Approval of GM maize that kills rootworm larvae using RNAi

A new pesticide which halts gene expression in one of North America’s most damaging pests is the first of its kind to be approved by the EPA.

Final regulatory approval has been granted to a genetically modified corn known as SmartStaxPro in North America. SmartStaxPro has been engineered to produce DvSnf7, a pesticide that kills the parasite Western corn rootworm. Rootworms lay their eggs in cornfields, where the larvae feast on the crop’s roots – this species alone is estimated to cost American farmers over $1 billion annually in lost yield and pest control. DvSnf7 aims to overcome widespread resistance to standard pesticides in the rootworm population by harnessing a recent biotech advance known as RNA interference (RNAi).

RNAi is a naturally occurring mechanism for silencing genes and is found in many species. Genes, made up of specific sequences encoded in DNA, provide the instructions for building proteins, which make up the structures and machinery in cells. RNA is a single stranded molecule which acts as an intermediate between DNA and protein by carrying the instructions (in the form of a sequence that matches that of the gene) from the nucleus, where genes are stored, to the cytoplasm, where proteins are built. Some viruses have their genes in the form of double stranded RNA, so in order to stop a viral infection, cells use the viral double stranded RNA as a template to guide the RNA-destroying complex RISC to any viral RNA in the cell with the same sequence.

Scientists can harness this mechanism to silence any gene by engineering cells to express RNA that has the same sequence as the target gene and that will fold into a double stranded structure. DvSnf7 forms a double stranded RNA in the corn plant which silences the gene snf7 in rootworm larvae when they eat the plant by causing its RNA intermediate to be degraded. snf7 is essential for breaking down worn-out proteins in the larvae’s cells so silencing it is fatal.

RNAi has generated much excitement in the agricultural industry for its ability to specifically kill pests without harming pollinators. RNAs also cannot pass into humans as they are broken down by the gut, so there should be no risk to consumers. Even so, the EPA has faced some criticism from environmental groups concerned that RNAi could pass into humans (although the only paper to propose this has since been widely discredited) or could harm pollinators, with the National Honeybee Advisory Board describing the decision as ‘more naïve than our use of DDT [a highly toxic insecticide] in the 1950s’. How justified these concerns are remains to be seen, but it is clear that RNAi technology is allowing innovative new methods of crop protection, something much needed if we want to ensure future food security.