HYPOSENSITIVE TO LIGHT, an Alpha/Beta Fold Protein, Acts Downstream of ELONGATED HYPOCOTYL 5 to Regulate Seedling De-Etiolation

Ambient light has profound effects on early seedling de-etiolation through red and far-red light-absorbing phytochromes and blue and UV-A light-absorbing cryptochromes. Subsequent integration of various light signal transduction pathways leads to changes in gene expression and morphogenic responses. Here, we report the isolation of a new Arabidopsis light-signaling component, HYPOSENSITIVE TO LIGHT or HTL. Both htl-1 and htl-2 alleles displayed a long hypocotyl phenotype under red, far-red, and blue light, whereas overexpression of HTL caused a short hypocotyl phenotype under similar light conditions. The mutants also showed other photomorphogenic defects such as elongated petioles, retarded cotyledon and leaf expansion, reduced accumulation of chlorophyll and anthocyanin pigments, and attenuated expression of light-responsive CHLOROPHYLL A/B BINDING PROTEIN 3 and CHALCONE SYNTHASE genes. HTL belongs to an alpha/beta fold protein family and is localized strongly in the nucleus and weakly in the cytosol. The expression of HTL was strongly induced by light of various wavelengths and this light induction was impaired in elongated hypocotyl 5. HY5 directly bound to both a C/G-box and a G-box in the HTL promoter but with a greater affinity toward the C/G-box. HTL, therefore, represents a new signaling step downstream of HY5 in phy- and cry-mediated de-etiolation responses.