New gene-altering strategy tested on corn

As a European-led outcry against genetically engineered food gains momentum, scientists continue to look for new—and potentially safer—ways to tinker with the genes of plants.

Genetically altered corn leaves (above) suffer less damage when treated with herbicide than normal leaves (below) do. Tong Zhu et al./Nature Biotech.

In a method that may prove more acceptable to a reluctant public, investigators have now created herbicide-resistant corn by subtly altering one of the plant’s own genes rather than by adding a new gene.

To endow commercial crops with a novel trait, such as herbicide resistance or freeze tolerance, genetic engineers traditionally first find some other plant with the desired quality.

They then isolate the gene responsible and transfer it into the crop. Yet opponents of genetically modified plants argue that this method can have unintended consequences if the introduced gene disrupts other genes or if its actions are not properly controlled.

Similar concerns have troubled physicians seeking to add genes to patients in the attempt to cure a disease. Gene therapists therefore have recently taken an interest in a technique that fixes mutated genes rather than replacing them.

The method combines RNA and DNA into molecules called chimeras. The DNA enables the hairpin-shaped constructs to home in on a specific gene, while the RNA seems to stabilize the molecule. In a way that remains poorly understood, a cell treated with these chimeras uses its DNA-repair machinery to swap some of the DNA of the chimera with a segment of the cell’s natural gene.

If the chimera contains an intact DNA sequence, it can correct a small defect in the native gene. Or if the chimera harbors a mutation, it can disable a working gene. The latter approach, while not medically useful, offers scientists an additional way to generate mutations in laboratory animals.

The chimeras don’t seem to alter nearby genes, the researchers say. The altered gene also stays under the same regulatory control as the original gene.

Biologists have for the past few years experimented with the RNA-DNA chimeras in mammalian cells. Last year, two research teams showed that the strategy also can alter the genes of plant cells.

One team, led by Chris L. Baszczynski of Pioneer Hi-Bred International in Johnston, Iowa, has now extended that work. The biologists created chimeras that seek out the gene for an enzyme called acetohydroxyacid synthase. From studies of a mustard plant, they knew that a single change in this gene’s DNA sequence would alter the enzyme and make the cells resistant to imidazolinones, a popular class of herbicides.

Chimeras harboring the resistance-endowing DNA sequence successfully altered the maize cells, and the researchers have grown several generations of herbicide-resistant corn from the cells. Baszczynski’s team describes this work in the May Nature Biotechnology.

The new paper “shows the stability of these conversions over generations, something one anticipates but that’s nice to see,” says Gregory D. May of the Samuel Roberts Noble Foundation in Ardmore, Okla., who has used chimeras to alter tobacco genes.

Both Baszczynski and May note that the efficiency of the chimera strategy remains poor; it converts its target gene only in about 1 out of every 1,000 cells. To improve the process, May and his colleagues are using a cellfree system to study how the chimeras work.

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