Applications in agriculture
Tissue culture, genetically modified organisms, Bt cotton and RNA interference.
The essentials
Three options can raise food production: agrochemical-based farming, organic farming and genetically engineered crop-based farming. The Green Revolution tripled food supply, but further gains need new approaches. Plants, bacteria, fungi and animals whose genes have been altered by manipulation are called genetically modified organisms (GMOs). GM plants have been made to be more tolerant to abiotic stresses (cold, drought, salt, heat), to reduce reliance on chemical pesticides, to reduce post-harvest losses, to use minerals more efficiently and to be more nutritious (for example Golden rice, rich in vitamin A).
Plant tissue culture grows whole plants from explants on a nutrient medium under sterile conditions (totipotency). Micropropagation produces thousands of genetically identical plants (somaclones). Healthy plants can be recovered from virus-infected plants by culturing the meristem, which is free of virus. Protoplasts from two varieties can be fused to form somatic hybrids, such as the pomato.
Bt cotton: Bacillus thuringiensis forms protein crystals containing an insecticidal protein. The Bt toxin exists as an inactive protoxin; in the alkaline pH of the insect gut it is solubilised and activated. The active toxin binds to the surface of midgut epithelial cells and creates pores, causing swelling, lysis and death of the insect. The toxin is coded by cry genes. The proteins coded by cryIAc and cryIIAb control cotton bollworms, while cryIAb controls the corn borer.
Pest-resistant plants by RNAi: The nematode Meloidogyne incognita infects tobacco roots. Using Agrobacterium vectors, nematode-specific genes were introduced into tobacco so that both sense and anti-sense RNA were produced. These form double-stranded RNA (dsRNA), which triggers RNA interference and silences the specific mRNA of the nematode. The parasite cannot survive in a transgenic host expressing this interfering RNA. RNAi is a natural defence in all eukaryotes, and the dsRNA may come from viral infection or transposons.
Common mistakes
- The Bt bacterium is not harmed by its own toxin because the toxin is an inactive protoxin until it meets alkaline gut pH.
- RNAi silences mRNA; it does not delete the gene from the DNA.