VOLUME 12, NUMBER 21 • MAY 4, 2001

Cutting Out Public Concern for Safety of Genetically Modified Crops

Professor Chua’s lab also is working on ways to improve methods for genetically modifying crops.

Current protocols depend on the use of "marker genes" to identify which strains of plants have taken up the gene of interest, such as a pest-resistance gene or a gene that allows crops to tolerate higher levels of salt in the soil. But these marker genes often code for antibiotic resistance, and some health officials worry that bacteria will acquire these genes when they encounter genetically modified foods in our gut. These feared "superbugs" would be capable of evading today’s already dwindling arsenal of effective antibiotics.

In 1999, Chua came up with a new marker gene. Instead of conferring resistance to antibiotics, this new gene, the isopentenyl transferase gene (ipt), promotes shoot growth in plants when activated by the chemical dexamethasone (dex). By placing transformed plant cells on a surface of dex, transgenic plants can be readily detected by the appearance of shoots.

Nevertheless, this technique might not completely allay public concern, because the ipt gene remains in the final plant product. Though scientists believe this gene to be safe, some people remain skeptical.

Recently, Chua and colleagues, including Jianru Zuo and Simon Geir Moller, both postdoctoral fellows, and Qi-Wen Niu, a visiting researcher, developed a simple and efficient way to eliminate extraneous marker genes altogether. In the February issue of Nature Biotechnology, they described a new chemical-inducible DNA removal system, in which all nonessential genes are cut out after the transgenic plants have been created.

"All the unwanted genes are cut out after they have done their business," says Chua. "The whole system self-destructs."

This system takes advantage of a protein called Cre recombinase, which cuts out all of the DNA that lies between two sites, termed loxP sites, and reseals the dangling ends. But in Chua’s system, it will do this only when activated by another chimeric protein called XVE, which, in turn, is activated only in the presence of beta-estradiol, a mammalian hormone that does not appear to have any physiological effects on plant growth and development.

By placing the antibiotic-resistance gene, the XVE gene and the Cre recombinase gene between two loxP sites on a strand of DNA, such that the target gene lies outside of the loxP sites, the researchers can excise all of the unnecessary genes simply by adding beta-estradiol to the cells. The only thing left is one foreign gene and a genetically enhanced plant.

The two techniques invented by Chua’s lab are not mutually exclusive: a researcher could use the ipt gene as a marker gene, then later cut it out with Cre-based system. Either way, these new techniques are bringing marker gene removal one step closer to commercial implementation.