RNAi for insect-proof plants
Author(s): Karl H J Gordon ; Peter M Waterhouse 
Plants and the insects that feast on them have waged war for hundreds of millions of years. Insect pests cost billions of dollars in the form of crop losses and insecticides, and farmers face an ever-present threat of insecticide resistance, fueling a continual search for alternative pest-control strategies. Two papers in this issue [1, 2] show the potential of using RNA interference (RNAi) induced by hairpin RNAs as a new line of defense against coleopteran and lepidopteran pests.
Among transgenic approaches to insect-pest management, Bacillus thuringiensis (Bt ) toxin has shown spectacular success, replacing chemical insecticides in a range of crops. But many important insect pests are not amenable to Bt protection, and there is an imminent danger of at least some species developing Bt resistance. RNAi has been exploited in plants for applications ranging from functional genomics to provision of valuable crop traits, such as resistance against viruses, bacteria and nematodes [3, 4]. However, it has not yet been harnessed to defend crops against insects.
In animals and especially in insects, RNAi is usually accomplished by injection of double-stranded RNA (dsRNA) , but in nematodes it may also be triggered by a diet containing dsRNA . This latter observation, combined with the demonstration of transgene-encoded RNAi in plants , has long prompted speculation that plants could be protected from herbivorous insects by engineering them to express dsRNAs targeting vital insect genes. However, the passage of years without reports of success using this approach seemed to suggest that simply expressing hairpin RNA in plant material to be ingested by an insect would not provide sufficient levels of intact dsRNA to trigger potent RNAi in the pest.
Experimental validation of this strategy, however, has now been achieved by Mao et al . --working with cotton bollworm--and Baum et al . --working with a range of insects that includes western corn rootworm (WCR), southern corn rootworm and Colorado potato beetle. Their results suggest that transgene-encoded ingestible dsRNA may one day stand alongside Bt transgenes in insect management programs.
Baum et al . first used feeding assays, in which larvae were fed an artificial diet supplemented with specific dsRNAs, to screen a large number of essential insect genes. These assays identified...