Plants might not get colds, but they do get viruses — and viral diseases in crops cause enormous economic damage each year. New research, however, suggests that plant “vaccines,” developed at Rockefeller University, may be a new way of helping fend off viral attackers.

“Plants possess several innate mechanisms to resist viruses,” says Nam-Hai Chua, Andrew W. Mellon Professor and head of the Laboratory of Plant Molecular Biology, “but many viruses are able to overcome these barriers. Transgenic technology offers the possibility to genetically modify plants with genes encoding virus tolerance and/or resistance.” Chua’s research, published in November in Nature Biotechnology, shows that the new method can confer resistance against two turnip viruses.

Chua and colleagues took advantage of a pathway that is normally used by plants to regulate their own development. MicroRNAs (miRNAs) are small 21-nucleotide strands of RNA that regulate the production of proteins from messenger RNAs (mRNAs) by interacting with, and most often directing the destruction of, the mRNAs. Each miRNA is made from a longer precursor gene.

“There was evidence that several nucleotides within the 21-nucleotide sequence could be altered to target a specific transcript,” says Qi-Wen Niu, co-first author of the paper and a research assistant in Chua’s lab. “We thought it might be possible to modify a plant miRNA sequence to target viral RNA.”

Working with the turnip mosaic virus and the turnip yellow mosaic virus, the researchers chose to target the viral suppressor proteins, which help the viruses elude the plant’s natural defenses. They picked sequences that were conserved across multiple strains and that had no homology to any plant genes. Their artificial miRNAs would not interact with any plant mRNAs, but they did give the plant resistance to infection.

“Giving the plants just one artificial miRNA precursor transcript, they were able to make miRNAs against both viruses and resist infection,” says Shih-Shun Lin, also co-first author and a postdoc in the Chua lab. “And the resistance was hereditable.”

The researchers also showed that unlike the plant’s normal defense system, which shuts down in low temperatures, their artificial miRNAs remained active even when the environment was not ideal. “Many crop plants, like turnips, need low temperatures for optimum growth, and the viruses can infect them in cold weather,” says Lin. “Our artificial miRNAs still function, and confer resistance, when other methods of plant resistance have shut down.”

“There are many concerns surrounding other transgenic methods that use viral genes,” says Chua, “but our artificial miRNA strategy minimizes the perceived risks as no viral genes are used. We think that our new method is applicable to other viruses, and can be used to engineer broad-spectrum resistance to several viruses through the co-expression of appropriately designed multiple artificial miRNAs.”

Nature Biotechnology 24(11): 1420-1428 (November 2006)