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2.9.6 Callus culture
Literature review
Explants, when cultured in the appropriate medium, usually with both auxin and
cytokinin, give rise to a mitotically active, but unorganized mass of cells. It is thought
that, under the right conditions, any plant tissue can be used as an explant (SLATER
et al., 2003). Callus culture concerns the initiation and continued proliferation of
undifferentiated parenchyma cells from explant tissue on clearly defined semi-solid
media (BROWN, 1990).
Callus initiation is the first step in many tissue culture experiments (BROWN, 1990;
SMITH, 2000b). In vivo, callus is a wound tissue produced in response to injury or
infestation (BROWN, 1990; MINEO, 1990; SMITH, 2000b). Not all the cells in an
explant contribute to callus formation (SMITH, 2000b). Only certain callus types,
which are competent to regenerate organised structures, display totipotency (SMITH,
2000b).
The level of plant growth regulators is a major factor that controls callus formation in
the culture medium (BROWN, 1990; SMITH, 2000b). The correct concentration of
plant growth regulators depends on the species, individual and explant source
(SMITH, 2000b). Other culture conditions such as light, temperature and media
composition are also important for callus formation and development (SMITH,
2000b).
Explants can be taken from various plant organs, structures and tissues (MINEO,
1990). Young tissues of one or a few cell types are most often used as explants
(MINEO, 1990). The pith cells of a young stem are regarded as a good source of
explant material for callus initiation (MINEO, 1990).
Callus growth is maintained, provided that the callus is subcultured onto a fresh
medium periodically. During callus formation there is a degree of de-differentiation in
both morphology (usually unspecialised parenchyma cells) and metabolism. As a
consequence most plant cultures lose their ability to photosynthesise (SLATER et al.,
2003). This means that the metabolic profile does not match that of the donor plant
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