What is plant growth hormone? Describe different types of plant hormones.

A plant growth hormone is a specific chemical substance produced in one part of a plant and transported to other parts which control the growth and some other functions of the plant. Plant hormones are also known as growth regulators or growth substances.


                          According to Thimann (1948-52), a hormone may be defined as " an organic substance produced naturally in higher plants,controlling growth or other physiological functions at a site remote from its place of production and active in minute amounts."

                   
Following are some major plant hormones:

1. AUXIN :

The name auxin was given to the growth hormone produced by the tip of coleoptile. Kogl and Haagen-Smit (1931) first discovered the active principle of this growth hormone which has been termed as auxin-a. The chemical identification of it shows that this substance is a nitrogen free compound known as auxenotriolic acid. Shortly afterwards, Kogl  et al (1934), isolated another compound, auxenolonic acid which they termed as auxin-b. Both auxin-a and auxin-b are light and heat stable and can be easily oxidised.

        In the same year Kogl et al discovered the third substance which they named as hetero-auxin whose chemical nature has been proved to be 3- indole acetic acid or IAA.

         Besides IAA, a number of naturally occurring auxins have been identified racently.

Different physiological effects of Auxins:


(A) ON GROWTH:
The most characteristic effect of auxin is to promote growth by cell enlargement. The action of auxin in controlling growth appears to be a complex of many functions.

(B)  ORGAN DIFFERENTIATION:
 Auxins and other plant hormones not only affect the growth by elongation, but also affect the morphological type of growth. Thus by applying auxins, a young stem may develop cluster of cells differentiated into callus roots, vegetative buds and sometimes even flowers.

(C) FRUIT DEVELOPMENT:
 The role of auxin in the normal process of fruit set has been thoroughly investigated by Gustafson (1936). This discovery of fruit set by auxin treatment has a great value in commercial practice, in producing fruit set when natural set is difficult.

2. GIBBERELLINS: 

(A)DORMANCY AND SEED GERMINATION:
 The gibberellins are involved in the phenomena of dormancy, flowering and fruiting. In many plants dormancy can be broken with gibberellins.

(B) SEX EXPRESSION:
Gibberellins have a definite role in the sex expression of Cucurbitaceae (Mitchell and Wittwer, 1962). Gibberellins causes formation of male flowers whereas, low concentration of gibberellins favours the formation of female flowers.

(C) PARTHENOCARPIC FRUIT-SET:
Gibberellins also induce parthenocarpic fruit-set in a number of plants and such fruits are usually seedless.

3. CYTOKININS: 

Cytokinins cause changes in the protein and nucleic acid components of tissues which are the basis of the cytokinins effects on cell division (Guttaman, 1956) as well as growth and mobilization actions. Cytokinins bring about the mobilization of various solutes including amino acids, auxins, and phosphorus.