UV‐B Action Spectrum for UVR8‐Mediated HY5 Transcript Accumulation in Arabidopsis

Abstract

Abstract Arabidopsis thalianaUV RESISTANCE LOCUS8 (UVR8) is a UV‐B‐specific signaling component that mediates low fluence, photomorphogenic responses to UV‐B. It is required for UV‐B‐induced expression of the gene encoding the ELONGATED HYPOCOTYL5 (HY5) transcription factor. HY5 is a key effector of responses mediated by UVR8. In mature leaf tissue, HY5 transcript accumulation occurred rapidly in response to a brief UV‐B treatment and no induction was observed in a uvr8 mutant over a broad range of UV wavelengths. In response to monochromatic light, maximal transcript accumulation occurred in wild‐type plants at wavelengths 280–300 nm. HY5 transcript accumulation showed reciprocity between the fluence rate and duration of UV‐B exposure, and on this basis conditions were chosen to generate an action spectrum for the UVR8 signaling pathway. Dose–response curves were produced for a range of UV wavelengths using 20 min exposure to UV and harvesting tissue 2 h after the start of illumination. Experiments using mutants defective in sinapate ester and flavonoid biosynthesis indicated that the presence of UV‐absorbing compounds did not affect the construction of an action spectrum under the conditions employed. The action spectrum for the induction of HY5 by the UVR8 pathway showed a main peak at 280 nm with a smaller peak at 300 nm. The data are discussed in relation to the proposed mechanisms of UV‐B photoreception.