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Growth Regulators - Four broad classes of growth regulators, namely auxins, cytokinins, gibberellins, and abscisic acid, are important in tissue culture. The growth, differentiation and organogenesis of tissues become feasible only on the addition of one or more of these classes of hormones to a medium.
The ratio of hormones required for root or shoot induction varies considerably with the tissue, which seems directly correlated to the quantum of hormones synthesized at endogenous levels within the cells of the explant.
i) Auxins: Media are supplemented with various auxins: 1 H indole-3-a,cetic acid (IAA), 1-naphthaleneacetic acid (NAA), 1 H indole-3-butyric acid (ISA), 2,4-dichlorophenoxyacetic acid (2,4-0), and naphthoxyacetic acid (NOA). IAA occurs naturally in the plant tissues.

There are other auxins which have proven particularly effective in plant cell culture. These include 4 chlorophenoxyacetic acid (4-CPA) or p-chlorophenoxyacetic acid (PCPA), 2, 4, 5-trichlorophenoxyacetic acid (2,4,5- T), 2-methyl-4-chlorophenoxyacetic acid (MCPA), 4,amino-3,5,6-trichloropicolinic acid (picloram), and 3, 6-dichloro-2-methoxybenzoic acid (dicamba).
A common feature of auxins is the property of inducing cell division. In nature the hormones of this group are involved with such activities as elongation of stem, internodes, tropism, apical dominance, abscission and rooting.

ii) Cytokinins: Cytokinins are adenine derivatives mainly concerned with cell division, modification of apical dominance and shoot differentiation in the tissue culture. The most frequently used cytokinins are 6-benzylaminopurine (SAP) or 6-benzyladenine (SA), 6+t-dimethylaminopurine or 2-N6_(P2-isopentenyl) adenine,(2-IP) N-(2-furfurylamino)-1-H-purine-6-amine (kinetin), and 6-(4-hydroxy-3-methyltrans-2-butanylamino) purine (zeatin).
Zeatin and 2-IP are naturally occurring cytokinins while SA and kinetin are synthetically derived cytokinins. They are generally dissolved in dilute HCI or NaOH. It was recently discovered that N,N-diphenylurea(OPU), thidiaziron, N-2­chloro-4-puridyl-N-phenylurea (CPPU), and other derivatives of diphenylurea show cytokinin type activity.

The ratio of auxins and cytokinins is important with respect to morphogenesis in the culture system. For embryogenesis, callus initiation, and root initiation the requisite ratio of auxins to cytokinin is high, while the reverse leads to axillary and shoot proliferation. The mechanism of cytokinin action is not clearly understood although there are reports about the presence of compounds with cytokinin activity in transfer RNA (tRNA). Cytokinins have also been shown to activate RNA synthesis and to stimulate protein and enzyme activity in certain tissues.

iii) Gibberellins and Abscisic Acid: These growth compounds are occasionally used in tissue culture. In some species these hormones are required to enhance and in others to inhibit growth. GA is the .most common gibberellin used of the over 20 known ones. It promotes the growth of cell cultures at low density, enhances callus growth and induces dwarf or stunted plantlets to elongate. Abscisic acid (ASA) in the culture medium either stimulates or inhibits callus growth depending on the species.
 

 
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