Figure 2. Overview of RNAi approaches for insect-resistant transgenic plants. Double-stranded RNA (dsRNA) produced in planta can lead to targeted gene silencing in
Lepidoptera and Coleoptera pest species [12,13]. dsRNAs corresponding to specific insect targets are expressed in planta and are cleaved by endogenous plant Dicer
enzymes to produce short interfering RNAs (siRNAs) of around 21 nucleotides. Large dsRNA and siRNA cleavage products are expressed throughout plant tissues and are
orally delivered to insect herbivores feeding on transgenic plant material. For gene-silencing to initiate in targeted insect pests, large dsRNAs and siRNAs must persist in the
insect gut, and sufficient quantities must be present for uptake into cells in contact with RNAs (the exact uptake mechanism in target insects remains unknown). Approach
(a): a gut-specific cytochrome monooxygenase, CYP6AE14, has been identified (i) whose expression correlates with larval growth on diets containing gossypol (ii), a cotton
secondary metabolite. CYP6AE14 is presumably involved in detoxification of gossypol (iii) because specific knockdown of this gene product by dsRNAs delivered in artificial
diet and by transgenic plant material increases larval sensitivity to gossypol [14]. Approach (b): a related study [13] used a screening approach to identify a lethal phenotype
in Diabrotica virgifera virgifera when midgut V-type ATPase A (V-ATPase) (iv) was downregulated by dsRNAs delivered in artificial diet feeding trials and transgenic corn.
Although no direct evidence was presented for the deleterious effects observed in larvae, it is tempting to speculate that knockdown of V-type ATPase A results in disruption
of electrochemical gradient across the gut epithelia, which results in high larval mortality.