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Phytochrome: Definition, Meaning, Examples, Function,Types & Forms

Phytochrome: Definition, Meaning, Examples, Function,Types & Forms

Irshad AnwarUpdated on 02 Jul 2025, 07:05 PM IST

What is Phytochrome?

Phytochrome is a photoreceptor pigment of plants, absorbing chiefly the red and far-red wavelengths of visible light and is involved in the control of most numerous particularities of plant growth and development. It controls photomorphogenesis, a development process in plants by which form and structure are determined by light. It is needed for the germination of seeds, elongation of stems, and the expansion of leaves and regulates the time of flowering. This way it will help plants adapt to the light environment to have appropriate growth and reproduction as well.

Commonly Asked Questions

Q: What is phytochrome and why is it important for plants?
A:
Phytochrome is a photoreceptor protein found in plants that helps them detect and respond to different light conditions. It's crucial for plant growth and development, regulating processes like seed germination, stem elongation, leaf expansion, and flowering time. Phytochrome allows plants to adapt to their environment by sensing changes in light quality and duration.
Q: What is the evolutionary significance of phytochrome in plants?
A:
The evolution of phytochrome has been crucial for plant adaptation to terrestrial environments. It allows plants to:
Q: How do phytochromes contribute to the regulation of circadian rhythms in plants?
A:
Phytochromes help regulate plant circadian rhythms by entraining the internal clock to external light-dark cycles. They provide input about day length and light quality to the central oscillator of the circadian clock. This interaction allows plants to synchronize their internal processes with the external environment, optimizing various physiological and metabolic activities.
Q: What is phytochrome-mediated de-etiolation?
A:
Phytochrome-mediated de-etiolation is the process by which dark-grown seedlings transition to light-grown plants upon exposure to light. It involves several morphological changes, including the inhibition of hypocotyl elongation, expansion of cotyledons, development of chloroplasts, and activation of light-regulated genes. This process is crucial for seedlings emerging from soil into sunlight.
Q: What role does phytochrome play in photomorphogenesis?
A:
Phytochrome is a key regulator of photomorphogenesis, the light-mediated development of plants. It influences:

Structure of Phytochrome

The structure of phytochrome is described below-

Chemical Structure

The phytochromes are a family of photoreceptors made up of a protein moiety linked to a light-absorbing chromophore called phytochromobilin.

Active and Inactive Forms

The two interconvertible forms of the phytochromes are the red light-absorbing form.

Conversion between Forms (Pr and Pfr)

Red light mediates the conversion of Pr to Pfr, while far-red light does the reverse step of converting Pfr back into Pr. This controls several light-dependent processes in plants.

Commonly Asked Questions

Q: How does phytochrome detect light?
A:
Phytochrome detects light through its ability to switch between two forms: Pr (red-light absorbing) and Pfr (far-red light absorbing). When Pr absorbs red light, it converts to Pfr. Conversely, when Pfr absorbs far-red light, it reverts to Pr. This interconversion allows plants to sense the ratio of red to far-red light in their environment, providing information about sunlight quality and shade conditions.
Q: What is the difference between light-dependent and dark-reversion reactions in phytochrome?
A:
Light-dependent reactions refer to the conversion of Pr to Pfr by red light absorption, and Pfr to Pr by far-red light absorption. These are rapid processes. Dark-reversion, on the other hand, is the slow, light-independent conversion of Pfr back to Pr that occurs in darkness. This dark-reversion is crucial for processes like measuring night length in photoperiodism.
Q: How do phytochromes interact with cryptochromes and phototropins?
A:
Phytochromes interact with other photoreceptors like cryptochromes (blue light receptors) and phototropins in several ways:
Q: How does temperature affect phytochrome activity?
A:
Temperature can affect phytochrome activity in several ways:
Q: What is phytochrome-interacting factor (PIF) and its significance?
A:
Phytochrome-Interacting Factors (PIFs) are a family of transcription factors that interact directly with phytochromes. They are significant because:

Types of Phytochromes

The types of phytochrome is described below-

Phytochrome A (PhyA)

This phytochrome is most sensitive to continuous far-red light. It plays a critical role in seedling development under shade from the canopy.

Phytochrome B (PhyB)

PhyB normally absorbs red light and performs different regulatory activities such as seed germination and shade avoidance.

Other Phytochromes (C, D, E)

The phytochromes C, D, and E have partly overlapping functions with PhyA and PhyB, thus giving an idea about the fine-tuning effects on the response of plants toward light.

Commonly Asked Questions

Q: What are the two main forms of phytochrome?
A:
The two main forms of phytochrome are:
Q: How does phytochrome signaling differ between monocots and dicots?
A:
While the basic mechanism of phytochrome action is similar in monocots and dicots, there are some differences:
Q: What is the "end-of-day far-red" effect?
A:
The "end-of-day far-red" effect refers to the growth response triggered in plants when exposed to a brief period of far-red light at the end of the day. This exposure converts a large portion of phytochrome to the Pr form, mimicking shade conditions. It often results in increased stem elongation and other shade-avoidance responses, even if the plant is not actually shaded during the day.
Q: How do different wavelengths of light affect phytochrome activity?
A:
Different wavelengths of light affect phytochrome activity as follows:
Q: What is the "very low fluence response" (VLFR) in phytochrome action?
A:
The Very Low Fluence Response (VLFR) is a highly sensitive phytochrome-mediated response that occurs with extremely low amounts of light. It's characterized by:

Functions of Phytochrome

The functions of phytochrome are described below-

Seed Germination

It refers to how phytochromes perceive the light signals for seed germination, where Pr is converted into Pfr which will break dormancy and promote growth.

Stem Elongation

Etiolation and De-Etiolation: In dark situations, phytochromes are in the Pr form and promote etiolation, i.e., greater elongation of the stem. On exposure to light, Pr is converted into Pfr, and de-etiolation is followed by normal growth of the stem.

Leaf Expansion and Unfolding

Phytochromes regulate the expansion and unfolding of leaves. So, therefore, it's done by regulating the size and shape of leaves according to light.

Flowering Time Regulation

The role of phytochromes as major regulators of time to flowering allows the plant to have a way it can be capable of flowering at the right moment through regulating the length of a day, photoperiod.

Shade Avoidance

Plants' response to the changes in light quality created by shading because of their phytochromes is by elongation of stems and leaves to grow above other plants to receive more light.

Commonly Asked Questions

Q: How does the ratio of red to far-red light affect plant growth?
A:
The ratio of red to far-red light provides plants with information about their light environment. A high red to far-red ratio indicates direct sunlight, promoting normal growth. A low ratio suggests shade conditions, often triggering shade-avoidance responses like increased stem elongation. Plants use this information to adjust their growth and development strategies.
Q: What is the "shade avoidance response" in plants?
A:
The shade avoidance response is a set of plant growth changes triggered by low red to far-red light ratios, typically experienced in shaded conditions. These changes include increased stem elongation, reduced leaf expansion, and accelerated flowering. This response helps plants compete for light in crowded environments by growing taller to reach sunlight above neighboring plants.
Q: How does phytochrome influence seed germination?
A:
Phytochrome plays a crucial role in seed germination by detecting light conditions. In many plant species, the Pfr form promotes germination while the Pr form inhibits it. A brief exposure to red light can convert Pr to Pfr, triggering germination. This mechanism ensures seeds germinate when light conditions are favorable, typically when they're close to the soil surface.
Q: What is photoperiodism and how is it related to phytochrome?
A:
Photoperiodism is the response of plants to the length of day and night cycles, which influences processes like flowering. Phytochrome is key to this process, as it can measure the duration of darkness. The slow conversion of Pfr to Pr in darkness acts as a "night-length timer," allowing plants to determine the season and adjust their flowering time accordingly.
Q: How does phytochrome affect plant height?
A:
Phytochrome regulates plant height primarily through its influence on stem elongation. In high red light conditions (high Pfr levels), stem elongation is typically inhibited, leading to shorter, more compact plants. In low red light or shaded conditions (low Pfr levels), stem elongation is promoted, resulting in taller plants as part of the shade avoidance response.

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Frequently Asked Questions (FAQs)

Q: How do phytochromes contribute to plant competition and neighbor detection?
A:
Phytochromes help plants detect and respond to neighboring vegetation through:
Q: What is the role of phytochrome in seed dormancy?
A:
Phytochrome plays a crucial role in regulating seed dormancy:
Q: How do phytochromes influence plant immune responses?
A:
Phytochromes can influence plant immune responses by:
Q: What is the significance of phytochrome nuclear translocation?
A:
The nuclear translocation of phytochrome is significant because:
Q: How do phytochromes regulate stomatal development and function?
A:
Phytochromes regulate stomatal development and function by:
Q: What is the role of phytochrome in phototropism?
A:
While phototropins are the primary photoreceptors for phototropism, phytochromes also play a role:
Q: How do phytochromes influence root development?
A:
Phytochromes influence root development in several ways:
Q: What is the significance of phytochrome C in flowering regulation?
A:
Phytochrome C (phyC) has specific significance in flowering regulation:
Q: How do phytochromes interact with other plant hormones?
A:
Phytochromes interact with various plant hormones to regulate growth and development. For example, phytochrome signaling can affect the production or sensitivity to hormones like auxins (influencing stem elongation), gibberellins (affecting seed germination and stem growth), and ethylene (impacting fruit ripening and senescence). These interactions allow for complex and fine-tuned responses to environmental conditions.
Q: How do phytochromes influence flowering in long-day and short-day plants?
A:
Phytochromes influence flowering differently in long-day and short-day plants:
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