Phytochrome: Definition, Mechanism, Role, Functions

Phytochrome: Light plays a vital role in photosynthesis. Similarly, it is also very essential for plant growth and development. Light controls multiple developmental processes in the plant life cycle, including seed germination, seedling de-etiolation, leaf expansion, stem elongation, phototropism, stomata and chloroplast movement, shade avoidance, circadian rhythms, and flowering time, in addition to being the primary energy source for plants.

Photomorphogenesis is how plants respond to light signals by growing and developing. Red and blue light are very powerful in triggering a photomorphogenic response in plants. The proteinous pigment that controls photo-period detection was initially referred to as phytochrome, which means “plant colour.”

The classic phytochrome effect is that the seeds germinate better in red light and fail in far-red light compared to control seeds kept in darkness. In this article, we will learn about the Phytochromes in detail. Scroll down to learn more!

Photomorphogenesis

Photomorphogenesis is a type of light-mediated development in which the spectrum of light influences plants growth patterns. Photochromic sensory receptors such as phytochromes, cryptochromes, and phototropins limit the photomorphogenic impact of light to the UV-A, UV-B, blue, and red regions of the electromagnetic spectrum. Light is used as a source of energy in this procedure. It is a common aspect of development in fungi, protists, bacteria, and plants.

Photomorphogenesis is divided into two stages, according to Hans Mohr (1983).

1. Pattern specification occurs when cells and tissues gain a certain ability or competence to respond to light at a specific developmental stage, and
2. Pattern realisation, which is when the photo-response takes place.

Plant responses to light signals can be divided into two categories:

1. Photo-responses mediated by blue light or cryptochrome
2. Photo-responses mediated by phytochromes

1. Photo-responses mediated by blue light or cryptochrome

Blue light regulates an extensive range of photo-responses in plants, which are thought to be mediated by cryptochrome pigments. Algae, fungus, ferns, and higher plants have all been observed to react to blue light.

Cryptochrome absorbs light rays mostly in the violet-blue wavelength range (400–500 nm). It also absorbs long-wave UV radiation (320 to 400 nm). However, the majority of cryptochrome-induced photoresponses in plants are generated by absorption in the violet-blue area of the spectrum and are referred to as blue-light responses.

Plants responses to blue light include: Phototropism, the opening of the stomata, phototaxis, the sun is tracked by the leaves, hypocotyl elongation inhibition, carotenoids and chlorophyll synthesis stimulation, and movement of chloroplasts within cells.

2. Photo-responses mediated by phytochromes

Phytochrome is known to mediate a variety of photomorphogenic responses in plants. It is a proteinaceous pigment that absorbs red and far-red light and functions as a photoreceptor. It’s also a good absorber of blue light.

The phytochrome-mediated reaction can be categorised into three groups depending on how much light is absorbed. Very Low Fluence Responses (VLFRs), Low Fluence Responses (LFRs), and High Irradiance Responses (HIRs) are the three types of responses.

Phytochrome

Phytochrome is a photoreceptor in plants that regulates a variety of photomorphogenic processes. Plants, fungus, and different microorganisms such as bacteria, cyanobacteria, and proteobacteria have photoreceptors called phytochromes. Phytochrome is a photo morphogenetic chromoprotein that induces photomorphogenesis by strongly absorbing red and far-red light. Each subunit is made up of two parts: chromophore, a light-absorbing pigment molecule, and Apo protein, a polypeptide chain. Phycocyanobilin (the chromophore of the phycobiliproteins used by cyanobacteria and red algae to collect light for photosynthesis) and the bile pigment bilirubin are closely related to the phytochrome chromophore phytochromobilin. Plants produce five phytochromes: PhyA, PhyB, PhyC, PhyD, and PhyE.

Phytochrome Discovery

In the early 1950s, Harry A. Borthwick, Stirling B. Hendricks, and coworkers at the US Department of Agriculture proposed the presence of phytochrome. According to their studies, many plant responses were most successfully induced by red light, and this induction may be cancelled by brief exposure to far-red light. Following these findings, a pigment with two interconvertible forms was suggested – a red light-absorbing form and a far-red light-absorbing form. Using sensitive dual-wavelength spectrophotometry, a pigment with this photochromic characteristic was discovered for the first time in 1959. This spectrophotometric technique was then used to purify phytochrome.

Types of Phytochrome

Phytochrome comes in two photo reversible varieties or forms. Pr and Pfr are their types. The absorption peak of these two pigments is different.

1. Phytochrome red (Pr) or Red light absorbing form: This pigment absorbs the maximum light in the red region. It has a 660 nm wavelength. Pr is the inactive form that does not initiate biological responses.

2. Phytochrome far-red (Pfr) or Far-red light absorbing form: Its absorption peak is at 730nm in the far-red region. Physiologically, only Pfr phytochrome is an active form that initiates biological responses.

Red light (R) converts the Pr form of pigment to the Pfr form, while far-red light (PR) converts the Pfr form of pigment to Pr.

Fig: Types of Phytochrome

Difference Between Pr and Pfr Forms of Phytochrome

The difference between Pr and Pfr forms of phytochrome is as follows:

Phytochrome Functions

Some of the functions of phytochromes are given below:

1. Photomorphogenesis, photoperiodism, and cleistogamy are all developmental processes in which phytochrome plays a role.
2. It is involved in seed dormancy, leaf abscission, and the production of gibberellins, anthocyanins, and carotenoids, among other things.
3. Plant growth and development are regulated in different ways by phytochromes A and B.
4. PhyB is more sensitive to red and far-red light.
5. Some seeds require phytochrome for germination.
6. It promotes leaf production as well as regulates leaf size, quantity, and shape.

Role of Phytochrome in Flowering Plants

Phytochrome regulates flowering in plants by the following process.

(a) Phytochromes role in Short-day Plants

• Short Day Plants are those that require less than 10 hours of daylight and more than 12 hours of darkness to begin flowering. Rice, coffee, tobacco, and soyabean, are some examples of SDP.
• Pr is changed to Pfr form in many SDPs when the dark cycle is broken with a brief exposure (approximately 1 hr) to red light. Flowering is inhibited because of the accumulation of Pfr. Pfr is changed to Pr and the plant produces flowers if far-red light is supplied for a short time following red light treatment.
• During the winter months, far-red light is received on the earth’s surface in greater quantities than portions of red light reaching the ground. This changes a large portion of the Pfr form into Pr, causing SDPs to flower.
• In the summer, however, the ratio is reversed because more sunlight reaches the soil, preventing SDPs from flowering.

(b) Phytochromes role in Long-day Plants

• Long Day Plants are those that require more than 14-16 hours of daylight and 8–10-hour dark periods to begin flowering. Pea, sugar beet, radish, cabbage, and wheat are among the examples.
• LDPs will not flower if the photo-period is less than 14 hours of light and more than 8 hours of darkness. Light is critical for flowering in LDPs.
• The role of phytochrome in LDPs is more complicated, and a blue-light photoreceptor is also necessary for flowering control.
• During the summer, more red light reaches the earth’s surface, compared to portions of far-red light reaching the ground. This changes a large portion of the Pr form into the Pfr form, causing LDPs to flower.
• However, in the winter, the ratio is reversed because more far-red light reaches the land, keeping LDPs from flowering.

Fig: Role of Phytochrome in Long-day plants and Short-day plants

In both SDPs and LDPs, circadian cycles of change in Pr and Pfr concentrations are found in relation to light and dark periods. It also confirms that phytochrome has a role in plant photoperiodism.

Summary

Phytochrome is a type of photopigment. They are photosensitive in nature. Plants, fungus, and different microorganisms such as bacteria, cyanobacteria, and proteobacteria have photoreceptors called phytochromes. Plants produce five phytochromes: PhyA, PhyB, PhyC, PhyD, and PhyE. There are two interconvertible types of phytochrome. Pr is a blue form that absorbs red light (660 nm) the most, whereas Pfr is a blue-green form that absorbs far-red light (730 nm). Recent studies suggest that phytochrome acts as a temperature sensor because, in environmental conditions, phytochrome gets deactivated. Phytochromes regulate the growth and development of plant-like seed germination, stem elongation, elongation of seed leaves, movement of seeds, the flowering of plants, etc.

FAQs on Phytochrome

Q.1. What are the two forms of phytochrome?
Ans: There are two interconvertible types of phytochrome. Pr is a blue form that absorbs red light (660 nm) the most, whereas Pfr is a blue-green form that absorbs far-red light (730 nm).

Q.2. Where is phytochrome found in plants?
Ans: Phytochromes in plants are found in the cytoplasm in their dark condition and are transported into the nucleus when light is activated.

Q.3. What is the role of phytochrome in plant cells?
Ans: The phytochrome system functions as a natural light switch. It maintains track of the amount, intensity, duration, and colour of environmental light. Red light’s action can be reversed by immediately illuminating far-red light on the sample, transforming the chromoprotein to the inactive Pr form.

Q.4. Is phytochrome a plant hormone?
Ans: Phytochromes are the most significant sensors in plants, and they are members of the photoreceptor gene family. They are a group of chromo proteins with a chromophore, a linear tetrapyrrole. To control growth, phytochromes and phytohormones function together.

Q.5. Which light is absorbed by phytochrome?
Ans: In response to red and far-red light, phytochrome acts as a molecular switch. It exists in two reversible conformations (Pr and Pfr), each of which absorbs red (R) and far-red (FR) light.

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