Plant growth regulators (PGRs) such as auxins, cytokinins, gibberellins, and abscissic acid are essential components of plant tissue culture media as they stimulate cell division and regulate plant growth and differentiation. Auxins induce cell division and elongation and promote root initiation at low concentrations or callus formation at high concentrations. Cytokinins promote shoot initiation and growth by stimulating cell division and modifying apical dominance. The ratio of auxins to cytokinins determines whether roots, shoots, callus, or embryos will form, with a higher auxin ratio leading to root initiation and a higher cytokinin ratio producing shoots. Gibberellins and abscissic acid are less commonly used

1. By SREEREMYA.S

2.  PGRs: stimulate cell division and hence
regulate the growth and differentiation of
shoot and roots on explants and embryos in
semisolid or in liquid medium cultures. The
four major PGRs used are auxins, cytokinin,
gibberellins and abscissic acid and their
addition is must to the culture medium.

3.  Auxins: induce cell division, cell elongation,
apical dominance, adventitious root
formation, somatic embryogenesis. When
used in low concentration, auxins induce root
initiation and in high, callus formation
occurs.

4.  Commonly used synthetic auxins are 1-
naphthaleneacetic acid (NAA). 2,4
dichlorophenoxyacetic acid (2,4-D), indole-3
acetic acid (IAA), indolebutyric acid (IBA) etc.
Both IBA and IAA are photosensitive so the
stock solutions must be stored in the dark.
2,4-D is used to induce and regulate somatic
embryogenesis.

5.  Cytokinins: promote cell division and
stimulate initiation and growth of shoots in
vitro. Zeatin, 6- benzylaminopurine (BAP),
kinetin, 2-iP are the frequently used
cytokinins. They modify apical dominance by
promoting axillary shoot formation.

6.  When used in high concentration, CK inhibits
root formation and induces adventitious
shoot formation. The ratio of auxin and
cytokinin in the culture decides
morphogenesis. If this ratio is high, leads to
embryogenesis, callus initiation and root
initiation whereas if ck/auxin is high, it gives
rise to axillary and shoot proliferation.

7.  Gibbrellins and abscissic acid: are lesser used
PGRs. Gibbrellic acid (GA3) is mostly used for
internode elongation and meristem growth.
Abscissic acid (ABA) is used only for somatic
embryogenesis and for culturing woody
species.