HFR1, a phytochrome A‐signalling component, acts in a separate pathway from HY5, downstream of COP1 in Arabidopsis thaliana

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Abstract
Summary: HFR1, a basic helix–loop–helix protein, is known to be required for a subset of phytochrome A (phyA)‐dependent photoresponses. To investigate the role of HFR1 in light signalling, we have examined the genetic interaction between HFR1 and HY5, a positive regulator of light signalling, and COP1, a repressor of photomorphogenesis. Double mutant analysis suggests that HFR1 mediates phyA‐dependent inhibition of hypocotyl elongation independently of HY5. 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. Phenotypic analysis of the triple mutant cop1hy5hfr1 indicated that both HFR1 and HY5 are required for cop1‐mediated photomorphogenic seedling development in darkness, consistent with their additive roles in phyA‐dependent signalling. Taken together, these results suggest that HFR1 might act downstream of COP1, in a separate pathway from HY5, to mediate photomorphogenesis in Arabidopsis.

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