Overexpression of a mutant basic helix-loop-helix protein HFR1, HFR1-ΔN105, activates a branch pathway of light signaling in Arabidopsis

The HFR1, a basic helix-loop-helix protein, is required for a subset of phytochrome A-mediated photoresponses in Arabidopsis. Here, we show that overexpression of the HFR1-DeltaN105 mutant, which lacks the N-terminal 105 amino acids, confers exaggerated photoresponses even in darkness. Physiological analysis implied that overexpression of HFR1-DeltaN105 activated constitutively a branch pathway of light signaling that mediates a subset of photomorphogenic responses, including germination, de-etiolation, gravitropic hypocotyl growth, blocking of greening, and expression of some light-regulated genes such as CAB, DRT112, PSAE, PSBL, PORA, and XTR7, without affecting the light-responsiveness of anthocyanin accumulation and expression of other light-regulated genes such as CHS and PSBS. Although the end-of-day far-red light response and petiole elongation were suppressed in the HFR1-DeltaN105-overexpressing plants, flowering time was not affected by HFR1-DeltaN105. In addition, the HFR1-DeltaN105-overexpressing plants showed hypersensitive photoresponses in the inhibition of hypocotyl elongation, dependently on phytochrome A, FHY1, and FHY3 under FR light or phyB under R light, respectively. Moreover, our double mutant analysis suggested that the hypersensitive photoresponse is due to functional cooperation between HFR1-DeltaN105 and other light-signaling components including HY5, a basic leucine zipper protein. Taken together, our results of gain-of-function approach with HFR1-DeltaN105 suggest the existence of a complex and important basic helix-loop-helix protein-mediated transcriptional network controlling a branch pathway of light signaling and provide a useful framework for further genetic dissection of light-signaling network in Arabidopsis.