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Monsanto Moves RNAi-based Pest-resistant Corn into Final Stage of Development

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Ag-bio giant Monsanto this month announced that its most advanced product candidate incorporating RNAi for pest control — a strain of corn resistant to rootworms called Smart Stax Pro — has advanced into the final stage of development, putting it on track for a potential market launch by 2017.

Also this month, Monsanto researchers and collaborators at Kansas State University published new details about the target of Smart Stax Pro's gene-silencing component, showing how silencing the gene proves lethal to the insects.

As reported by Gene Silencing News, Smart Stax Pro, formerly known as corn rootworm III, is a transgenic corn variety that expresses widely used Bt proteins derived from the bacterium Bacillus thuringiensis along with dsRNA designed to silence a gene known as Snf7.

Monsanto has long been interested in the agricultural applications of RNAi and has already introduced to the market one product based on the technology — soybeans called Vistive Gold that are altered with RNAi to have lower levels of saturated fats. But Smart Stax Pro represents the company's first effort to create a pest-resistant plant that expresses dsRNAs.

As shown by Monsanto scientists in a 2012 publication, corn rootworms ( Diabrotica virgifera virgifera) belong to a group of insects that are sensitive to environmental RNAi wherein dsRNA can induce gene silencing after being ingested. The team also demonstrated that using this approach to inhibit DvSnf7 — the rootworm ortholog of Snf7 and an essential component of cellular machinery known as endosomal sorting complex required for transport (ESCRT) — is lethal to the insects.

This month, Monsanto investigators reported further on DvSnf7 inhibition, analyzing the ultrastructural changes in the midgut enterocytes in rootworms treated with dsRNA.

It had already been shown that DvSnf7 inhibition triggered deficiencies in protein de-ubiquitination and autophagy, leading to insect death, they wrote in PLoS One. But new research has revealed that silencing the gene results in "the appearance of irregularly shaped macroautophagic complexes consisting of relatively large lysosomes and multi-lamellar bodies, indicative of failure in autolysosome formation." Additionally, the RNAi effect leads to cell sloughing and loss of apical microvilli, and ultimately the "massive loss of cellular contents indicating loss of membrane integrity."

The findings indicate that the critical functions of Snf7 in insect midgut cells underlie "the conserved essential function of the ESCRT pathway in autophagy and membrane stability in other organisms," the team added.

These and other data, including the determination that the spectrum the DvSnf7 silencing effect via dsRNA is "narrow" and therefore unlikely to affect other animals in the corn rootworm environment, has led Monsanto to announce earlier this month that Smart Stax Pro has advanced into phase IV. This prelaunch stage includes the preparation and submission of data packages to the US Department of Agriculture and the Environmental Protection Agency, as well as various international regulatory agencies.

Previously, Monsanto had said that Smart Stax Pro was on track for commercialization by the end of the decade. According to the company, however, phase IV typically lasts between 12 and 36 months, which could see the product on the US market as early as 2015.

This timeline, however, is dependent on the time it takes regulators to review and sign off on Smart Stax Pro, and therefore imprecise, cautioned Ron Flannagan, corn insect traits platform lead at Monsanto and co-author of the latest PLoS One paper.

Also this month, Monsanto highlighted its next-generation corn rootworm control product called corn rootworm IV, which is currently in the phase I proof-of-concept stage of development. Notably, while Smart Stax Pro joins DvSnf7 RNAi with the established Bt traits known as Cry3Bb1 and Cry 34/35, the company said that corn rootworm IV would use novel protein and RNAi modes of action.

Flannagan noted that several RNAi targets are currently under evaluation, but Monsanto has yet to select one for use in the new product.

Also in phase I are four other previously disclosed RNAi programs, which are grouped under the company's so-called BioDirect initiative. These include an effort to develop a spray that can control weeds resistant to the herbicide glyphosate; a topical treatment for plants to combat Colorado potato beetle; a topical product to prevent tospovirus infections in specialty crops; and a topical treatment for bees to fight varroa mites and various viruses.

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