Plant Growth Hormones : Definition, Types and their Functions

What is a plant hormone

A plant hormone is a chemical substance produced in plants that helps to promote, inhibit or control the growth of different parts. Plant hormones are essential organic compounds but need in very small quantities. 

They are generally synthesized at apices of roots, stem, and in leaves from where these are transported to other parts.

Though the synthesis of growth regulators occurs under genetic control, it is also affected by environmental factors.

Most common types of plant growth hormones

1) Cytokinins

Cytokinins are the group of substituted purine derivatives capable of inducing plant cell division. It is essential for normal plant development. The work of Folke Skoog and Miller in the 1950s helped to explore the function of cytokinins. 

These are naturally synthesized by the young plant organs where active cell division takes place such as root tips, seeds, fruits, etc

It induces cell division in plants. It has the ability to induce cell division even in permanent cells. 

There is a process called Organogenesis in which three germ layers of tissues of the embryo which are ectoderm, mesoderm, endoderm, develop into the internal organs of organisms. 

To induce this process balanced combination of cytokinins and auxin is useful. 

If the amount of auxin is more than cytokinins, then root formation (rhizogenesis) is initiated. And if the amount of cytokinins is more than auxin, then shoot formation (caulogenesis) is initiated.

Cytokinins also help to initiate chloroplast formation in the presence of light, There is a phenomenon called senescence in which degradation of the plant an organ takes place. Cytokinins retard the senescence by preventing degradation of metabolites or by promoting their synthesis.

2) Gibberellins

Gibberellins are the large family of tetracyclic diterpenoid plant hormones. They help to promote cell division and elongation, seed germination, root growth, and much more.

Japanese botanist, Eiichi Kurosawa discovered Gibberellins as a causative agent for overgrowth symptoms in rice plant. This overgrowth in rice plants was caused by fungus Gibberella fujikuroi. 

This fungus caused "bakanae" (foolish seedling disease) in rice plants. The term "Gibberellins" first used by Teijiro Yabuta in 1935. The Gibberellins are abbreviated as GA

Up to now, 136 gibberellin molecules have been discovered. But only a few of them such as GA1, GA3, GA4, GA7 is bioactive.

In plants, gibberellins are synthesized from acetate units of acetyl coenzyme-A. The sites of their synthesis are apical buds, tips of growing roots, and young roots.

Gibberellins induce elongation of cells in the stem. 

It is used to increase the yield of sugar. How? Cane sugar (sucrose) is stored in parenchyma cells of the internodes of sugarcane. Spraying of gibberellins results in the elongation of internodes. Thus the production of sugar increases. It is used to increase the size of fruits. 

It is also useful in breaking seed dormancy. Seed dormancy due to low temperature and requirement for germination can be broken by treating seeds with gibberellins.

It is useful to promote seed germination in monocotyledons. In the seeds of wheat, barley, etc. gibberellin treatment induces the synthesis of amylase enzyme. Amylase converts insoluble starch into sugar which is made available to the embryo as food during seed germination.

Also, gibberellins are more effective than auxins for inducing parthenocarpy.

3) Auxins

Auxins are one of the most common types of naturally occurring growth hormones in plants. "Auxin" term was first used by a Dutch biologist, Frits Warmolt Went. Auxins promote cell elongation, inhibit the growth of lateral buds. 

Auxins are synthesized in shoot tips, root tips, young leaf primordia, etc from where they translocate to the region of elongation. The translocation of auxins is polar i.e from morphological apex to the base.

Auxins may be natural or synthetic. Naturally occurring auxins are indole 3- acetic acid (IAA) and its derivatives. Now Naphthalene Acetic Acid (NAA), Dichlorophenoxy Acetic Acid (DAA) are some synthetic auxins. 

Lower concentration of auxin promotes the growth of roots and auxiliary buds but a higher concentration of auxin is required for the growth of the stem.

When auxins like IAA and NAA are applied to the cuttings in lower concentration, it helps in the formation of adventitious roots. Auxins hence used in artificial vegetative propagation by stem or leaf cuttings.

4) Ethylene

Ethylene is the only special gaseous hormone produced naturally by plants. It is also known as 'the ripening hormone'. It is colorless, unsaturated hydrocarbon gas which is much lighter than air. 

Gane in 1934 reported that ethylene synthesized by plants promotes the ripening of fruits.

It helps in breaking seed dormancy of seeds and buds of many species and also overcomes the dormancy of potato tubers. It inhibits the elongation of stem and roots in dicots. It plays an important role in post-harvest technology.

It is used for the commercial ripening of fruits. The green and unripe fruits are picked from the field and transported to the desired destination in the CO2 rich medium. CO2 has inhibiting effects on ethylene. After removal of CO2 rich medium, the formation of ethylene is initiated and hence ripening of fruits.

References
1) https://www2.mcdaniel.edu/Biology/botf99/hormweb/hgibb.htm
2) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC540265/#bib7 
3) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4622167/
4) https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookPLANTHORM.html     
5) https://www.nature.com/articles/1341008a0