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

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.

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.

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.

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.

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