Sensory photobiology is an interesting area of how organisms perceive and react to light. It is an essential discipline for plants, which utilize light not only for photosynthesis but also to regulate growth, development, and behaviour. This blog post will explain the structure, function, and mechanisms of the major classes of photoreceptors: phytochromes, cryptochromes, and phototropins. The discussion also includes their role in stomatal movement, photoperiodism, and biological clocks.
Phytochromes
Phytochromes are red and far-red light-sensing photoreceptors. These kinds of proteins have a significant role in the regulation of plant development processes and growth, including seed germination, stem elongation, and flowering.
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| Attribution: Cindyhsieh, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons |
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| Attribution- Betsy True, Public domain, via Wikimedia Commons |
Structure
Phytochromes are dimeric proteins composed of identical subunits. There is a chromophore in each of the two subunits, which is a pigment molecule capable of absorbing light. Structurewise, linear tetrapyrrole is covalently linked to the protein for each apoprotein.
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| Attribution: Audrius Meskauskas, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons |
Function and Mechanism
Two interconvertible forms of phytochrome are known: Pr (absorbing red) and Pfr (absorbing far-red). Absorption of red light converts Pr to Pfr, which is bioactive. On the other hand, Pfr is converted back to Pr by absorption of far-red light or darkness. This process of photoconversion controls the induction of expression of genes involved in the light response.
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| Attribution: Bengt A. Lüers - BiGBeN_87_de, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons |
Critical Processes Regulated by Phytochromes:
Seed Germination: Phytochromes induce seed germination by initiating the breakdown of ABA (abscisic acid) and synthesis of GA (gibberellins), which will trigger growth.
Shade Avoidance: Phytochromes sense the light quality, promoting elongation growth within dense vegetation where filters out red light and far-red light prevails to outcompete neighbouring plants.
Flowering: Phytochromes control the expression of flowering genes and so regulate the time to flower as a response to day length (photoperiod).
Cryptochromes
Cryptochromes are photoreceptors for blue/UV-A light. They play an essential part in the regulation of circadian rhythms and processes of development, both in plants and animals.
Structure
Cryptochromes are flavoproteins, and their chromophore is a flavin adenine dinucleotide (FAD). They further contain a pterin cofactor, which is helpful in the absorption of light.
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| Fig. FAD |
The chromophores absorb blue light and, because of its absorption, bring about a change in conformation that allows it to interact with other proteins, thus influencing gene expression.
Key Processes Regulated by Cryptochromes:
Circadian Rhythms: Cryptochromes act at the core of the plant's circadian clock. They help in the synchronization of biological activities with the day-night cycles.
Growth and Development: Stem elongation, leaf expansion, and flowering time—partly in conjunction with phytochromes.
Phototropins
These are blue light receptors involved in mediating plant phototropism which is growth towards or away from light.
Structure
Phototropins are serine/ threonine kinases containing two LOV (Light, Oxygen, or Voltage) domains that bind flavin mononucleotide (FMN) as the chromophore.
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| Fig.- LOV2 domain of phototropin-2 protein from Arabidopsis thaliana |
Function and Mechanism
After absorbing blue light, phototropins undergo autophosphorylation. The former provides them resistance to photodestruction.
Key Processes Regulated by Phototropins:
Phototropism: Phototropins orient plant growth towards light by causing differential cell elongation on the side of the plant that remains in the shadow.
Stomatal Opening: Phototropins have a significant role in regulating the stomatal aperture to optimize gaseous exchange and water use efficiency. This is achieved by controlling the opening and closing of stomata appropriate to the blue light condition.
Chloroplast Movement: The chloroplast movement within the plant cell, controlled by phototropins, ensures that the light absorption is optimized while at the same time protecting the individual from over-excitation due to light.
Stomatal Movement: Stomata are tiny pores situated on the surface of the leaf, which aid the leaf in losing water vapour and gaseous exchange. Phototropins have been recognized to be implicated in quite a number of the effects on stomatal movement, mainly through light perception. In these blue light responses, phototropins activate proton pumps in the guard cells that then take up potassium and water for stomatal opening. The actions by phytochromes are, however, quite indirect at the level of stomata as they affect general plant physiology and water status.
Photoperiodism and Biological Clocks
Photoperiodism is the physiological response of organisms to the length of day or night, whereas biological clocks are the internal timing mechanisms that control these responses.
Photoperiodism Flowering
Plants use phytochromes and cryptochromes to measure day length, determining the right time for flowering. Long-day plants flower when days are long, and short-day plants flower when nights are long. Photoperiodism also controls other seasonal behaviours like dormancy and leaf senescence. Cryptochromes and phytochromes regulate the circadian rhythms of physiological processes according to the period of 24 hours, distinguishing between day and night. The regulation of such physiological activities helps properly synchronize with various metabolic activities, hormonal responses, and gene expressions.
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| Attribution: EarthDragon, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons |
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| Attribution: EarthDragon, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons |
Conclusion
An understanding of photobiology in sensory perception and the role of phytochromes, cryptochromes, and phototropins in plants provides deep insights into how plants interact with their environment. These photoreceptors not only mediate growth, development, and stress responses but also accommodate plant needs for acclimation to changing light conditions to maximize survival and productivity. Research in this field is being carried out further to illuminate the delicate mechanisms causing plant behaviour to make way for future applications in agriculture and horticulture, increasing crop yield and resilience.