GM crops

Genetically modified organisms (GMOs) have had their genes altered. GM crops can:

• tolerate stress such as cold, drought, salt and heat;
• depend less on chemical pesticides, because they resist pests;
• lose less food after harvest;
• use soil minerals more efficiently, keeping soils fertile;
• be more nutritious: Golden rice has been enriched with vitamin A.

Bt cotton. The soil bacterium Bacillus thuringiensis (Bt) makes proteins that kill certain insects: the larvae of butterflies and moths (lepidopterans, such as the cotton bollworm), beetles (coleopterans) and flies and mosquitoes (dipterans).

Why doesn't the toxin kill the bacterium? It is made as an inactive protoxin, stored as crystals. When an insect eats it, the alkaline pH of the insect's gut dissolves the crystals and activates the toxin. The active toxin binds to the cells lining the midgut and makes pores in them. The cells swell and burst, and the insect dies.

The toxin is coded by cry genes, and each kills particular pests. The genes cryIAc and cryIIAb control cotton bollworms; cryIAb controls the corn borer. Putting such a gene into a crop lets the plant make its own insecticide, so farmers need to spray far less.

Pest-resistant tobacco by RNA interference. A nematode (roundworm), Meloidogyne incognita, infects the roots of tobacco plants and greatly reduces yield. The solution used RNA interference (RNAi), a natural defence in eukaryotic cells: a double-stranded RNA that matches a particular mRNA binds to it and stops it being translated. This silences the gene.

Using Agrobacterium vectors, scientists put nematode-specific genes into the tobacco plant, arranged so that the plant made both the normal (sense) and the complementary (anti-sense) RNA. These paired up into dsRNA, which silenced the matching mRNA in the nematode when it fed on the roots. The parasite could not survive, and the plant was protected.