Key regulator of far red / red (FR/R) spectrum-specific responses essential for the adaption to changing light conditions (e.g. de-etiolation), essentially by regulating PHYA shuttling from the cytoplasm to the nucleus and by directly regulating the expression of some target genes, depending on light conditions and phosphorylation status (PubMed:22582101). Binds chromatin at target genes promoters, especially in FR light conditions (PubMed:25071219). Can activate transcription of different genes, some being in a phytochrome A (PHYA)-dependent and other in a PHYA-independent manners (PubMed:15469493, PubMed:11726703, PubMed:25071219). Controls specific aspects of plant development, such as the inhibition of seed germination under FR during salt stress (PubMed:25071219). Essential for light-regulated PHYA nuclear accumulation and subsequent PHYA phototropic signaling processes involved in photomorphogenesis (PubMed:17566111, PubMed:21969386, PubMed:25071219, PubMed:22374392, PubMed:19482971). Mediates the association of PHYA with HFR1 and LAF1 in the nucleus in response to FR conditions (PubMed:19482971). PHYA-specific signal transducer in response to continuous FR lights (PubMed:15469493, PubMed:16045472, PubMed:19482971, PubMed:11711433, PubMed:11726703, PubMed:8364355). Contributes to inhibition of hypocotyl elongation in continuous blue light (B) (PubMed:16045472). Homodimer and heterodimer with FHL (PubMed:16045472). Interacts with underphosphorylated PHYA, especially upon far-red (FR) light illumination (PubMed:19208901, PubMed:18722184, PubMed:21884939, PubMed:19482971). Binds to LAF1 and HFR1. Forms PHYA/FHY1/HFR1 complex in darkness but dissociates from PHYA and HFR1 in response to continuous FR light (FRc) (PubMed:19482971). In hypocotyls, mostly present in nucleus of dark (cD) grown plants, but also detected at low levels in cytoplasm of cD and continuous far red (cFR) grown plants. Disappearance of the nuclear pool after exposition of etiolated plants to the light is faster in red (R), blue (B) and white light (WL) than in FR (PubMed:11711433). Shuttles reversibly from cytoplasm to nuclear bodies when dephosphorylated in FR but stay in the cytoplasm when phosphorylated by PHYA in other light conditions (PubMed:18980642, PubMed:22582101, PubMed:21884939). Expressed in hypocotyl cells of etiolated plants. Accumulates in young seedlings with a peak three days after seed germination (PubMed:11726703). Mostly abundant in young seedlings grown in darkness, but quickly down-regulated during further seedling development and by light exposure (at protein level) (PubMed:16244150). Activated by FHY3 and FAR1 under far-red light (FR); HY5 prevents this activation (PubMed:16045472, PubMed:21097709, PubMed:18033885). Down-regulated by FR, red (R) and blue (B) lights (PubMed:18033885, PubMed:16045472). Accumulates in dark (D)-grown but not in R and FR-grown hypocotyl cells. Repressed in etiolated plants transferred to FR conditions in a FHY3-dependent manner (PubMed:15469493, PubMed:11711433). Inhibited by light (PubMed:11726703). Repressed by PHYA-mediated phosphorylation in R illumination; the phosphorylated form is a substrate for ubiquitin/proteasome-mediated degradation (PubMed:22582101, PubMed:19208901). PHYA-dependent down-regulation by light is largely at post-transcriptional level, primarily mediated through the 26S proteasome-dependent protein degradation (PubMed:16244150). Inactivated by rapid reversible PHYA-mediated phosphorylation at Ser-39 and Thr-61 in red light (R), thus inhibiting PHYA signaling in a negative feedback loop; this ensures the seedling deetiolation process in response to a R-enriched light condition (PubMed:19208901, PubMed:22582101). Subsequent exposure to far-red light (FR) after the R conditions leads to dephosphorylation (PubMed:19208901). The phosphorylated form is cytoplasmic only and unable to bind to chromatin at direct target genes whereas the unphosphorylated form can shuttle from cytoplasm to nucleus (PubMed:22582101). Partially blind to far-red (FR) (PubMed:15469493, PubMed:11711433, PubMed:11726703, PubMed:19482971, PubMed:16045472). Impaired inhibition of hypocotyl elongation and cotyledons expansion under continuous FR light conditions (PubMed:11711433, PubMed:11726703, PubMed:8364355, PubMed:22582101, PubMed:19482971, PubMed:16045472). Absence of FR-induced killing response (PubMed:11726703). Increased seed germination rate in salt stress conditions (PubMed:25071219). In plants lacking FHY1 and FHL, altered phototropism (e.g. phototropic bending) associated with abnormal consitutive cytosolic localization of PHYA (PubMed:22374392, PubMed:17566111). In the double mutant fhl fhy1 several PHYA-dependent phototropic responses are altered (e.g. hypocotyl elongation and cotyledon opening under high-irradiance conditions and seed germination under very-low-fluence conditions), but not for some PHYA-dependent responses such as the abrogation of negative gravitropism in blue light and red-enhanced phototropism (PubMed:17566111). Hyposensitivity to blue light (B) (PubMed:16045472). Belongs to the FHY1 protein family. Extended N-terminus.