Nature

Hormones are chemicals produced in one part of the body that has a target in another part in the body. While a large number of hormones are present in animal bodies, here are five main classes of hormones in plants Auxin, Cytokinin, Gibberellin, Ethylene and Abscisic acid. These plant hormones, together with enzymes, serve to help regulate normal body functioning in plants and one important aspect of this is obtaining food or nutrients. Overall plant hormones affect flowering, aging, root growth and many other conditions associated with plant physiology. It is also important to take note that only very small concentrations of plant hormones can produce major changes in growth.

Three Major Divisions of the Plant Hormones
Cowan (n.d.) states that the five main plant hormones can be classified into three main categories Growth hormones which include Auxin and Cytokinins Stress hormones which include Ethylene and Gibberellins and Survival hormones that include Abscisic Acid. Growth hormones are mainly located in young meristematic cells, stress hormones in mature cells, and survival in all cells (Cowan,n.d.).

    Apart from the five major groups and the three major categories, there are other hormones which are fairly new and only partly understood. They include Jasmonates, Brassinosteriods, and Salicylates (Cowan, n.d.).

The Five Main Plant Hormones
Auxins. The hormone Auxin is considered the major hormone involved in growth. It is initiated in any cell which has an excess of sugar and gases that are needed for survival during peak activity (Cowan, n.d.). This hormone is also the active ingredient of most rooting compounds into which stem cuttings are dipped during vegetative propagation (Plant hormones, n.d.).

Location. Auxin is found in the embryo of the seed, in young leaves and in the meristems of apical buds (Farabee, 2007).

Major Functions. Farabee (2007) states that auxin functions as a stimulant of cell elongation. It is also involved in gravitropism, phototropism, apical dominance as well as vascular differentiation. It also acts as an inhibitor of abscission before an abscission layer is formed. Auxin also stimulates the synthesis of ethylene. It also stimulates the formation of the flowers and the development of the fruit. Moreover, it induces the formation and growth of adventitious roots in stem cuttings.

Specific Chemical Forms. Kende and Zeevaart (1997) states that the primary form of Auxin found in plants is indole-3-acetic acid, or IAA. Other compounds that have auxin activity include phenyl acetic acid, indole-3-butryric acid and 4-chloro-IAA.

Cytokinins. Cytokinins are different from other hormones because they are both found in plants and animals. Cytokinins usually trigger mitosis that is why they are used in sterile media for growing plants from tissue culture (Plant hormones, n.d.).
Location. Cytokinins are synthesized in the roots and then transported to the other organs (Farabee, 2007).

Major Functions. Cytokinins first of all stimulate cell division, promote the growth of shoots, reverse apical dominance and delay leaf sequence (Farabee, 2007). They also delay aging and senescence in plants (Plant hormones, n.d.).

Specific Chemical Forms. Kende and Zeevaart (1997) states that naturally-occurring cytokinins in plants include N6-substituted adenine derivatives. There are also several bacteria that produce cytokinins, such as the Agrobacterium.
Ethylene. Ethylene is a plant hormone in the gaseous form. It is in fact a gas produced by ripe fruits. (Farabee, 2007).
Location. Ethylene is produced in the tissues of ripening fruits, in senescent leaves and flowers and in the nodes of stems (Farabee, 2007). There is often an increase in the ethylene production in plants in response to stress, and high concentrations of the plant hormone are often found at the end of a plants life (Plant Hormones, n.d.).
Major Functions. Ethylene functions for the stimulation of the ripening of fruits, sprouting of potato buds, seed germination, stem elongation in rice, flower formation in certain species, and leaf and flower senescence (Farabee, 2007). Ethylene causes epinasty, or the drooping of leaves. It also causes abscission, or the dropping of leaves especially during the fall (Plant Hormones, n.d.).
Specific Chemical Forms. Ethylene is an alkene with a molecular structure of H2CCH2 (Ethylene, 2009).

Gibberellins. Gibberellins are plant hormones that function for mitosis and germination of seeds. Gibberellic acid (GA) solution is even used to hasten the germination of certain seeds. Gibberellins are actually the first type of hormones ever to be discovered (Farabee, 2007).
Location. Gibberellins are located in the meristems of apical buds and roots as well as in young leaves and embryos (Farabee, 2007).

Major Functions. Gibberellins function for the stimulation of shoot elongation, stimulation of bolting and flowering in biennials, and the regulation of the production of hydrolytic enzymes in grains (Farabee, 2007). Gibberellins also cause root expansion (Cowan, n.d.).
Specific Chemical Forms. Gibberellins usually occur in the form of Gibberellic acid (GA) with the following structure (Gibberellins, 2009)


Abscisic Acid. Abscisic Acid, or ABA, is generally an inhibitor of plant growth. ABA also serves to indicate if environmental conditions are poor such as in the case of very low soil temperature that would hamper the growth of the roots. ABA activity would then lower and Auxins would go higher in order to cause sugar production to be used for growth (Cowan, n.d.).
Location. Generally, Abscisic Acid is found in the leaves, stems and green fruit of plants (Farabee, 2007).

Major Functions. The main function of Abscisic Acid is the stimulation of stomatal closure (Farabee, 2007). Abscisic Acid also induces dormancy and unlike Gibberellins, it prevents seeds from germinating. It also causes abscission of leaves, flowers, and fruits (Plant hormones, n.d.). Other functions of ABA include inducing seeds for the synthesis of storage proteins, inhibiting the action of Gibberellins on the stimulation of the de novo synthesis of -amylase, and inducing gene transcription especially for proteinase inhibitors as a response to wounding. This may explain the apparent role of Abscisic Acid in pathogen defense (Abscisic Acid, n.d.). ABA is above all considered a major player in helping the plant to adapt to stress caused by drought, freezing, and exposure to salinated soil (Abscisic Acid, 2009).
Specific Chemical Forms. Abscisic Acid has the molecular form C15H20O4 with the chemical structure (Abscisic Acid, 2009)