Plant Hormones

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Introduction

Coordination of all the activities taking place within an organism is essential for the maintenance of life. This coordination among various cells of the body can be achieved in two ways; nervous coordination and chemical coordination. Chemical coordination is brought about by hormones.

Hormones are the chemical substances capable of affecting the various chemical processes taking place within a cell. they perform their action by binding to specific cellular receptors. They are the part and parcel of chemical coordination which is found in lower organisms. Nervous coordination is only seen in highly developed multicellular organisms included in the kingdom Animalia.

Plants coordinate various activities such as photosynthesis, cellular respiration, storage of starch, etc.  by the action of plant hormones. These are the chemical substances produced by plant cells that act on other plant cells to bring about coordination among various activities. In this article, we will focus on some important hormones found in plants.

Major Classes

Plant hormones are the organic compounds that can influence various physiological processes taking place within the plants. They help regulate and coordinate processes like photosynthesis, respiration, transpiration, etc.

Based on their chemical nature and occurrence, plant hormones are divided into the following major classes.

  • Auxins
  • Gibberellins
  • Cytokinins
  • Ethylene
  • Polyamines
  • Abscisic Acid
  • Salicylic Acid
  • Brassinosteroids
  • Jasmonates

The above-mentioned classes of plant hormones will be discussed separately in the rest of our article.

Auxins

It is one of the major classes of plant hormones that influence plant growth as well as the development of new plants.

Chemical Nature

Auxins include indole acetic acid (IAA) and its derivatives. They have an indole group attached to the acetic acid. Indole-3-acetic acid is the most important auxin found in the majority of the plants.

Other auxins may be formed by conjugation reactions of IAA with various other groups. Artificial auxins can also be made according to the needs of the plant using IAA.

Synthesis in Plants

Auxins are made by plants using tryptophan or indole amino acids as precursors. Its synthesis occurs in leaves and seeds of plants.

Mode of Transport

Auxins travel in plants via cell to cell transport. They can easily diffuse from one plant cell to the other. Phloem, a component of the transport system in plants, is used when auxins are to be sent to the root hair cells.

Physiological Effects

Auxins are involved in the following physiological processes taking place in plants.

  • They are responsible for cell growth and enlargement during stem growth.
  • Auxins stimulate cell division in association with other plant hormones like cytokines
  • They are responsible for the differentiation of vascular tissues in plants.
  • Auxins cause the root initiation, development, and differentiation in tissue culture and stem cutting techniques.
  • They cause the phototropic and gravitropic responses of plant stems and roots.
  • They delay the leaf senescence and fruit ripening in plants.
  • They stimulate the growth of flowers and the flowering process.
  • They promote the female characters in some flowers.

Gibberellins

It is a family of plant hormones including 125 organic compounds all having structural similarities. Gibberellic acid 1 and 3 (GA1 and GA3) are the most commonly found gibberellins in plants.

Chemical Nature

They are the cyclic compounds having three six-carbon rings in their structure. a carboxylic group and hydroxyl groups are essential features of these compounds.

Synthesis in Plants

Glyceraldehyde-3-phosphate acts as a precursor for gibberellins. They are made in the growing stems and seeds of the plants. The synthesis of these compounds occurs in the chloroplast of plant cells.

Mode of Transport

The hormones included in this class are transported via transport systems of plants which include specialized tissues xylem and phloem.       

Physiological Effects

The major physiological roles performed by gibberellins are mentioned below.

  • GA1 causes stem elongation by promoting cell division and cell elongation.
  • They cause stem elongation in response to long summer days, a phenomenon called bolting.
  • They are responsible for seed germination in the absence of light or other favorable conditions.
  • They are involved in the induction and synthesis of enzymes during seed germination.
  • Although gibberellins are not directly involved in the process of fruit growth, external applications of these compounds can cause growth in the case of some species.
  • Unlike auxins, they promote the male features of flowers.

Cytokinins

It is another important category of plant hormones. Organic compounds having a structural resemblance to these hormones are also found in animal cells. they perform completely different roles in animal bodies.

Chemical Nature

These are the derivatives of adenine, a purine base. Ribosides and ribotides made from adenine are also included in this class. Zeatin is the most common cytokinin found in the majority of the plants.

Synthesis in Plants

Various cytokinins are made by biochemical modifications of adenine. These reactions take place in the roots and seeds.

Mode of Transport

Cytokinins are made in roots and are transported to the higher parts of the plant body via xylem.

Physiological Effects

The major function of cytokinins is the induction of cell division. They also perform the following functions:

  • Cytokinins induce cell division both endogenously as well as on exogenous application.
  • Cytokinins are the organic compounds found in tumors, suggesting that they have a role in tumor formation.
  • They induce cell division in association with auxins. Both these hormones have synergic action in cell division.
  • Cytokinins are involved in morphogenesis and bud formation.
  • They can cause the growth of lateral buds.
  • Cytokinins cause leaf expansion, delay leaf senescence, and promote chloroplast development in the leaves.
  • They also have a role in stomatal openings in some species of plants.

Ethylene

Ethylene gas plays several important roles in plant growth and development. It can also be provided to plants in artificial ways to meet their needs.

Chemical Nature

Ethylene or ethene is a hydrocarbon belonging to the class of alkenes having molecular formula C2H4.

Synthesis in Plants

Plants make ethylene from methane gas. The synthesis of ethylene occurs under stress conditions. It mainly takes place in tissues that are in the process of ripening or senescence.

Mode of Transport

Ethylene moves to different parts of plants by the process of diffusion. Being a gas, it does not need some specific medium for transport.

Physiological Effects

Ethylene is not essential for the normal growth of plants. Most of the plants can undergo normal development in the absence of ethylene. However, it is essential for some physiological processes listed below.

  • It limits the effect of injury by activating defense mechanisms
  • It is responsible for the development of root and shoot
  • Ethylene causes flower induction and flower opening in some species
  • It is also responsible for fruit and leaves abscission
  • Ethylene is a powerful agent that promotes fruit ripening. It is used in the artificial ripening of fruits

Polyamines

It is another large group of organic compounds that play essential roles in the growth of some plants.

Chemical Nature

Polyamines are aliphatic amines that are derived from amino acids. The most important polyamine include putrescene, spermidine, and spermine. Other chemical derivatives are also found.

Synthesis in Plants

They are made by the decarboxylation of some amino acids like arginine and ornithine. They can also be obtained by the decarboxylation of S-adenosyl methionine. They are present in all cells without having any specific site of biosynthesis. 

Mode of Transport

No specific transport mechanisms have been identified in the case of polyamines.

Physiological Effects

Polyamines perform the following functions in plant cells.

  • These are the regulatory hormones that control the growth and development of the plant cells.
  • They are necessary for normal cell division and morphology.

Abscisic Acid

This class includes only one organic compound called abscisic acid.

Chemical Nature

It is an organic compound having a complex structure. One carboxylic group, hydroxyl group, and a benzene ring are essential components of the abscisic acid molecule. 

Synthesis in Plants

The synthesis of abscisic acid occurs in grown-up roots and leaves of plants. Glyceraldehyde-3-phosphate is used as a precursor for its synthesis. A large amount of abscisic acid is found in seeds, although they are not the site of its production.

Mode of Transport

 It is transported to all parts of a plant’s body via xylem and phloem.

Physiological Effects

The major physiological functions of abscisic acid are mentioned below.

  • It is responsible for the induction and maintenance of dormancy in some seeds
  • The closure of stomata due to water shortage occurs via the abscisic acid pathway.
  • It is involved in the defense system against the attack of insects
  • It inhibits stem growth in response to water stress
  • It causes the synthesis of some storage proteins in seeds

Salicylic Acid

Salicylic acid and its derivates have also been recognized recently to exert some regulatory effects on physiological processes taking place in plants.

Chemical Nature

They are the acidic compounds having a benzene ring.

Synthesis in Plants

Salicylic acids are made by plants using phenylalanine, an amino acid,  as a precursor.

Physiological Effects

The physiological effects of salicylic acid are as follows.

  • It protects the plants against pathogens.
  • It induces the synthesis of specific proteins in response to pathogen attack that would protect the other parts of the plant from pathogenesis
  • It starts thermogenesis in some flowers
  • It inhibits ethylene synthesis and seed germination
  • It is believed to increase the life of flowers

Brassinosteroids

It is another important group of plant hormones that include more than 60 steroidal compounds.

Chemical Nature

All the compounds included in this category have a steroid nucleus. Derives of this st6eriod nucleus can be made via various chemical reactions.

Physiological Effects

The major physiological effects of these steroidal compounds are as follows.

  • They increase the rate of cell division
  • They are involved in vascular differentiation
  • They also cause elongation of plant cells in roots and stem
  • They are needed for fertility and reproduction in plants
  • They can inhibit the growth and development of roots

Jasmonates

These include jasmonic acid and its ester derivatives. It is named after the jasmine plant in which they are the important component of compounds that produce a scent.

Chemical Nature

They are lipid in nature having a carboxylic group and a benzene ring in their structure.

Synthesis in Plants

Jasmonic acid is derived from linolenic acid, a polyunsaturated fatty acid found in plant cells. The methyl ester derivatives are made by the process of esterification.

Physiological Effects

The important physiological functions of these compounds are listed below.

  • They are a part of the plant defense system against pathogens
  • They are the inhibitory hormones and can inhibit plant growth as well as seed germination
  • They play an important role in the development of male reproductive parts in some plants

Summary

Plant hormones are the organic compounds that regulate and coordinate the physiological and chemical processes taking place in plants. They are divided into different classes based on their structure.

Auxins are the derivative of indole acetic acids, made in leaves and seeds. They are the growth stimulatory hormones transported via diffusion of phloem.

Gibberellins are the cyclic acids having a great diversity in structure. They are made in the stem and seeds of plants. They are also growth stimulatory in nature.

Cytokinins are the nitrogenous bases derived from adenine. They promote cell division in plants and can also cause tumor formation.

Ethylene is a hydrocarbon gas not needed for the normal development of plants. However, it has a major role in the ripening of fruits.

Polyamines are the aliphatic derivatives of amino acids having one or more amino groups. They have a regulatory role in plant growth.

Abscisic acid is a cyclic carboxylic acid having an inhibitory role in plant growth. It also improves the immunity of plants.

Salicylic acid is another acidic hormone that has a role in preventing plants from pathogens.

Brassinosteroids are the steroid compounds that increase stem growth but inhibit root growth in some plants.

Jasmonates are the derivatives of linolenic acid that were first identified in the jasmine plant. They are growth inhibitors and part of the defense system in plants.

References

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