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Literature review
organ is then produced on this medium. Successful determination is partially
dependent on the chemical and physical environment to which they have been
exposed. The result of failure of explant tissues to express totipotency is a failure of
the explant tissues to achieve the state of competence for induction. This makes
investigation of the effects of physical and chemical parameters difficult. The use of
biochemical or genetic markers that can clearly indicate the developmental
disposition of the primary explant tissue have not yet been discovered (SCHWARTZ
et al., 2004).
In the next phase the morphological differentiation and development of the nascent
organ occurs (SCHWARTZ et al., 2004). Organ initiation involves a rapid shift in
polarity followed by a smoothing of this shift into a radially symmetrical organization
and the concurrent growth along the new axis to form a characteristic bulge
(SCHWARTZ et al., 2004). There is not an absolute certainty as to which tissues are
involved and the number of cells involved in meristem initiation (SCHWARTZ et al.,
2004).
The initiation of adventitious roots occurs in four stages (SCHWARTZ et al., 2004). A
meristematic locus is first formed by the dedifferentiation of a stem or other cells.
These cells then multiply to form a spherical cluster. Further cell multiplication occurs
with the initiation of planar divisions to form a recognizable bilateral root meristem.
Lastly, the cells located in the basal part of the developing meristem elongate,
resulting in the eventual emergence of the newly formed root (SCHWARTZ et al.,
2004). The production of a functional adventitious root system depends on the
selection of a microcutting of the appropriate developmental stage and the ability of
the in vitro environment to initiate the sequence of events described above
(SCHWARTZ et al., 2004).
2.9.8 Somatic embryogenesis
In plants, morphologically and functionally correct nonzygotic embryos can arise from
an array of cell and tissue types at a number of different points within both the
gametophytic and sporophytic phases of the plant life cycle (GRAY, 2005). Somatic
(or asexual) embryogenesis involves the formation of embryo-like structures, which
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