Phytochrome A Null Mutants of Arabidopsis Display a Wild-Type Phenotype in White Light

Abstract

Phytochrome is a family of photoreceptors that regulates plant photomorphogenesis; the best-characterized member of this family is phytochrome A. Here, we report the identification of novel mutations at three Arabidopsis loci (fhy1, fhy2, and fhy3) that confer an elongated hypocotyl in far-red but not in white light. fhy2 mutants are phytochrome A deficient, have reduced or undetectable levels of PHYA transcripts, and contain structural alterations within the PHYA gene. When grown in white light, fhy2 mutants are morphologically indistinguishable from wild-type plants. Thus, phytochrome A appears to be dispensable in white light-grown Arabidopsis plants. fhy2 alleles confer partially dominant phenotypes in far-red light, suggesting that the relative abundance of phytochrome A can affect the extent of the far-red-mediated hypocotyl growth inhibition response. Plants homozygous for the recessive fhy1 and fhy3 mutations have normal levels of functional phytochrome A. The FHY1 and FHY3 gene products may be responsible for the transduction of the far-red light signal from phytochrome A to downstream processes involved in hypocotyl growth regulation.

Combinatorial interaction of light‐responsive elements plays a critical role in determining the response characteristics of light‐regulated promoters in Arabidopsis

“…The photoreceptor mutant alleles used were phyA-1 (Whitelam et al, 1993), phyB-B064 (Koornneef et al, 1980), and hy4-2.23N (Ahmad and Cashmore, 1993;Koornneef et al, 1980). The cop1-4 McNellis et al, 1994a) and det1-1 (Chory et al, 1989;Pepper et al, 1994) mutations used for this study have been described previously as indicated.…”

“…Additionally, a number of genetic loci, HY5, FHY1, FHY3, PRC1, RED1 and PEFs, have been reported that are involved in multiple or specific phytochrome-mediated signal transduction pathways (Ahmad and Cashmore, 1996;Desnos et al, 1996;Koornneef et al, 1980;Wagner et al, 1997;Whitelam et al, 1993). Moreover, genetic studies suggested that the pleiotropic COP/DET/FUS genes act as repressors of photomorphogenic development and light-induced gene expression, and that light signals from multiple photoreceptors inactivate the repressive action of these gene products (Chory, 1993;McNellis and Deng, 1995;Wei and Deng, 1996).…”

An Arabidopsis Mutant Hypersensitive to Red and Far-Red Light Signals

“…We found that the effect of psi2 on hypocotyl length reduction under far-red light, but not red light, was abolished by the fhy1 mutation (Table 3). The latter has been shown to specifically control signal transduction downstream of phyA (Whitelam et al, 1993;Barnes et al, 1996). The absence of PSI2 induces a slight increase in sensitivity to blue light in a fluence-dependent manner.…”

“…A double mutant was also constructed between psi2 and fhy1, a mutant that is impaired in a branch of the phytochrome A signaling pathway (Whitelam et al, 1993; Barnes et al, 1996). In the four cases, we isolated F 3 lines homozygous for the luciferase and the long-hypocotyl phenotype.…”

“…Mutants impaired in far-red light perception but not in phytochrome A levels ( fhy1 and fhy3 ; Whitelam et al, 1993) seem to lack functional proteins involved in regulating secondary branches of the phyA signal transduction pathway. In addition, the cue1 mutation of Arabidopsis (Li et al, 1995) has been implicated in the regulation of plastid development and light-responsive gene expression, and the DOC gene product is hypothesized to be a repressor of chlorophyll a / b binding protein ( CAB ) genes in the dark (Li et al,1994).…”