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Concept
Phytochrome
Summary
Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or Type II that are activated by red light. Recent advances have suggested that phytochromes also act as temperature sensors, as warmer temperatures enhance their de-activation. All of these factors contribute to the plant's ability to germinate.
Phytochromes control many aspects of plant development. They regulate the germination of seeds (photoblasty), the synthesis of chlorophyll, the elongation of seedlings, the size, shape and number and movement of leaves and the timing of flowering in adult plants. Phytochromes are widely expressed across many tissues and developmental stages.
Other plant photoreceptors include cryptochromes and phototropins, which respond to blue and ultraviolet-A light and UVR8, which is sensitive to ultraviolet-B light.
In addition to their roles in nature, the light-induced interaction between a plant phytochrome and phytochrome-interacting factor (PIF) was used in 2002 to control gene transcription in yeast. This was the first example of using photoproteins from another organism for controlling a biochemical pathway.
Phytochromes consist of a protein, covalently linked to a light-sensing bilin chromophore. The protein part comprises two identical chains (A and B). Each chain has a PAS domain, GAF domain and PHY domain. Domain arrangements in plant, bacterial and fungal phytochromes are comparable, insofar as the three N-terminal domains are always PAS, GAF and PHY domains. However C-terminal domains are more divergent. The PAS domain serves as a signal sensor and the GAF domain is responsible for binding to cGMP and also senses light signals. Together, these subunits form the phytochrome region, which regulates physiological changes in plants to changes in red and far red light conditions.
Official source
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