After germination, plant growth is characterized by a the urgent need for the seedling to reach the light. During this period of growth, the seedling is throwing all of its resources into finding light. This kind of development, known as skotomorphogenesis, is characterized foremost by rapid elongation of the shoot, which is coupled with minimal elongation or further development of the root system. The shoot elongation that occurs during this period is strongly gravitropic, as this is one of the few cues available to the young seedling for orienting its growth. In dicot species, the energy-rich cotyledons are protected throughout this process of rapid stem elongation by the formation of an apical hook that tucks the cotyledons downward. Finally, dark-driven development is characterized by the lack of pigment biosynthesis, due in large part to the requirement for light as a catalyst in the production of chlorophyll. The seedling that develops in the dark is said to be etiolated.
Upon exposure to light for the first time, a number of changes occur to the seedling. The most obvious change is in the rate of stem elongation, which immediately slows and gives way to radial expansion. This is a very rapid response, occuring within seconds of the seedling sensing blue light. Almost concurrently with this, the primary root begins to elongate and will go on to begin producing lateral roots over the ensuing days. No longer needed for protection, the apical hook straightens and the cotyledons begin expanding and becoming green due to the synthesis of chlorophyll and other pigments required for photosynthesis.
The activation of such dramatic change in the transition from dark- to light-grown development requires many factors including several photoreceptors and many downstream signaling components. The rapid inhibition of stem elongation is mediated by a blue-light photoreceptor called cry1 (for cryptochrome). Many of the other responses associated with light are mediated by the phytochromes.
To identify other signal components of the photomorphogenesis pathway, mutant screens have been employed. One such screen was carried out to identify mutants that developed as though they had been exposed to light even though they hadn’t. These mutants were designated the constitutive photomorphogenesis (cop) mutants. The cop1 mutant was shown to have a lesion in a gene encoding an E3 ubiquitin ligase that targets transcription factors for degradation. These transcription factors regulate light-dependent genes, and by targeting them for degradation, COP1 acts as a negative regulator of photomorphogenesis. Many of the other COP genes encode components of a signalosome involved in regulating the localization of COP1 to either the nucleus or cytoplasm.