Photomorphogenesis – Definition, Meaning, Stages, Importance, Facts

What Is Photomorphogenesis?

Photomorphogenesis is the plant growth and development in response to light signals. It is a very paramount process in plant biology that controls the most imperative functions undertaken by plants, including seed germination, stem elongation, or epicotyl elongation, expansion of the leaf, and flowering.

Light as a cardinal factor in the environment sets these processes into interrogation through light-driven alteration of growth patterns and physiological responses using photoreceptors like phytochromes.

Structure and Function of Phytochromes

There exists a chromophore and a polypeptide moiety in each phytochrome protein. This light binding to the chromophore results in the formation of a holo-protein that may be thought of as a single pigment-absorbing colour. Phytochrome's perception of red and far-red light greatly controls plant development, including seed germination.

Activation and Deactivation Mechanism

There exist two forms of phytochromes: one inactive, Pr, and the other representing the active state, Pfr. They are induced by red light as a result of the conversion of Pr to its active form, Pfr, and turned off by far-red light in which the Pfr is reverted to Pr. It is due to phytochromes that plants can respond to any fluctuations in light exposure.

Role of Phytochromes in Seed Germination

Phytochromes are implicated in the induction of seed germination. The Pfr form senses red light as an indicator of a favourable condition for growth. Pfr initiates the expression of genes that result in breaking the dormancy of seeds and allows seed germination.

Phytochrome Regulation of Gene Expression

On activation, phytochromes in the Pfr form enter the nucleus. They regulate gene transcription by interacting with transcription factors. Another such example of regulation is the transition from seedling growth to flowering.

Cryptochrome-Mediated Photomorphogenesis

The details are given below:

Structure and Function of Cryptochromes

Cryptochromes are part of a class of photoreceptors that contain a flavin chromophore, which may be activated through blue light. Hence, this forms a significant controller in the processes for the plant's circadian rhythm, photoperiodism, and other growth processes.

Mechanisms of Cryptochrome Activation

Blue light activates cryptochromes by causing conformational changes in them to physically interact with other proteins which are controlling gene expression. This allows the plant to coordinate its internal clock with the prevailing illumination in the environment.

Role in Circadian Rhythms and Photoperiodism

The cryptochromes appear to play a role in the active circadian rhythm maintenance by controlling the clock genes' expression. Further, they take part in photoperiodism by enhancing the ability of a plant to measure day length. This is to correctly time flowering and other seasonal responses.

Phototropin-Mediated Photomorphogenesis

The details are given below:

Structure and Function of Phototropins

In addition to this, the LOV domain-containing phototropins are photoreceptors for blue light and mediate several light-dependent physiological responses, including phototropism, chloroplast movement, and stomatal opening.

Role of Phototropism

This is because the cell elongation in the shaded part of the plant stem enhances, making it lean towards the light. Differential tissue growth in plants leads them to bend towards light for optimising light capture by leaves, due to the stimulated phototropins as a result of sensing blue light.

Phototropins and Stomatal Opening

Phototropins are the principal cause of stomata opening by blue light. They activate the proton pumps, increasing the turgor pressure of the guard cells, thus opening the stomata. This facilitates gaseous exchange and transpiration.

UVR8-Mediated Photomorphogenesis

UVR8 is a UV-B receptor that gives plants the ability to respond to ultraviolet light. It controls UV-B-induced photomorphogenic and stress responses, thus safeguarding them from UV-B damage by the activation of protective pathways.

Structure and Function of UVR8

UVR8 is a UV-B receptor that helps plants detect and respond to ultraviolet radiation. It initiates protective responses to prevent UV-induced cellular damage.

Summary – Roles of Photoreceptors in Photomorphogenesis

The summary of the role of photoreceptors is given in the table below:

Phytochrome NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Structure and Function of Phytochromes

• Cryptochrome-mediated Photomorphogenesis

Practice Questions for NEET

Q1. Phytochrome is a

1. Flavoprotein

2. Glycoprotein

3. Lipoprotein

4. Chromoprotein

Correct answer: 4) Chromoprotein

Explanation:

Phytochromes and their effects -

• It is observed that that a brief exposure with red light during critical dark period inhibits flowering in a short day plant and this inhibitory effect can be reversed by a subsequent exposure to far-red light.

• Similarly, prolongation of the critical light period or the interruption of the dark period stimulates flowering in long-day plants.

• This inhibition of flowering in short day plant and stimulation of flowering in long day plants involves the operation of a proteinaceous pigment called phytochrome.

• Phytochromes are the chemicals which perceive the photoperiodic stimulus.

• The wavelength of light is perceived by the leaves.

• A defoliated plant does not flower.

• A single leaf is sufficient to stimulate the flowering.

• Phytochrome is present in roots, coleoptiles, stems, hypocotyls, cotyledons, petioles, leaf blades, vegetative buds, flower tissues, seeds and developing fruits of higher plants.

• Proteins having pigmented prosthetic group are referred to as chromoproteins. Phytochrome is a chromoprotein, e.g., phytochrome A and phytochrome B that regulate flowering in plants by under different light conditions by absorption of red or far-red light by their pigment.

Hence, the correct answer is option 4) Chromoprotein.

Q2. Photoperiodic stimulus is perceived by

1. Leaves

2. Roots

3. Flowers

4. Stem

Correct answer: 1) Leaves

Explanation:

• Phytochromes are the chemicals which perceive the photoperiodic stimulus.

• The wavelength of light is perceived by the leaves.

• A defoliated plant does not flower.

• A single leaf is sufficient to stimulate the flowering.

• Phytochrome is present in roots, coleoptiles, stems, hypocotyls, cotyledons, petioles, leaf blades, vegetative buds, flower tissues, seeds and developing fruits of higher plants.

Hence, the correct answer is option 1) Leaves.

Q3. Pfr activates flowering in

1. Short day plants

2. Long day plants

3. Day neutral plants

4. Both short day and long day plants

Correct answer: 2) Long day plants

Explanation:

Activation of flowering in long-day plants relies much on par, a far-red type of phytochrome. Since prolonged exposure to light causes a cumulative increase of pfr that tends to initiate flowering, such increased pfr does not exist at night; during this dark phase, gradual back-conversion to Pr will be the only mechanism operating to some extent, until red-light flash that temporarily brings back to the night's mixture the previously disappeared Pfr by converting some Pr to pfr. This mechanism enables long-day plants to flower during the seasons with long daylight hours.

Hence, the correct answer is option 2) Long-day plants.

Also Read:

• MCQs on Phytochrome

• Auxin Bioassay

• Plant Growth Regulators