Photomorphogenesis

What is Photomorphogenesis?

Photomorphogenesis is the development of plants where the pattern of plant growth responds to the spectrum of light. In this process, light is used as a source of energy.

Any change in the structure and function of an organism in response to changes in light intensity is known as photomorphogenesis. Along with plants, it is a common feature of development in fungi, protists, and bacteria.

There are two important stages of photomorphogenesis:

  • Pattern specification where the cells and tissues develop the ability to respond to light during some developmental stage.

  • Pattern realization during which the photoresponse occurs.

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

  • Phytochrome-mediated photoresponse

  • Blue-light response or cryptochrome-mediated photoresponse

Phytochrome-Mediated Photoresponse

Many photomorphogenic responses in plants are known to be mediated by phytochrome. It is a proteinaceous pigment that acts as a photoreceptor and absorbs red and far-red light. It also absorbs blue light.

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

  • Very Low Fluence Responses– These responses are non-photo reversible and are initiated by very low fluences.

  • Low Fluence Responses- These are photo reversible. It includes most of the red and far-red photo responses including the lettuce seed germination.

  • High Irradiance Responses- These require prolonged exposure to light of high irradiance. These saturate at much higher influences than low fluence responses and are non-photo reversible.

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

Photoreceptors are responsible for photomorphogenesis.

When the seed which was initially in an environment of complete darkness is exposed to light, 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 wavelength 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

The phytochrome is a protein covalently bonded to a chromophore. The wavelengths of red to far-red lights are detected by the phytochrome receptors.

A plant has multiple phytochromes that sometimes act independently of one another and sometimes are dependent either at the same time or at different times in the process of development.

Phytochrome exists in two forms-

  • Pfr

  • Pr

Pfr is in a biologically active form and absorbs far-red. Pfr is converted to Pr when far-red light is absorbed.

The red wavelengths are absorbed by Pr. When the red light is absorbed, Pr is converted to Pfr.

The red and far-red reversibility defines the responses of the phytochrome proteins. Photomorphogenesis like leaf expansion and stem elongation is mediated by phytochrome.

Cryptochrome

Cryptochrome perceives green, UVA and blue light.

It is a flavin protein with 2 chromophores one each for the blue and green light.

Flavin is used as a chromophore. Leaf expansion, stem elongation, circadian rhythms of plants, all are regulated by cryptochrome.

They are found in plants and animals and are involved in their circadian rhythms.

They can also sense magnetic fields in a number of species.

Phototropins

These are blue-light receptors that control all the responses that optimize the photosynthetic efficiency of the plants. These regulate the movement of the plant towards the light (phototropism), the opening of stomata with response to light, and the movement of chloroplast with the change in light intensity.

UVR8

It is an ultraviolet-B sensing protein found in plants and other sources. It can sense ultraviolet light in the range of 280-315 nm and initiates plant stress response.

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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