Beyond repression of photomorphogenesis: role switching of COP/DET/FUS in light signaling
Introduction
In addition to functioning as a source of energy for photosynthesis, light also acts as an informational stimulus that triggers photomorphogenic development in plants. The physical manifestation of photomorphogenesis includes the development of short hypocotyls, open and expanded cotyledons, and green chloroplasts. Arabidopsis thaliana seedlings have long been employed as a model system for genetic and molecular studies of photomorphogenesis in higher plants. Over the course of photomorphogenesis, light signals are perceived by multiple photoreceptors, and interpreted by a series of signaling intermediate factors for downstream transduction. Due to their regulatory involvement in the degradation of photomorphogenesis-promoting factors, CONSTITUTIVE PHOTOMORPHOGENIC/DE-ETIOLATED/FUSCA (COP/DET/FUS) proteins have been widely recognized as the central repressors of light signaling. Here, we provide an update of our current understanding of the COP1-centered COP/DET/FUS functions beyond the traditional photomorphogenesis triggered by far-red, red and blue light. Recent studies have also raised the novel concept that the COP10-DET1-DAMAGED DNA BINDING PROTEIN 1 (DDB1) (CDD) and COP9 signalosome (CSN) complexes behave as regulators of transcription. Owing to the space limitations of this piece, however, readers interested in the CDD and CSN complexes are encouraged to consult the following excellent reviews [1, 2, 3].
Section snippets
COP/DET/FUS: a historical remark
COP/DET/FUS are a group of pleiotropic genes that were first identified and characterized as central repressors of light-regulated development in Arabidopsis. Recessive mutations in COP/DET/FUS have been shown not only to result in constitutive photomorphogenesis in darkness [4, 5], but also to lead to a variety of developmental defects in plants and animals [1, 4, 5, 6]. Nine of the COP/DET/FUS loci were found to encode COP1, DET1, COP10, and CSN subunits 1–4, 7, and 8. All of these proteins
COP1: Janus shaped by the context
COP1 is a multifunctional RING E3 ubiquitin ligase that is found in both plants and animals. In addition to its essential role in seedling photomorphogenesis triggered by far-red, red, blue and ultraviolet-B (UV-B) light, Arabidopsis COP1 protein is also known to function in a variety of other developmental processes including organ development [24, 25, 26, 27, 28], responses to environmental stress [29, 30, 31], and crosstalk between light and phytohormone signaling pathways [32, 33, 34, 35] (
Light modulates COP1 activity
The light environments not only essentially determine the nature of COP1 actions, but also elaborately monitor the tuning of COP1 activity. Plants utilize several families of photoreceptors to sense a wide spectrum of light ranging from UV-B to far-red light (280–800 nm): phytochromes for far-red and red light, cryptochromes and phototropins for blue and UV-A light, and UVR8 for UV-B light. COP1 behaves as a signaling core that coordinates the transfer of light information from all the
Conclusions
While COP/DET/FUS's role in repressing photomorphogenesis via proteolysis has long been established, recent advances have shed light on additional functions of COP/DET/FUS proteins. Here our discussion focuses on COP1's functions in photomorphogenesis induced by various light signals. These studies have not only extended our understanding on COP/DET/FUS as central regulators of photomorphogenesis, but have also raised a number of challenging questions to be addressed in the future.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
We thank Abigail Coplin for her critical reading of the manuscript. Apologies to our colleagues whose work cannot be discussed because of space constraint. Relevant research in our laboratory is supported by the National Natural Science Foundation (31330048), the National Basic Research Program of China (2012CB910900), the National Institute of Health (GM47850), the Natural Science Foundation (MCB-0929100), the State Key Laboratory of Protein and Plant Gene Research and the Peking-Tsinghua
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