Plant Mediated RNAi: An Alternative Approach to Pest Control

Justin Overcash is a working on his Ph.D. in Dr. Zach Adelman’s laboratory at Texas A & M University in the area of vector biology and gene drive.  More About the Author

In their recent review in Trends in Biotechnology, Zhang et al 2017 provide an overview of how the RNAi pathway can be utilized to control pests, how different insect pests provide unique challenges to overcome and how those challenges can be overcome.

Adult Diabrotica virgifera virgifera. The larvae are soil dwelling and feed on plant roots, making this a serious pest of corn. Image credit

Plant mediated RNAi may allow plants to defend themselves against insecticide resistant strains and could allow for an overall reduction in the use of insecticides. The first successful report of utilization of nuclear mediated RNAi was reported by a Monsanto group where dsRNA expression in maize targeted a vital gene of western corn rootworm (Diabrotica virgifera). In the same journal issue, another report was published where the Cotton Bollworm (Helicoverpa armigera) fed transgenic cotton designed to target an enzyme believed to detoxify the Gossypol toxin, lead to stunted larval growth.

RNA interference beginning with long dsRNA or short hairpin RNA. Chloroplasts lack this pathway and therefore can be used to express and accumulate high concentrations of long dsRNA that can be ingested by a target insect herbivore.

One of the great challenges associated with plant mediated RNAi is generating enough dsRNA to affect the targeted insect. This challenge is complicated by the generation of transgenic plants in which the dsRNA-producing transgene is located in the nuclear genome.  In this situation the plant’s RNAi system readily converts long dsRNAs into small siRNAs and because these are relatively unstable, it becomes difficult to achieve the concentrations of silencing RNAs needed to impact the target insect.

To avoid this problem transgenes can be inserted into and expressed within the genomes of plastids (usually chloroplasts). The plastid genome lacks core RNAi machinery and in the absence of Dicer activity, the expression and accumulation of high concentrations of long dsRNA is possible.  This results in a significant increase in the amount of dsRNA delivered to the target insect.

In addition, Zhang et al 2017 covers topics which may influence the effectiveness of transplastomic plants such as gut physiology and the selection of a potent target gene(s) for silencing.

Image result for chloroplast transformation

This is a summary figure illustrating the process of chloroplast transformation and is from and excellent review by  Adem M, Beyene D, Feyissa T. Recent achievements obtained by chloroplast transformation. Plant Methods. 2017 Apr 19;13:30. doi: 10.1186/s13007-017-0179-1.

Lastly, the role that the pest’s environment, as well as the evolution of resistance is addressed.

In summary, Zhang et al 2017 provides a concise history of the use of transgenic plants to control insect pests, recent advancements which have been made in the field, and a plethora of additional factors which should be considered before attempting to generate a transplastomic plant for pest control

Zhang, J., Khan, S. A., Heckel, D. G., Bock, R. (2017). Next-generation insect-resistant plants: RNAi-mediated crop protection. Trends in Biotechnology. doi:10.1016/j.tibtech.2017.04.009.

Cited References
Control of coleopteran insect pests through RNA interference.
Baum JA, Bogaert T, Clinton W, Heck GR, Feldmann P, Ilagan O, Johnson S, Plaetinck G, Munyikwa T, Pleau M, Vaughn T, Roberts J.
Nat Biotechnol. 2007 Nov;25(11):1322-6. Epub 2007 Nov 4.

Mao YB, Cai WJ, Wang JW, Hong GJ, Tao XY, Wang LJ, Huang YP, Chen XY.
Silencing a cotton bollworm P450 monooxygenase gene by plant-mediated RNAi
impairs larval tolerance of gossypol. Nat Biotechnol. 2007 Nov;25(11):1307-13.

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