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Literature review
The essential function of auxins and cytokinins is to reprogram somatic cells that
were in a state of differentiation (GABA, 2004). Such reprogramming causes
dedifferentiation and then redifferentiation into a new developmental pathway (GABA,
2004).The mechanism of dedifferentiation is not understood (GABA, 2004).
The use of 2,4-D should be avoided, as it may induce mutations (PIERIK, 1997). This
growth regulator is however important in callus initiation for many species.
2,4-D is widely used for callus induction, while IAA (0.6-60 µM), IBA (2.5-15 µM) and
NAA (0.25-6 µM) are mainly used in root initiation (SMITH, 2000b; GABA, 2004).
Higher than optimum levels of auxins causes callus production and a reduction in
root growth and root quality (GABA, 2004). Combinations of auxins at low
concentrations can sometimes produce better results than using individual auxins
(GABA, 2004). High concentrations of auxins are sometimes necessary to induce
rooting (GABA, 2004). This can however have undesirable side effects such as
growth inhibition of induced roots (GABA, 2004). In such cases the elevated auxin
levels should be administered as a pulse treatment (GABA, 2004). To do this the
shoot is incubated with auxin for several days before it is transferred to a medium
with no plant growth regulators to allow root growth and development (GABA, 2004).
Somatic embryogenesis is typically induced by auxins, sometimes in combination
with cytokinins (GABA, 2004). 2,4-D is commonly used at this stage, although other
auxins can be used (GABA, 2004). Auxins induce the cells to become embryogenic
and promote subsequent repetitive cell division of embryogenic cells (GABA, 2004).
High concentrations of auxins prevent subsequent cell differentiation and embryo
growth (PIERIK, 1997).
2.9.3.2 Cytokinins
As the name suggests, cytokinins cause cell division (GABA, 2004). Such cell
division can lead to shoot regeneration in vitro, by stimulating the formation of shoot
apical meristems and shoot buds (PIERIK, 1997). This cell division caused by
cytokinins can also cause the production of undifferentiated callus (GABA, 2004). A
high concentration of cytokinins can cause the release of shoot apical dominance
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