Plants produce chemical compounds, called hormones, that act as chemical signals between cells and tissues in different parts of the plant. Some hormones stimulate growth activity in the plant, while others inhibit growth activity. Because they regulate growth, plant hormones are often called plant growth regulators.

There are currently five recognized groups of plant hormones:

  • Auxins,
  • Gibberellins,
  • Cytokinins,
  • Abscisic acid (ABA)
  • Ethylene.

Stimulatory Hormones:

Hormone Selected Key Functions Where Produced or Found in Plant
Auxins • stimulate cell division and elongation in stems and roots
• regulate cell expansion in plant responses to light and gravity
• developing leaves and seeds
• shoot tips
Cytokinins • stimulate cell division
• prevent ageing of leaves
• root tips
Gibberellins • stimulate cell elongation
• stimulate seed germination
• young shoots
• developing seeds

Inhibitory Hormones:

Hormone Selected Key Functions Where Produced or Found in Plant
Ethylene • promotes ripening of fruit • all parts, especially when under stress, ageing, or ripening
Abscisic acid • induces and maintains seed dormancy
• inhibits shoot growth
• closes stomata
• mature leaves
• plants under stress

Auxins: 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). 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: 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 (2 iP) and kinetin. Cytoknins often inhibit embryogenesis and root induction.

Gibberellins: 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: 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.


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