High impact information on HFR1

• Taken together, our results show that HFR1 is ubiquitinated by COP1 E3 ligase and marked for post-translational degradation during photomorphogenesis [1].
• We describe a new mutant, hfr1 (long hypocotyl in far-red), that exhibits a reduction in seedling responsiveness specifically to continuous far-red light (FRc), thereby suggesting a locus likely to be involved in phytochrome A (phyA) signal transduction [2].
• Thus, HFR1 may function to modulate phyA signaling via heterodimerization with PIF3 [2].
• HFR1 encodes an atypical bHLH protein that acts in phytochrome A signal transduction [2].
• Under blue light, shb1 suppressed LONG HYPOCOTYL IN FAR-RED LIGHT1 (HFR1) expression and showed several deetiolation phenotypes similar to hfr1-201 [3].

Biological context of HFR1

• Here we show that overexpression of Arabidopsis SPA1 results in a hyperetiolation phenotype and reduced accumulation of HY5 and HFR1 [4].
• The degradation of HFR1, a putative bHLH class transcription factor involved in light signaling, is regulated by phosphorylation and requires COP1 [5].
• The Arabidopsis AT412 genotyping array and 32 F(2) plants were used to map the rsf1 mutation close to the top of chromosome 1 to an interval of approximately 500 kb [6].
• Molecular cloning and characterization of the REP1 gene revealed that it encodes a light-inducible, putative transcription factor containing the basic helix-loop-helix motif [7].

Anatomical context of HFR1

• In Arabidopsis, phytochrome A (phyA) is the major photoreceptor both for high irradiance responses to far-red light and broad spectrum very low fluence responses, but little is known of its signaling pathway(s). rsf1 was isolated as a recessive mutant with reduced sensitivity to far-red inhibition of hypocotyl elongation [8].

Physical interactions of HFR1

• We show that SPA1 physically interacts with HFR1 in a yeast two-hybrid assay and an in vitro co-immunoprecipitation assay [9].

Enzymatic interactions of HFR1

• Both HFR1 and HFR1(DeltaN) can be ubiquitinated by COP1, although with different efficiencies [1].

Regulatory relationships of HFR1

• In transgenic plants, HFR1 levels are significantly elevated upon induced expression of a dominant-negative COP1 mutant that interferes with endogenous COP1 E3 activity [1].
• We also show that phosphorylation of HFR1 is promoted by light and that a predicted CKII site, Ser(122), represents a major phosphorylation site of HFR1 under both dark and light conditions [10].

Other interactions of HFR1

• HFR1 was shown to be necessary for a subset of cop1-triggered photomorphogenic phenotypes in the dark, including inhibition of hypocotyl elongation, gravitropic hypocotyl growth, and expression of the light-inducible genes CAB and RBCS [11].
• Double mutant analysis suggests that HFR1 mediates phyA-dependent inhibition of hypocotyl elongation independently of HY5 [11].
• HFR1, a basic helix-loop-helix protein, is known to be required for a subset of phytochrome A (phyA)-dependent photoresponses [11].

Analytical, diagnostic and therapeutic context of HFR1

• Cloning of the Arabidopsis RSF1 gene by using a mapping strategy based on high-density DNA arrays and denaturing high-performance liquid chromatography [6].
• We isolated the gene responsible for the recessive mutation rsf1 (for reduced sensitivity to far-red light) in the Arabidopsis Columbia accession by using classical genetic analysis and two recently developed technologies: genotyping high-density oligonucleotide DNA array and denaturing high-performance liquid chromatography (DHPLC) [6].

References