HFR1 encodes an atypical bHLH protein that acts in phytochrome A signal transduction

Phytochromes are informational photoreceptors through which plants adapt th eir growth and development to prevailing light conditions. These adaptation s are effected primarily through phytochrome regulation of gene expression by mechanisms that remain unclear. We describe a new mutant, hfr1 (long hyp ocotyl in far-red), that exhibits a reduction in seedling responsiveness sp ecifically to continuous far-red light (FRc), thereby suggesting a locus li kely to be involved in phytochrome A (phyA) signal transduction. Using an i nsertionally tagged allele, we cloned the HFR1 gene and subsequently confir med its identity with additional alleles derived from a directed genetic sc reen. HFR1 encodes a nuclear protein with strong similarity to the bHLH fam ily of DNA-binding proteins but with an atypical basic region. In contrast to PIF3, a related bHLH protein previously shown to bind phyB, HFR1 did not bind either phyA or B. However, HFR1 did bind PIF3, suggesting heterodimer ization, and both the HFR1/PIF3 complex and PIF3 homodimer bound preferenti ally to the Pfr form of both phytochromes. Thus, HFR1 may function to modul ate phyA signaling via heterodimerization with PIF3. HFR1 mRNA is 30-fold m ore abundant in FRc than in continuous red light, suggesting a potential me chanistic basis for the specificity of HFR1 to phyA signaling.