HFR1

HFR1, or LONG HYPOCOTYL IN FAR-RED 1, is a gene in Arabidopsis thaliana that encodes a basic helix-loop-helix (bHLH) transcription factor. The HFR1 protein localizes to the nucleus and participates in the plant’s light-signaling network, with a particular role in responses to shade and far-red light environments.

In the shade-avoidance response, HFR1 interacts with other transcription factors in the PHYTOCHROME INTERACTING FACTOR (PIF) family to modulate growth. Genetic analyses show that loss-of-function hfr1 mutants exhibit elongated hypocotyls when grown in far-red light, indicating that HFR1 normally acts to restrain elongation under these conditions. By contrast, overexpression of HFR1 can dampen the elongation response and alter the expression of several light-regulated genes.

Regulation of HFR1 is dependent on light quality. Active phytochromes, particularly phyB, promote the degradation of HFR1 under red light, while far-red or low red to far-red conditions stabilize the protein, enabling it to influence transcriptional programs. HFR1 activity is also integrated with the plant ubiquitin-proteasome system and may function alongside other light-signaling components, such as COP1-SPA complexes, to shape growth responses through interactions with PIFs.

HFR1 homologs exist in other plant species, where related bHLH factors contribute to light-regulated development. The gene was identified in studies of shade avoidance in Arabidopsis and has since been incorporated into models of PHY-PIF signaling networks that govern seedling de-etiolation and morphological adaptation to light quality.