A plant's ability to survive long days in the sun may rely in part on its ability to repair the resulting genetic damage after dark, according to a new report by University of California, Davis, researcher Anne Britt. Plants need sunlight for photosynthesis -- the energy-producing chemical process that supports most life on earth -- but ultraviolet rays lurking in the light, responsible for certain skin cancers in humans, pose a danger to plants as well. Scientists have known that certain beneficial solar rays help plants repair the most common kind of DNA damage during the daytime. Britt's study suggests that plants need another kind of repair, which can take place in light or darkness. In a research report in the Sept. 17 issue of the journal Science, Britt and her colleagues identify a gene critical to this "dark" repair process. They isolated for the first time a UV-sensitive mutant strain of the plant Arabidopsis, or mouse-eared cress, valued for its genetic research applications. Missing the genetic function necessary for dark repair, these mutant plants irrevocably suffered after laboratory exposure to only a small amount of ultraviolet B light. According to Britt, the mutants were unable to grow, apparently because they lost an efficient and effective type of genetic repair service. Meanwhile, the mutant plants protected from these UV rays grew to a typically healthy size and shape. "This paper shows that one of the ways plants can resist the toxic effects of UV light is by getting rid of the photoproduct through this 'dark' repair pathway," says Britt, an assistant research geneticist in plant biology at UC Davis. Searching for the detailed story of how mutations occur in plants, Britt is especially interested in finding other DNA repair pathways that are sloppier in fixing genetic damage than the efficient light-independent repair pathway. Poor genetic repair may lead to new and different plant species, she says. The new technique of isolating mutants that are missing specific DNA repair pathways is expected to help other researchers shed light on other unknown DNA repair processes in plants, most of which are still a mystery to scientists. "Paradoxically, DNA repair processes are important because they are a source of genetic diversity," Britt says. "The damage itself is not a mutation; a mutation happens when the damage is repaired inaccurately, causing a change in the genetic code." Britt was assisted by UC Davis graduate student Ju-Juin Chen and undergraduate Dennis Wykoff, now a graduate student at Stanford University. Dennis Mitchell, a researcher at The University of Texas M. D. Anderson Cancer Center, evaluated the DNA damage products and rates of repair in mutant and regular plants.