Mutated plant sheds light on DNA repair.

If DNA mutations caused by excessive sunbathing can promote skin cancer in humans, how do plants manage to get away without harm? Rooted in place, many plant species take in much higher doses of ultraviolet (UV) radiation than human sun-lovers do during a tanning marathon.

Though plant cells are adept at amending the damage caused by UV-B, the most dangerous portion of sunlight, scientists have been in the dark about how they do it. Now, geneticists have created a mutant strain of cress that is unable to repair UV-induced DNA damage.

Ann B. Britt, a geneticist at the University of California, Davis, and her co-workers have pinpointed a gene in Arabidopsis thaliana, or mouse-eared cress, that in healthy plants is involved in clipping out marred DNA pieces, restoring the original genetic code. By displaying the way in which its DNA repair is hampered, this mutant strain of cress sheds light on how the repair process works in the wild-type plant.

Says Britt, "We know much about DNA repair and mutation in bacteria, some animals, even humans, but virtually nothing in flowering plants," largely because higher plants have stubbornly resisted scientists' attempts to generate DNA-report-defective mutants. Her group's report in the Sept. 17 SCIENCE helps fill that gap and introduces a technique for breeding other mutant strains. "This study is taking the first step toward dissecting one pathway of DNA repair in plants," says Allen Smith, a Stanford University molecular biologist.

It may also help explain how plants are coping with the intensifying UV irradiation of Earth that accompanies the degradation of the planet's UV-absorbing ozone layer. "The ozone depletion has focused much interest on what increasing amounts of UV can do to crop plants and to the ecosystem in general. So it is time that we begin to understand how plants protect themselves from UV radiation," says Smith.

To create a plant impaired in its ability to repair DNA, Britt's group developed a "neat and clever technique," says Smith. The researchers treated seeds of A. thaliana with a chemical mutagen and grew them into stable lines. They placed seeds from these plants on round, vertical plates and allowed roots to emerge and grow downward along the plate for three days.

Next came the task of finding the needle in the haysack -- spotting a UV-sensitive individual among the thousands of sprouting rootlets. To do that, the geneticists irradiated the baby roots with a short but intense dose of UV-B, strong enough to "give us a good sunburn0 in 30 seconds," Britt says. Then they rolled the plates 90 degrees. Wild-type roots continued to grow and, in response to gravity, took a sharp turn downward again. In contrast, the roots of the UV-sensitive mutants were no longer able to grow after the UV pulse. Thus, by never forming the right angle, they betrayed their genetic defect.

The mutant cress plants tolerate very little UV radiation, the researchers report. Although they develop normally in the absence of UV, the plants wither and turn black with dying tissue when exposed to even a small dose of UV, such as that emitted by the fluorescent lamps commonly used in offices. This sensitivity shows how crucial the DNA-repair gene is for the wild type to survive, Britt says.

Studies of this type might eventually help breed more UV-resistant crop plants, but Britt says she is more interested now in learning how DNA repair mechanisms can cause mutations in plants. "There are several DNA repair pathways," she says, "some that make no mistakes and some that are prone to make errors. These errors end up as DNA mutations in plants and thus may contribute to plant evolution."

The gene described in the current study belongs to the error-free group, but having identified it makes it easier to find other genes involved in DNA repair, including the error-prone ones. By mutating the mutant strain, Britt plans to knock out additonal DNA repair genes and isolate strains that are even more vulnerable to UV. "This way we can gradually flesh out the repair pathways at work in higher plants," she says.