Photomorphogenesis

Table of Contents

• What is Photomorphogenesis?
• Photoreceptors

Photomorphogenesis is a developmental process in plants in which the incident light determines the growth of the plant. During this process, the pattern of plant growth is controlled by the spectrum of light available to the plant as energy.

The changes which occur in the development of an organism in response to the intensity of incident light on the organism are known as photomorphogenesis. Photomorphogenesis is a process which is also seen in organisms other than plants, such as fungi, protists, and bacteria.

There are two important stages of photomorphogenesis:

• The stage where the cells or tissues of the organisms learn the ability to respond to the incident light is known as pattern specification.

• The stage of photomorphogenesis during which the photoresponse occurs is known as pattern realization.

The plant responds to light signals in the following two ways:

• Phytochrome-mediated photoresponse

• Blue-light response or cryptochrome-mediated photoresponse

Phytochrome-Mediated Photoresponse

Phytochromes are light-receptors that plants use to detect the presence of incident light. These phytochrome receptors are used by plants for several photomorphogenic responses. Phytochromes are plant pigments which are protein by nature. These pigments absorb sunlight in the red and the far-red spectrum of light, as well as the blue spectrum of light.

The phytochrome-mediated response can be divided into three categories depending upon the amount of light absorbed.

• Very Low Fluence Responses– The responses which are initiated even by very low fluences of incident light are known as Very Low Fluence Responses or VLFRs. These responses are highly sensitive to light and are non-photo reversible.

• Low Fluence Responses- These responses are also highly sensitive to low fluences of light but are less sensitive than the VLFRs. These are photo reversible in nature.

• High Irradiance Responses- The responses which need a long exposure to incident light are known as High Irradiance Responses or HIRs. HIR are non-photo reversible in nature and have very low sensitivity towards incident light.

Also read: Photoperiodism

Blue Light Response or Cryptochrome-Mediated Photoresponse

These photoresponses are controlled by blue light and are mediated by a group of pigments called cryptochromes. These responses have been reported in fungi, algae, and ferns.

Some of the blue-light responses in plants are:

• Phototropism

• Stomatal opening

• Phototaxis

• Sun tracking by leaves

• Inhibition of hypocotyl elongation

• Stimulation of synthesis of carotenoids and chlorophyll

• Chloroplast movement within the cells

Photoreceptors are responsible for photomorphogenesis.

When the seed, which was initially in an environment of complete darkness, is exposed to light, it results in the activation of photoreceptors in the seed. This is because the seeds are exposed to electromagnetic radiation, especially to red or far-red wavelengths of light. The signals are transmitted into the nucleus by the receptors through a signal transduction pathway, which stimulates the genes responsible for growth and development.

A plant has the following types of photoreceptors:

• Phytochrome

• Cryptochrome

• Phototropin

• UVR8

Phytochrome

Phytochromes are proteinaceous plant pigments which are light-sensitive. These photo-sensitive pigments act as receptors which help the plant detect incident light.

There are several types of phytochromes present within a plant that either work independently of each other or can work together in the photomorphogenesis process.

Phytochrome exists in two forms-

• Pfr

• Pr

Pfr is the form which absorbs the light in the far-red spectrum. After Pfr absorbs the far-red light, it gets converted into another form known as Pf. Pf is the form which absorbs the light in the red spectrum of light. After it absorbs red light, it converts back to Pfr.

Cryptochrome

Cryptochromes are proteins which detect the light in the UV-blue spectrum of light. It consists of two chromophores which detect blue and green light, respectively.

Phototropins

Phototropins are photo-receptors which help to detect blue light. These help in enhancing the photosynthesis process. These also help in the movement of plants in response to incident light, the opening of stomata and light-dependent chloroplast movement.

UVR8

UVR8 is a protein which senses UV-B radiation in plants. It can detect UV light and signals the plant to initiate its stress response towards UV light.

Also read: Phototropism

For more information on Photomorphogenesis, keep visiting BYJU’S website or download BYJU’S app for further reference.

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