotton and Bt Crops

• Certain strains of Bacillus thuringiensis produce proteins that are toxic to particular insect orders such as Lepidoptera (moths and butterflies, e.g., cotton bollworm, armyworm), Coleoptera (beetles) and Diptera (flies, mosquitoes).
• Bt forms protein crystals that contain the insecticidal protein; these crystals are ingested by the target insect.
• The alkaline pH of the insect gut solubilises the crystals and activates the toxin.
• The activated toxin binds to the surface of midgut epithelial cells and creates pores, leading to cell lysis and death of the insect.
• Specific Bt toxin genes have been incorporated into crop plants; the choice of gene depends on the crop and the target pest because most Bt toxins are insect-group specific.
• The Bt toxin genes belong to a family named cry. Different cry genes encode proteins with different specificities; for example, the proteins encoded by cryIAc and cryIIAb are used to control cotton bollworms, whereas cryIAb is effective against corn borer.

Cotton Balls

Pest-Resistant Plants and RNA Interference (RNAi)

Biotechnology provides strategies other than Bt for controlling pests. One such strategy is based on RNA interference (RNAi), a natural mechanism in eukaryotic cells for silencing gene expression. RNAi can be used to reduce or eliminate the expression of essential genes in pests or pathogens that attack plants.

Root-knot nematodes are important plant parasites. A commonly encountered species that attacks many crops is Meloidogyne incognita, which infects roots and causes severe yield reduction in crops such as tobacco.

Pest Resistant Plants

RNAi operates when a double-stranded RNA (dsRNA) molecule, complementary to a target mRNA, triggers degradation of that mRNA or prevents its translation, thereby silencing the corresponding gene. In nature, dsRNA may originate from viral replication intermediates or transposable elements; in biotechnology, dsRNA can be produced by the plant to target parasite genes.

Using plant transformation vectors (for example, those derived from the bacterium Agrobacterium), researchers can introduce DNA constructs into a host plant so that the plant produces RNA sequences corresponding to essential genes of the pest. By arranging the introduced DNA to express both sense and antisense strands of the target sequence, the plant produces complementary RNAs that anneal to form dsRNA within plant tissues.

RNA and DNA

The dsRNA derived from the host plant is taken up by the feeding nematode or other pest; the dsRNA initiates RNAi in the pest and silences the targeted gene. If the targeted gene is essential for the survival or infectivity of the parasite, the parasite cannot develop or reproduce successfully on the transgenic host. In this way, host-generated dsRNA provides protection against nematode infestation.

The following figure illustrates the concept of host plant-generated dsRNA triggering protection against nematode infestation:

(a) roots of a typical control plants
(b) transgenic plant roots 5 days after deliberate infection of nematode but protected through novel mechanism 

Other Considerations and Management

• Transgenic crops are frequently produced by inserting a gene cassette containing a promoter (for example, the constitutive Cauliflower mosaic virus 35S promoter), the coding sequence of the transgene, and a selectable marker to identify transformed cells. Such transformed cells are regenerated into whole plants by tissue culture techniques.
• To delay the evolution of resistance in insect populations exposed to Bt crops, integrated pest management measures are recommended; one commonly used measure is the provision of refuges (areas planted with non-Bt crops) to maintain populations of susceptible insects.
• Potential environmental and biosafety issues such as gene flow to wild relatives, impact on non-target organisms, and development of pest resistance require careful assessment and regulatory oversight before commercial release of transgenic crops.
• Transgenic approaches are complementary to conventional breeding and agronomic practices and are most effective when used as part of integrated crop-health and soil-fertility management strategies.

Summary

Biotechnological methods in agriculture, such as gene transfer from Bacillus thuringiensis and host-generated RNA interference (RNAi), offer effective tools to increase crop productivity, reduce pesticide use and enhance nutritional quality. These technologies must be developed and deployed alongside agronomic best practices and appropriate biosafety measures to achieve sustainable food production and environmental protection.