Genetic and phenotypic characterization of cop1 mutants of Arabidopsis thaliana

Ten Arabidopsis lines that carry recessive mutations in the cop1 (constitutively photomorphogenic) locus have been isolated. These lines define at least four different alleles. All of the mutant lines produce dark-grown seedlings that mimic wild-type seedlings grown in the light. The phenotype of the dark-grown mutant seedlings includes: short hypocotyls, open and enlarged cotyledons, accumulation of anthocyanin, cell-type differentiation and chloroplast-like plastid differentiation in cotyledons. Moreover, in more prolonged dark-growth periods the mutants exhibit true leaf development that parallels that in light-grown siblings. The four mutant alleles represent two types of mutations: three alleles (cop 1-1, cop 1-2, and cop 1-3) have severely affected phenotypes whereas one allele (cop 1-4) has a less severe phenotype. Compared to the severe alleles, the cop 1-4 mutant has slightly longer hypocotyls in dark-grown seedlings and does not accumulate abnormal levels of anthocyanin. The cop1–1/cop1-4 hybrid seedlings are intermediate in many physiological properties under both dark- and light-growth conditions, relative to the two parents. These results may suggest that the extent of residual cop1 gene activity in the mutants dictates the degree to which the aberrant plant phenotype is expressed. Analysis of plants carrying both cop1 and hy, a mutation that results in a deficiency of active phyto-chrome, suggests that the cop1 gene product acts downstream of phytochrome. The differentiation of chloroplasts in the roots of light-grown cop1 plants but not in wild-type plants suggests that the wild-type cop1 gene product also normally plays a role in suppressing chloroplast development in the roots of light-grown plants. To aid the eventual molecular cloning of the cop1 locus, its chromosomal location has been mapped and a molecular marker that is located about 1 centimorgan away from the cop1 locus obtained.