Regulatory hierarchy of photomorphogenic loci: allele-specific and light-dependent interaction between the HY5 and COP1 loci.

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Abstract
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.