GM crops: insect resistance

Insect pests of the corn borer or the cotton boll worm represent a major threat to maize and cotton farmers respectively. Both insect’s larvae feed on the inside of the plant and are therefore difficult to reach by sprayed insecticides.

The Gram-positive soil bacterium Bacillus thuringiensis has been long known to produce a range of crystal (Cry) proteins. These proteins - commonly known as Bt toxins – become cleaved in the insect’s midgut bind specifically to cell-surface receptors. The cells are lysed by pore formation via an oligomerisation step after recognition. Subsequently, larvae stop feeding and starve. Due to high specificity the Cry proteins are toxic to a very limited range of insects and are harmless to other organisms; no toxicity to mammals, birds and fish.

In the pre-GM era, Bt toxins were used as a biological alternative to chemical insecticides, however, the costly production prevented a major breakthrough. The introduction of Cry genes into plants circumvents this problem, and a Cry-protein content as low as 0.1% of the total cell protein suffices to protect cultivars from certain insect pests. Accordingly, Cry genes were engineered into cotton and maize, and found successful application in the US (29% of the maize, 41% of the upland cotton), Australia, China, India and the Philippines, known as Bt variants.

As an unexpected positive side effect, the grains of Bt maize contain lower amounts of mycotoxins such as aflatoxin and fumicosin, which are risk factors for liver carcinoma.

Another approach to achieve insect resistance is based on protease inhibitors, which are produced by some plants protecting against animals. The protease inhibitors disrupt digestive processes leading to starvation, enabling broad insect resistance. On the downside, protease inhibitors work on a variety of species and the plants needs to be cooked before consumption to inactivate the protease inhibitors. This may also have a lot of ramifications on the surrounding fauna when bred outdoors.