uvhl, an Arabidopsis Mutant Hypersensitive to Ultraviolet Light and lonizing Radiation

A genetic screen for mutants of Arabidopsis that are hypersensitive to UV light was developed and used to isolate a new mutant designated uvhl. UV hypersensitivity in uvhl was due to a single recessive trait that is probably located on chromosome 3. Although isolated as hypersensitive to an acute exposure to UV-C light, uvhl was also hypersensitive to UV-B wavelengths, which are present in sunlight that reaches the earth’s surface. UV-B damage to both wild-type and uvhl plants could be significantly reduced by subsequent exposure of UV-irradiated plants to photoreactivating light, showing that photoreactivation of UV-B damage is important for plant viability and that uvhl plants are not defective in photoreactivation. A new assay for DNA damage, the Dral assay, was developed and used to show that exposure of wild-type and uvhl plants to a given dose of UV light induces the same amount of damage in chloroplast and nuclear DNA. Thus, uvhl is not defective in a UV protective mechanism. uvhl plants were also found to be hypersensitive to ionizing radiation. These results suggest that uvhl is defective in a repair or tolerance mechanism that normally provides plants with resistance to several types of DNA damage.