Bt Crops: Genetically Modified Plants for Pest Resistance

Bacillus thuringiensis is a naturally occurring, soil-dwelling bacterium that, during a particular stage of its life cycle, produces crystalline protein inclusions known as delta-endotoxins, or more commonly, Bt toxins. These crystal proteins are encoded by a family of related genes generally referred to as cry genes (crystal genes), and different cry genes produce Bt toxin variants that are effective against different specific groups of insect pests -- for example, certain Cry proteins are toxic specifically to lepidopteran pests (such as the cotton bollworm), while others target coleopteran (beetle) pests.

Inside the Bacillus thuringiensis bacterium itself, the Bt toxin exists as an inactive precursor form, called a protoxin, and it causes the bacterium itself no harm at all. The protoxin only becomes an active, toxic form under the very specific chemical conditions found inside the gut of a susceptible insect: once an insect larva ingests the protoxin (either by eating the bacterium directly, or, in the case of Bt crops, by feeding on plant tissue engineered to produce the protoxin protein), the highly alkaline environment of the insect's gut dissolves and chemically activates the protoxin into its toxic form. This activated toxin then binds to specific receptor molecules present on the cells lining the insect's midgut; this binding step explains the toxin's specificity, since only insects possessing the particular matching gut receptors are affected, while other insects, and non-target organisms such as mammals (including humans), which lack both the required alkaline gut environment and the matching receptors, are unharmed by exposure to the same protein. Once bound, the activated toxin creates pores in the midgut cell membranes, causing the cells to swell and burst; this disrupts the insect's gut function, leading to paralysis of the digestive system, cessation of feeding, and ultimately the death of the insect larva.

Genetic engineers have exploited this natural insecticidal mechanism by isolating the relevant cry gene from Bacillus thuringiensis and introducing it directly into the genome of a crop plant, creating what is called a Bt crop -- a plant that manufactures its own Bt toxin protein within its tissues, protecting itself against attack by the target insect pest without requiring external chemical insecticide spraying for that pest. Bt cotton, engineered using Cry protein genes (commonly Cry1Ac and/or Cry2Ab, depending on the specific commercial variety) to resist the cotton bollworm, a major pest of cotton crops, was among the earliest and most widely adopted Bt crops. As with the natural bacterium, the toxin protein produced inside the Bt cotton plant remains in its inactive protoxin form within the plant's own cells (which have a near-neutral pH), and is converted to its active, toxic form only after being ingested by a susceptible insect larva feeding on the plant, whose gut provides the alkaline environment needed for activation. …