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Photomorphogenesis is a branch of developmental biology in which the growth and development of plants are affected by the light they are exposed to. The process enables plants to make better use of light and space.
- Plants can sense the time of day and year by using various wavelengths of sunlight.
- Thus, we can say that plants can use light in order to track time.
- The sensing of light is important to plants as it is important for competition and survival in tough environments.
- Theophrastus of Eresus was the first one to research about photomorphogenesis in 371 to 287 BC.
- Chromophore is a type of photoreceptor that is composed of protein covalently bonded with light-absorbing pigment.
Key Terms: Photomorphogenesis, Stages of Photomorphogenis, Types of Photoreceptors, Blue-light responses, Cryptochrome, Phototropins, Zeaxanthi, UVR8
What is Photomorphogenesis?
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Photomorphogenesis is the term used to describe how we can use light as a source of illumination for plants. Light is an important environmental factor that is responsible for plant growth and development.
- Plants use light in various ways, which includes their ability to photosynthesise low-molecular-weight sugars using CO₂ and water.
- The research on photomorphogenesis is based on different plant studies, which involve the following kingdoms: Fungi, Monera, and Protista.
- The process makes use of tightly frequency-controlled light to grow the plants.
- Seed germination, seedling development, and phototropism are three stages of photomorphogenesis.
- We can restrict the photomorphogenic effect of light to the UV-A and UV-B portions of the electromagnetic spectrum.
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Stages of Photomorphogenesis
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The two critical stages of photomorphogenesis, according to Hans Mohr (1983), are:
- Pattern specification is the process by which plant cells and tissues develop a specific ability or competence to respond to light at different stages of development.
- The photo-response occurs during the pattern realization stage of photomorphogenesis.
Stages of Photomorphogenesis
There are two kinds of plant responses to light signals.
- Phytochrome Mediated Photoresponses
- Blue-light Responses or Cryptochrome Mediated Photo-responses.
Phytochrome Mediated Photoresponses
Phytochrome, a proteinaceous pigment that acts as a photoreceptor and absorbs red and far-red light, mediates several photomorphogenic responses in plants.
- Blue light can also be absorbed by it.
- Based on the amount of light absorbed, the phytochrome-mediated response can be divided into three categories:
Very Low Fluence Responses (VLFR)
Very Low Fluence Responses (VLFR) are non-photo reversible and are initiated by very low fluences (0.1 to 1 nmol m-2) saturating at 50 nmol m-2. In dark-grown oat seedlings, for example, a brief flash of red light with fluence as low as 0.1 nmol m-2 can stimulate coleoptile growth while inhibiting mesocotyl growth. Similarly, red light with a fluence of 1-100 nmol m-2 is sufficient to stimulate Arabidopsis seed germination.
Low Fluence Responses (LFRs)
Low Fluence Responses (LFRs) are photo-reversible and require fluencies of at least 1.0 nmol m-2, saturating at 1000 nmol m-2. This category contains the majority of the red/far-red photo responses, including lettuce seed germination.
High Irradiance Responses (HIRs)
High Irradiance Responses (HIRs) are non-specific and require continuous or prolonged exposure to light of relatively high irradiance saturating at much higher fluencies (at least 100 times more) than LFRs and are non-photo reversible.
Blue-Light Responses or Cryptochrome Mediated Photo-Responses
We can examine that plants have a large number of photoresponses that are regulated by blue light and mediated by pigments called cryptochrome (crypto from cryptogams), the latter acting as a photoreceptor in such responses.
- Algae, fungi, ferns, and higher plants have all shown blue light responses.
Some of the most common and well-known blue-light responses in plants are:
- Phototropism
- Opening of the stomata
- Phototaxis
- Hypocotyl elongation inhibition
- The movement of chloroplasts within cells
- The sun is tracked by leaves.
- Stimulation of carotenoids and chlorophyll synthesis.
Types of Photoreceptors
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The different types of photoreceptors are as follows:
Phytochromes
A phytochrome is a protein that is covalently bonded to a chromophore. A plant has multiple phytochromes, which sometimes act independently of one another and sometimes are dependent on one another, either at the same time or at different times during the development process.
Phytochrome is found in two forms:
- Pfr is biologically active and absorbs light in the far-red range.
- When far-red light is absorbed, it is converted to Pr.
- Pr absorbs wavelengths in the red range.
- Pr is converted to Pfr when it absorbs red light.
Cryptochrome
Cryptochrome is a type of protein that is used for the detection of light in the UV-blue spectrum of light. It also plays a role in determining the length of the flowering time.
- It helps in the identification of flavoprotein involved in the inhibition of hypocotyl elongation in response to blue light.
- Cryptochrome is divided into two chromophores that are used for the detection of blue and green light, respectively.
Phototropins
Phototropins are blue-lighted receptors that help plants move their chloroplasts and phototropism. The process alsi help in enchancing the process of photosynthesis.
- It also helps in the opening of stomata and light-dependent chloroplast movement.
- The process was discovered in the late 1980s that blue light stimulated the phosphorylation of a 120 kDa protein.
- These areas were also the most sensitive to phototropic stimuli.
- Phototropin is a flavoprotein that contains two flavin mononucleotide (FMN) chromophores.
- A carboxy-terminal domain of the protein contains a serine/threonine kinase activity.
- The chromophores are attached to two domains called LOV domains (of about 100 amino acids each) in the amino-terminal half.
UVR8
UVR8 is another type of protein which we can use to examine the UV-B radiation in plants. It is used for the detection of UV light and signals the plant initiates stress response towards UV light.
Importance of Photomorphogenesis
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Plant growth and development are influenced by a variety of environmental factors, including light. Light, on the other hand, causes a variety of responses in the plant body in addition to photosynthesis.
- These responses have a significant impact on the course of plant growth and the final plant appearance.
- They are morphogenic responses to light.
- Many plant seeds, for example, do not germinate unless they are exposed to light.
- Seed germination in light demonstrates that seedlings require light to grow.
- Phototropic responses of seedlings and mature plant leaves are also photomorphogenic processes.
- Older plants rely on photomorphogenesis responses as well.
Many of these responses are in response to the relative lengths of day and night by forming reproductive structures or dormant buds that can withstand a cold winter (i.e., the phenomenon of photoperiodism and vernalization).
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Things to Remember
- Aside from photosynthesis, in which light is harvested, photomorphogenesis in plants is the growth and development of plants in response to light.
- The three different photoreceptors are: Cryptochrome, Phototropins and Zeaxanthin.
- The two critical stages of photomorphogenesis, according to Hans Mohr (1983), are: pattern specification and pattern realization.
- There are two kinds of plant responses to light signals. Photo-responses mediated and Blue-Light Reactions or Photoresponses.
- A plant goes through at least three stages of photomorphogenesis in its life.
Sample Questions
Ques: What are phytochromes? (2 marks)
Ans: A phytochrome is a protein that is covalently bonded to a chromophore. Its receptors are capable of detecting wavelengths ranging from red to far-red. A plant has multiple phytochromes, which sometimes act independently of one another and sometimes are dependent on one another, either at the same time or at different times during the development process.
Ques: Explain the mechanism of zeaxanthin? (5 marks)
Ans: Blue-light excitation of zeaxanthin in guard cells is thought to initiate a signal transduction pathway that includes:
- Zeaxanthin isomerization
- Apoprotein conformational changes
- A secondary messenger (most likely Ca++, phosphatases, calcium-binding protein calmodulin, and inositol triphosphate) transmits a blue-light signal across the chloroplast membrane (IP3)
- The activation of H+-ATPases at the plasma membrane of the guard cell results in the pumping of protons across the membrane and the intake of K+ ions, followed by Cl– ions.
- Turgor accumulation in the guard cell and stomatal opening.
Ques: What are the features of cryptochrome? (2 marks)
Ans: The features of cryptochrome are:
- It is a flavin protein with two chromophores (One for blue and one for green light)
- Cryptochrome regulates physical activities such as leaf expansion, leaf expansion, plant circadian rhythm, and so on.
- It can detect magnetic energy in a variety of plant species.
Ques: What are the arguments in favor of carotenoids? (3 marks)
Ans: The arguments in favor of carotenoids are as follows:
- Three-peaked (three-finger) action spectra resemble carotenoids' absorption spectra.
- Some action spectra have a low or no UV maximum.
- It is possible to transfer energy from a UV-absorbing pigment to carotenoids.
- Carotenoids from diatom mutants do not respond to blue light.
Ques: Explain photoperiodism? (2 marks)
Ans: Photoperiodism refers to a plant's ability to use light to track time. Plants can tell the time of day and the season by sensing and utilizing different wavelengths of sunlight. Phototropism is a directional response that allows plants to grow in the direction of, or even away from, sunlight. Plants need to be able to detect light in their surroundings in order to compete and survive. Different photoreceptors mediate a plant's response to light.
Ques: Explain the salient features of phototropins? (3 marks)
Ans: The salient features of Phototropins are:
- Phototropins are blue-lighted receptors that regulate all plant responses that improve photosynthetic efficiency.
- They also control phototropism.
- Other physical activities that they can control include the opening of stomata in response to light, the movement of chloroplasts, and so on.
Ques: What is meant by phytochrome-mediated photoresponses? (4 marks)
Ans: Phytochrome, a proteinaceous pigment that acts as a photoreceptor and absorbs red and far-red light, mediates several photomorphogenic responses in plants. Blue light can also be absorbed by it.
- Very Low Fluence Responses (VLFR): These responses are non-photo reversible and are initiated by very low fluences (0.1 to 1 nmol m-2) saturating at 50 nmol m-2. Similarly, red light with a fluence of 1-100 nmol m-2 is sufficient to stimulate Arabidopsis seed germination.
- Low Fluence Responses (LFRs): These responses are photo-reversible and require fluencies of at least 1.0 nmol m-2 saturating at 1000 nmol m-2
- High Irradiance Responses (HIRs): These responses are non-specific and require continuous or prolonged exposure to light of relatively high irradiance saturating at much higher fluencies (at least 100 times more) than LFRs and are non-photo reversible.
Ques: What are the importance of photomorphogenesis? (4 marks)
Ans: The importance of photomorphogenesis:
- Plant growth and development are influenced by a variety of environmental factors, including light.
- Light, on the other hand, causes a variety of responses in the plant body in addition to photosynthesis.
- These responses have a significant impact on the course of plant growth and the final plant appearance.
- Older plants rely on photomorphogenesis responses as well.
- Many of these responses are in response to the relative lengths of day and night by forming reproductive structures or dormant buds that can withstand a cold winter (i.e., the phenomenon of photoperiodism and vernalization).
Ques: What is blue light responses? (2 marks)
Ans: Cryptochrome absorbs light rays primarily in the violet-blue spectrum (400 – 500 nm). In the UV-A region, it also absorbs long-wave ultraviolet rays (320 to 400 nm). However, the majority of cryptochrome-induced photoresponses in plants result from absorption in the violet-blue region of the spectrum, but they are simply referred to as blue-light responses.
Ques: What is the process of cryptochrome? (5 marks)
Ans: Arabidopsis produced the first protein with blue-light receptor properties in 1993. It was discovered that the Arabidopsis hy4 mutant had lost the ability to respond specifically to blue light, as evidenced by an elongated hypocotyl even when exposed to blue light.
- The hy4 gene (later renamed cryl) encoded a 75 kDa protein called crypto-chrome 1 (CRY1) with remarkable sequence similarity (homology) to DNA photolyase in having two chromophores: a flavin adenine dinucleotide (FAD) and a pterin attached to the apoprotein
- Cryptochrome is distinguished from photolyase primarily in two ways.
- First, unlike photolyase, cryptochrome lacks photolyase activity.
- Second, unlike photolyase, it has an extended carboxy-terminal domain with kinase activity.
- A second cryptochrome 2 (CRY2), which also has two chromophores like CRY1, has been isolated from Arabidopsis (Lin 2000).
- CRY2 regulates hypocotyl elongation, cotyledon expansion, and anthocyanin production in response to blue light.
Ques: What are the evidence that supports zeaxanthin's role in the stomatal opening? (5 marks)
Ans: The following evidences strongly support zeaxanthin's role in the stomatal opening:
- Zeaxanthin's absorption spectrum closely resembles the action spectrum of blue-light stimulated stomatal opening.
- The incident radiation, zeaxanthin concentration in guard cells, and stomatal apertures have been found to be directly correlated during the stomatal opening in intact leaves.
- As the concentration of zeaxanthin in guard cells increases, so does their blue-light sensitivity.
- A 3mM solution of dithiothreitol (DTT), a potent inhibitor of the enzyme that converts violaxanthin to zeaxanthin, completely inhibits blue-light stimulation of stomatal opening.
- Salt accumulation causes a shift in carbon metabolism from C3 to CAM in facultative CAM plant species such as MeSembryanthemum crystallinum. Guard cells accumulate zeaxanthin and respond to blue light in C3 mode.
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Comments
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Hormones such as Cytokinin promote photomorphogenesis. While, Auxin, Barrsinosteroids (BRs) and Gibberlins (GAs) oppose photomorphogenesis. Abscisic acid (ABA) helps in maintaining the etiolated growth. Ethylene can either promote or inhibit the photomorphogenetic growth in a tissue.
Phytochrome is responsible for the Photomorphogenesis. The Pr form of phytochrome absorbs red light up to a maximum of 60 nm and the Pfr form of phytochrome absorbs far-red light at 730nm. The red light absorbed by Pr is converted to Pfr which is active and initiates the biological response. When far-red light is absorbed by Pfr, it is converted to Pr form which is inactive.
Auxin is the plant hormone that plays an important role in regulating plant growth and development and photomorphogenesis. During Photomorphogenesis, Auxin regulates gene expression and physiological responses to light signals. It also helps in cell elongation, regulates phototropic responses and promotes leaf expansion.
Photoperiod refers to the time during which the plants receive sunlight. Photoperiod plays a significant role in Plant Growth and Development. It also aids in physiological processes Based on the photoperiod plants are categorized into three groups:
No, both phototropism and photomorphogenesis are different mechanisms. The phenomenon by which plants or plant organs bend towards the light is called Phototropism. Photomorphogenesis is a process by which light mediates the growth of the plant.
The process of using light as a source of illumination for plants is called Photomorphogenesis. The important factor influencing Photomorphogenesis is light and it is responsible for plant growth and development. It also helps plants in photosynthesis to prepare Carbon dioxide and water using low molecular weight sugars.
Phytochrome is a pigment involved in the initiation and regulation of Photomorphogenesis. It absorbs red and far-red light and signals for the signals for photomorphogenesis response.