Credit: VPC Travel Photo/Alamy

The Colorado potato beetle (CPB) is a notorious insect pest of not only potatoes but also tomatoes and eggplants. Both adults and larvae feed on leaves and can skeletonize entire plants, causing devastating effects on crop yields. Insecticides are currently the main method of beetle control, but the CPB is rapidly developing resistance. Transgenic plant technology offers an alternative strategy for crop protection, and one approach has been the expression of exogenous double-stranded (ds)RNAs that target essential insect genes to trigger a lethal RNA-interference (RNAi) response in feeding bugs. However, this tactic is hampered because insecticidal dsRNAs are unstable and are processed by the plants' own RNAi machinery into short interfering RNAs, which are less effective in causing a systemic RNAi response in pests. Bock and colleagues now report a new strategy that circumvents the problem. They stably expressed long dsRNA from the genome of chloroplasts, plant organelles evolutionary derived from cyanobacteria, which lack the RNAi pathway. In this manner, dsRNAs accumulate to high levels, without causing a visible phenotype in the potato plants in regard to growth and tuber production. But are these plants protected against the hungry mouths of the potato beetles? Indeed, Bock and colleagues showed that chloroplast-transformed plants producing dsRNA targeting the CPB β-actin gene were lethal to CPB larvae and were protected against herbivory, even from adult beetles. Plants that expressed the same dsRNAs from the nuclear genome were less well shielded and caused no larval mortality, although larval growth was still retarded. The authors confirmed that the toxic effects were due to repression of the target gene, with obvious consequences on the CPB actin cytoskeleton, especially in cells of the midgut. The potential for RNAi technology in the control of insect pests is great, and shifting dsRNA synthesis from the plant nucleus to the chloroplast is a major step forward for optimization of dsRNA delivery from plants to insects. (Science 347, 991–994, 2015)