RNA interference (RNAi) technology has been widely proposed to control insect pests by targeting their vital genes. Small interfering RNA (siRNA)-assisted RNAi has been reported in several previous studies. However, Agrawal et al 2015 report microRNA-assisted plant-mediated RNAi to control the devastating insect pest Helicoverpa armigera (fig1).
miRNA are small non-coding RNA (usually 19-24 nucleotide long) molecules known to regulate expression of gene/s post-transcriptionally. miRNA are well conserved molecules and they are found in plants, animals and some viruses. miRNA recognizes and binds to the target mRNA based on the sequence similarity, followed by disruption of the targeted mRNA resulting in gene silencing (fig1).
Agrawal et al 2013 previously identified miR-24, which down-regulates the expression of H. armigera chitinase gene. In the present follow-up study Agrawal et al 2015 has successfully demonstrated that miR-24 could be exploited to silence the chitinase gene and thereby control the growth and development of H. armigera larvae.
The authors used Solanum lycopersicon (Sly-miR-159) pre-miRNA as a backbone to modify and generate the artificial miRNA-24 (amiR-24) which targets H. armigera chitinase. The mutated amiR-24C, which has reduced binding efficiency was also generated to use as control. The chemically synthesized amiRs were cloned into an expression vector and agro-transformed into tobacco plants. Transgenic plants accumulating amiR-24 and amiR-24C and their corresponding transcripts in highest levels were selected for the insect feeding experiments. H. armigera larvae fed on such transgenic plants were found to have reduced accumulation of chitinase transcripts, reduced growth and experience up to 50% mortality.
Popularity of RNAi technology to control insect pests has been accumulating rapidly. Transgenic plants expressing dsRNA for specific target genes is a common approach, however Agrawal et al 2015 demonstrates an additional and effective strategy by overexpressing the miRNA, which regulates the vital insect pest gene.
Validation of new and previously identified miRNAs, to use as insecticidal miRNAs in other insect pest species, is warranted. Since single miRNA may regulated multiple genes, specificity of the miRNA-assisted gene silencing (off-targets), inter species targets and ecological consequences of such approach should be addressed to enhance the potentiality of this technology.
Agrawal A1, Rajamani V, Reddy VS, Mukherjee SK, Bhatnagar RK. Transgenic Research 2015. Transgenic plants over-expressing insect-specific microRNA acquire insecticidal activity against Helicoverpa armigera: an alternative to Bt-toxin technology DOI 10.1007/s11248-015-9880-x.
Agrawal NB, Sachdev B et al, Scientific reports 2013. Development associated profiling of chitinase and microRNA of Helicoverpa armigera identified Chitinase repressive microRNA, 3:2292. doi:10.1038/srep02292