How to Pick an Engineered Seedling

A new lab procedure may someday eliminate the risk of accidentally creating antibiotic-resistant bacteria during the development of transgenic plants. The technique, described in this month's issue of Nature Biotechnology, accelerates the growth of transgenic tobacco and lettuce seedlings.

To grow a plant with better salt tolerance or pest resistance, scientists must first add genes to the embryo. But not all cells take up foreign genes. The easiest way to select plant cells containing introduced genes is to kill the cells that didn't accept the gene. Biologists traditionally accomplish this feat by pairing up the desired gene, or transgene, with another gene that makes the cell resistant to antibiotics. Bathing all the cells in an antibiotic wipes out the nonresistant ones. The method is so effective in so many kinds of plants that "all previous technology uses antibiotic resistance genes," says Michael Syvanen, a microbiologist at the University of California, Davis.

But there is a danger: Experiments have shown that some transgenes can slip from an adult plant into bacteria. This has raised fears that native bacteria could take up the antibiotic resistance genes. No one has seen this phenomenon occur in nature, says Syvanen, but in theory it can happen. "It's just a matter of time," he says.

Now, a team led by microbiologist Tim Kunkel of The Rockefeller University in New York City has shown that at least two crops--lettuce and tobacco--can be engineered without using antibiotic resistance genes. Instead, Kunkel's group linked the transgenes to isopentenyltransferase, an enzyme that accelerates shoot formation in seedlings. By bathing the young plants in a steroid that triggers shoot growth, the team was able to pick out the transgenic seedlings and dispose of the rest. How well the strategy will work in other plants, says Kunkel, "remains to be seen."