Five Recognized Groups of Plant Hormones

Plant growth regulators (also called plant hormones) are numerous chemical substances that profoundly influence the growth and differentiation of plant cells, tissues and organs. Plant growth regulators function as chemical messengers for intercellular communication . There are currently five recognized groups of plant hormones: auxins, gibberellins, cytokinins, abscisic acid (ABA) and ethylene. They work together coordinating the growth and development of cells. Ethylene is mainly involved in abscission and flower secscence in plants and is rarely used in plant tissue culture. In addition to the five principal growth regulators, two other groups sometimes appear to be active in regulating plant growth, the brassinosteroids and polyamines.

Auxins stimulate cell elongation and influence a host of other developmental responses, such as root initiation, vascular differentiation, tropic responses, apical dominance and the development of auxiliary buds, flowers and fruits. Auxins are synthesized in the stem and root apices and transported through the plant axis. The principal auxin in plants is indole-3-acetic acid (IAA). Several other indole derivatives, all as precursors to IAA, are known to express auxin activity, probably by converting to IAA in the tissue. Auxins in plant tissue culture are used to induce callus from explants , and cause root and shoot morphogenesis . Auxins are often most effective in eliciting their effects when combined with cytokinins.

Cytokinins are able to stimulate cell division and induce shoot bud formation in tissue culture. They usually act as antagonists to auxins. (Cytokinins are N6 substituted derivatives of the nitrogenous purine base adenine.) Cytokinins most used in tissue culture include zeatin, adenine, 6-(g,g-dimethylallylamino)purine and kinetin. Cytoknins often inhibit embryogenesis and root induction.

The main effect of gibberellins in plants is to cause stem elongation and flowering. They are also prominently involved in mobilization of endosperm reserves during early embryo growth and seed germination. Gibberellins are an extensive chemical family based on the ent-gibberellane structure. There exit over 80 different gibberellin compounds in plants but only giberrellic acid (GA3) and GA4+7 are often used in plant tissue culture. In tissue culture, gibberellins are used to induce organogenesis, particularly adventitious root formation.

Abscisic Acid:
Abscisic acid (ABA) in plants is a terpenoid involved primarily in regulating seed germination, inducing storage protein synthesis and modulating water stress. In plant tissue culture, it is used to help somatic embryogenesis, particularly during maturation and germination.

Ethylene is a simple gaseous hydrocarbon with the chemical structure H2C=CH2. Ethylene is apparently not required for normal vegetative growth. However, it can have a significant impact on development of root and shoots. Usually, ethylene is not used in plant tissue culture.

Class Function(s) Practical uses
Auxins Shoot elongation Thin tree fruit, increase rooting and flower formation
Gibberellins Stimulate cell division and elongation Increase stalk length, increase flower and fruit size
Cytokinins Stimulate cell division Prolong storage life of flowers and vegetables and stimulate bud initiation and root growth
Ethylene generators Ripening Induce uniform ripening in fruit and vegetables
Growth inhibitors Stops growth Promote flower production by shortening internodes
Growth retardants Slows growth Retard tobacco sucker growth