Key Points
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The phytochromes comprise a small family of sensory photoreceptors (with five members in Arabidopsis, designated phyAâphyE), which monitor informational light signals in the environment and modulate appropriate growth and development responses through induced changes in gene expression.
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Individual family members have different, albeit frequently partially overlapping, photosensory and/or physiological functional roles in regulating plant responses to light.
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Five principal developments in recent years have substantially influenced current concepts of the cellular, molecular and biochemical mechanisms by which the phytochromes perceive and transduce light signals to photoresponsive nuclear genes:
â light-induced nuclear translocation of phytochrome molecules;
â direct interaction of phytochrome molecules with a DNA-bound transcription factor;
â transcription-factor genes are early targets of phytochrome signalling;
â phytochrome-associated protein kinase activity;
â post-translational regulation of signalling through targeted degradation of a key transcriptional regulator.
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Evidence so far indicates that the phytochromes might use at least two pathways to signal to photoresponsive genes: a primary, early-response pathway, which involves transcriptional regulation by direct targeting of light signals to responsive promoters; and a secondary, longer-term pathway, which involves abrogation of targeted proteolytic degradation of a key transcription factor.
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Future research efforts can be expected to exploit the unprecedented power that is offered by the combination of mutants and microarrays for the dissection of photosensory signalling and transcriptional networks.
Abstract
Light is life for plants. To continuously assess and adapt to fluctuations in the quality and quantity of this essential commodity, plants deploy sensory photoreceptors, including the phytochromes. Having captured an incoming photon, the activated phytochrome molecule must relay this information to nuclear genes that are poised to respond by directing appropriate adjustments in growth and development. Defining the intricate intracellular signalling networks through which this sensory information is transduced is an area of intense research activity.
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Acknowledgements
I thank the members of my laboratory for stimulating discussions; E. Huq and M. Hudson for helpful comments on the manuscript; E. Schäfer and S. Kircher for the images in Figure 3; J. Tepperman for preparing the figures; and R. Wells for manuscript preparation and editing. Research is supported by grants from: National Institutes of Health, United States Department of Energy, Torrey Mesa Research Institute, Syngenta Research Technology and United States Department of Agriculture.
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Glossary
- PHOTOMORPHOGENESIS
-
Changes in growth and development in response to light. These occur throughout the plant life cycle, from seed germination, and seedling de-etiolation, through vegetative architectural adaptations and flower induction, and are observable at all levels of organization, from visible phenotype to gene expression.
- CHROMOPROTEIN
-
A molecule that consists of two moieties: a polypeptide and a chromophore, the latter of which absorbs light of specific wavelengths and causes the molecule to be coloured.
- Pr
-
The biologically inactive form of the phytochrome molecule that absorbs red light (P for phytochrome; r for red-light-absorbing).
- Pfr
-
The biologically active form of the phytochrome molecule that absorbs far-red light (P for phytochrome; fr for far-red-absorbing).
- HYPOCOTYL
-
The equivalent of the stem in a young seedling. The vegetative organ between the root and the cotyledons, which is responsible for initial upward elongation growth.
- COTYLEDONS
-
Referred to as 'seed leaves', these organs initially remain small and yellow in colour, and provide storage reserves for seedling growth in darkness (heterotrophic growth), but rapidly expand and turn green in response to light as chloroplasts develop and photosynthesis begins (autotrophic growth).
- F-BOX PROTEIN
-
A class of protein that contains a conserved sequence motif referred to as an F-box domain. The particular F-box variant in a given SCF-type E3-ubiquitin-ligase complex confers target-protein substrate specificity on the SCF complex.
- SCF-TYPE E3 UBIQUITIN LIGASE
-
Enzyme complex that catalyses the ubiquitylation of target proteins. The complex contains three main components, termed Skp1, a member of the cullin family (Cul1) and F-box proteins.
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Quail, P. Phytochrome photosensory signalling networks. Nat Rev Mol Cell Biol 3, 85â93 (2002). https://doi.org/10.1038/nrm728
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DOI: https://doi.org/10.1038/nrm728
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