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New 'BioClay' Shows Potential as RNAi Delivery System for Plants

NEW YORK (GenomeWeb) – A team of Australian researchers this week reported on the development of a new method for the topical delivery of RNAi molecules to plants, opening the door for a potentially new way to use the gene-silencing molecules in agriculture.

The ability of RNAi to silence the expression of specific genes has made it an attractive technology for both crop protection and trait selection. A number of RNAi-modified foods are already on the market including potatoes, apple, and soybeans, and ag-bio firm Monsanto is nearing the launch of a corn variety that expresses RNAi molecules — double-stranded RNA (dsRNA) — that are lethal to corn rootworms.

Topically applied dsRNA are also under development as pesticides by various companies and academic groups, but suffer from a number of limitations related to stability, cellular uptake, and duration of protection. To address these problems, scientists at the University of Queensland explored the potential of positively charged layered double hydroxide (LDH) nanosheets as carriers for dsRNAs.

In a paper appearing in Nature Plants, the team described the synthesis of non-toxic, degradable LDH clay nanosheets — dubbed BioClay — which were loaded with dsRNAs and sprayed onto tobacco and Arabidopsis leaves. The dsRNAs were shown to remain on BioClay under environmental conditions, could be detected on the leaves up to 30 days after application, and were taken up by plant cells to silence their intended target.

To examine the potential of the technology for crop protection, the investigators tested BioClay containing dsRNAs that were designed against two plant viruses — pepper mild mottle virus (PMMoV) and cucumber mosaic virus (CMV).

In one set of experiments, cowpea plants were sprayed with either CMV-targeted dsRNAs contained in BioClay, naked dsRNAs, or LDH alone, and then challenged with the virus. After five days, RNAi-treated plants showed a significant reduction in the formation of the necrotic lesions that characterize CMV infection compared to the LDH controls.

In other experiments, tobacco leaves were sprayed with either PMMoV dsRNA-loaded BioClay or naked PMMoV dsRNAs. When challenged with the virus five days after treatment, both sets of leaves demonstrated similar levels of protection. However, when the plants were challenged with the virus 20 days after spraying, only the BioClay-treated plants showed high levels of viral resistance. Additionally, the scientists found evidence of viral protection in new unsprayed leaves that emerged weeks after BioClay treatment.

Overall, the findings suggest that "if a stable and constant supply of dsRNA can be maintained on the leaf surface using the LDH delivery system, it can provide protection against viruses in sprayed and unsprayed new leaves," the study's authors concluded. "There is also the potential to use BioClay to target insect pests that serve as vectors for transmission of viruses. BioClay has the capacity to change the way we protect plants with the potential of reducing pesticide usage and overcoming the obstacles faced by genetically modified crops."