Evidence That Heat and Ultraviolet Radiation Activate a Common Stress-Response Program in Plants That 1s Altered in the uvh6 Mutant of Arabidopsis thaliana’

l h e uvh6 mutant of Arabidopsis was previously isolated in a screen for increased sensitivity to ultraviolet (UV) radiation. uvh6 mutant plants were killed by incubation at 37°C for 4 d, a treatment not lethal to wild-type plants. Furthermore, under permissive conditions, uvh6 plants were yellow-green with an approximately onethird lower chlorophyll content. Cenetic analysis of the uvh6 mutant strongly suggested that all three mutant phenotypes were due to mutation at the same genetic locus. To understand UVH6 function more fully, the response of wild-type plants to growth at elevated temperatures and exposure to UV radiation was analyzed. Wild-type plants grown at 30°C were as UV-hypersensitive and yellow-green as uvh6 mutant plants grown at 24°C. Mutant uvh6 plants induced heat-shock protein HSP21 at a lower threshold temperature than wild-type plants, indicating that the uvh6 mutant was exhibiting signs of heat stress at a 4 to 5°C lower temperature than wild-type plants. We propose that UV damage and heat induce a common stress response in plants that leads to tissue death and reduced chloroplast function, and that the UVH6 product is a negative regulator of this response.