By Karthik Reddy
RNA interference (RNAi) is an endogenous regulatory process in eukaryotes, concerned with restricted or reduced mRNA expression in a sequence-specific manner which might be incorporated into the conventional plant protection calendar. In the host-induced RNAi (HI-RNAi) approach, a crop plant is engineered with an RNAi vector to produce dsRNA, a trigger component of RNAi against the target gene of insect pests. Topical application of double-stranded RNA (dsRNA) can be used as a narrow-spectrum alternative to chemical insecticides with increased accuracy and specificity. Upon feeding on treated plant parts, dsRNA enters into the insect gut, leading to the induction of RNAi machinery resulting in silencing of the target gene. Consequently, a non-functional phenotype is produced, which no longer can survive or damage the plant. Also, this method negates the need for integration of dsRNA-expressing constructs as transgenes into the plant, eliminating the limitations of genetic modification and related controversies.
With the population likely to reach 9 billion by 2050, the demands on global agriculture are expected to skyrocket in the coming decades. Many supplementary factors are expected to aggravate the worsened condition, including climate change, reduction in arable land, decreased efficacies of chemical insecticides due to resistance and resurgence of pests coupled with poor agronomic practices and increased consumption of more nutritious foods in the developing nations.
The South American pinworm Tuta absoluta (Meyrick) (Gelechiidae: Lepidoptera) is a major destructive pest of tomato worldwide. Intensification of domestic and international trade and human movement has been critical for the invasiveness of the pest in tomato-growing regions of the world. This moth has been one of the most important tomato pests in South America since the 1960s. In India, the pest was initially detected in Pune (Maharashtra) on tomato plants grown in a polyhouse and fields during October 2014 and soon spread to the rest of the tomato-growing regions of the country (https://icar.org.in/node/6084). Younger larvae produce blotch-type mines in leaf epidermis, and older ones bore into the fruits through calyx, often infesting 100% fruits in a field. The pinhead-sized entry holes often go unnoticed until the larvae produce a brown to black frass pile at the edge of entry points.
Since it has a high reproduction capacity and short generation cycle, it is more likely to develop resistance to insecticides, including new generation molecules. So far, it has developed resistance against Avermectins and Milbemycins (IRAC Group 6), Pyrethroids and Pyrethrins (Group 3A), Nereistoxin analogues (Group 14), Benzoylureas (Group 15), and Oxadiazines (Group 22A).
Identifying a good gene target is critical to the success of RNAi in pest control. Only 48 percent of the 130 lepidopteran genes evaluated for RNAi silencing were silenced at a significant level, while 14 percent of the attempts were unsuccessful.
Juvenile hormone (JH) regulates many physiological processes in insect growth, diapause, and reproduction. The carrier partner protein of JH, juvenile hormone binding protein (JHBP), is required to balance JH titre to govern insect metamorphosis and development, which could be an excellent target for RNAi mediated suppression of metamorphosis. Another potential target is Vacuolar ATPases, ATP-dependent proton pumps that acidify intracellular compartments and transfer protons across the plasma membrane. They keep the midgut lumen alkaline and energise secondary amino acid uptake in Lepidoptera. Interestingly, in a recent study conducted by a team of scientists led by ICAR-Indian Institute of Horticulture Research, Bengaluru successfully produced dsRNA for vacuolar ATPase subunit B and JHBP from T. absoluta (Ramkumar et al. 2021).
Different dsRNA concentrations were tested in tomato leaflet feeding bioassays, and mortality rates were recorded at different feeding access periods. Treatment of dsRNA to tomato leaves (50 g dsRNA/cm2 leaf) and oral intake of the same resulted in RNAi silencing and a significant increase in larval mortality. These RNAi biopesticides were discovered to have high specificity for the target pest while posing no threat to beneficial non-target species that may feed on the treated pests, conserving predators, parasitoids, and pollinators. With all of these advances in RNAi for pest control, the researchers concluded that the discovery of two dsRNA triggers, v-ATPase subunit B and JHBP, may provide acceptable targets for the development of RNAi-based control of Tuta absoluta.
The ability to use RNAi to control T. absoluta by exogenously treated leaves, crops, or diets with bacterial-expressed dsRNA gives enough proof to move forward with RNAi biopesticides to manage this economically destructive lepidopteran pest. Furthermore, creating an exogenously administered formulation that could be applied to a wide range of agricultural plants would be more effective and economical than treating a specific plant species. Based on these findings, RNAi techniques using exogenously applied double-stranded RNAs could be a great alternative to conventional pesticides targeting T. absoluta larvae. Furthermore, growing public acceptance and a proven track record of safety towards human health and biodiversity support their use in the sustainable management of insect pests.
So, will the discovery of RNAi pave the way to sustainable global agriculture, or is it just another technology that holds promise but cannot be practical or palatable enough to become part of our conventional pest management practices? Well, we can expect some answers within the next few years as more empirical pieces of evidence accumulate regarding the pros and cons of RNAi based pest management approaches. In a developing country like India, with agriculture being the backbone of the economy, RNAi-based research has massive scope to control devastating agricultural and forest insect pests sustainably.
For further details, please refer to:
Ramkumar, G., Asokan, R., Prasannakumar, N. R., Kariyanna, B., Karthi, S., Alwahibi, M. S., Elshikh, M. S., Abdel-Megeed, A., Senthil-Nathan, S., Kalaivani, K., Hunter, W. B., Krutmuang, P. (2021). RNA Interference Suppression of v-ATPase B and Juvenile Hormone Binding Protein Genes Through Topically Applied dsRNA on Tomato Leaves: Developing Biopesticides to Control the South American Pinworm, Tuta absoluta (Lepidoptera: Gelechiidae). Frontiers In Physiology. https://www.frontiersin.org/articles/10.3389/fphys.2021.742871/full
Karthik Reddy, a Masters student at the Division of Entomology, ICAR-IARI, New Delhi-110012, India.
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