HFR1, a putative bHLH transcription factor, mediates both phytochrome A and cryptochrome signalling

Plants are very sensitive to their light environment. They use cryptochromes and phytochromes to scan the light spectrum. Those two families of photoreceptors mediate a number of similar physiological responses. The putative bHLH (basic Helix Loop Helix) transcription factor long hypocotyl in far-red (HFR1) is important for a subset of phytochrome A (phyA)-mediated light responses. Interestingly, hfr1 alleles also have reduced de-etiolation responses, including hypocotyl growth, cotyledon opening and anthocyanin accumulation, when grown in blue light. This phenotype is particularly apparent under high fluence rates. The analysis of double mutants between hfr1 and different blue light photoreceptor mutants demonstrates that, in addition to its role in phyA signalling, HFR1 is a component of cryptochrome 1 (cry1)-mediated light signalling. Moreover, HFR1 mRNA levels are high both in blue and in far-red light but low in red light. These results identify HFR1 as a positively acting component of cry1 signalling and indicate that HFR1 integrates light signals from both phyA and cry1.