What is the mechanism of RNAi-based approach?
RNA interference (RNAi) is a biological process that inhibits the expression of target genes. Eukaryotic organisms including insects, possess this RNAi mechanism for sequence specific gene silencing that is triggered by the introduction of double-stranded RNA (dsRNA). A long dsRNA can be either artificially or naturally introduced into the cell. Once inside the cell, the dsRNA is cleaved into ~21 bp small interfering RNA (siRNA) by an enzyme called Dicer (Bernstein et al., 2001; Hamilton & Baulcombe, 1999, Science 286: 950), thus producing multiple “trigger” molecules (siRNA) from the original single dsRNA. One strand of each siRNA is loaded into Agoraute, an endonuclease, to form an RNA-induced Silencing Complex (RISC), thus guiding the RISC to the target mRNA resulting in the effective cleavage and subsequent degradation of the target mRNA. The mechanism of RNAi is illustrated in Fig. 1.
Degradation of the mRNA leads to the silence of its gene. When the important gene of the insect is silenced, the insect will be killed.
Fig. 1 RNAi mechanism
Why do we use topical application approach instead of transgenic approach?
Because transgenic approach may raise concerns about environment and food safety, and topical application of effector molecules (such as dsRNA or siRNA) can also trigger the RNAi mechanism, this approach is convenient to operate and does not involve transgenic plants, thus is easier to be accepted by people.