They are groups of plant hormones, widely distributed in higher plants and known to exert profound influences on the growth and development of plants. They exert their effect at very low concentrations and regulate a range of cell activities such as cell growth, cell division, differentiation, morphogenesis, dormancy, and senescence. Following the discovery of their crucial role in shooting and rooting of plant cells in cultures, they became essential additions to culture media. They are used in vegetative propagation, micropropagation, and in tissue culture. Their requirements in culture vary with tissue and their endogenous levels in cells. Variations in relative proportions of plant growth regulators in tissue culture media affect metabolism. Also there are several interesting reports of enhanced secondary metabolite production due to modulation of growth hormone levels of the media.

Auxins

Auxins are a group of compounds associated with stem and internode elongation, tropism, apical dominance, adventitious root production, fruit setting, rooting, and in similar functions in whole plants. They promote production of other hormones. Indole acetic acid (IAA) is a major auxin found in actively growing tissues with many of its potential precursors reported from several plants. In tissue culture, auxins at low concentrations are required for cell elongation and root initiation, and at high concentrations, they suppress morphogenesis. Several synthetic auxins are being widely used, and the most commonly used ones in tissue culture are IAA, Indole-3-butyric acid (IBA), and Naphthalene acetic acid (NAA) for rooting and along with cytokinin for shoot proliferation. 2, 4-Dichlorophenoxy acetic acid (2, 4-D) and 2, 4, 5 -trichlorophenoxyacetic acid are used for callus initiation and growth in somatic embryogenesis. Naphthoxy acetic acid (NOA) and p-chlorophenoxy acetic acid (p-CPA) are the other auxins used.

Auxins have been reported to enhance ergot alkaloid production in submerged culture and also anthraquinone production in Morinda citrifolia cell suspension cultures.