THE PLANT CELL, Vol 6, Issue 5 613-628, Copyright © 1994 by American Society of Plant Biologists
Regulatory Hierarchy of Photomorphogenic Loci: Allele-Specific and Light-Dependent Interaction between the HY5 and COP1 Loci
L. H. Ang and X. W. Deng
Department of Biology, Osborn Memorial Laboratories, OML 301, Yale University, 165 Prospect Street, New Haven, Connecticut 06511
Previous studies suggested that the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1)
gene product represses photomorphogenic development in darkness and that
light signals reverse this action. In this report, we used genetic analysis
to investigate the regulatory hierarchical relationship of COP1 and the
loci encoding the photoreceptors and other signaling components. Our
results showed that cop1 mutations are epistatic to the long hypocotyl
mutations hy1, hy2, hy3, and hy4, suggesting that COP1 acts downstream of
the phytochromes and a blue light receptor. Although epistasis of a
putative null cop1-5 mutation over a hy5 mutation implied that COP1 acts
downstream of HY5, the same hy5 mutation can suppress the dark
photomorphogenic phenotypes (including hypocotyl elongation and cotyledon
cellular differentiation) of the weak cop1-6 mutation. This, and other
allele-specific interactions between COP1 and HY5, may suggest direct
physical contact of their gene products. In addition, the synthetic
lethality of the weak deetiolated1 (det1) and cop1 mutations and the fact
that the cop1-6 mutation is epistatic to the det1-1 mutation with respect
to light control of seed germination and dark-adaptative gene expression
suggested that DET1 and COP1 may act in the same pathway, with COP1 being
downstream. These results, together with previous epistasis studies,
support models in which light signals, once perceived by different
photoreceptors, converge downstream and act through a common cascade(s) of
regulatory steps, as defined by DET1, HY5, COP1, and likely others, to
derepress photomorphogenic development.
