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The below mentioned article provides a study note on organogenesis.
In plant tissue culture, organogenesis is a process of differentiation by which plant organs like roots, shoots, buds etc. are formed from the unusual points of origin of an organized explants where a preformed meristem is lacking. Plant development through organogenesis is the formation of organs either de novo or adventitious in origin and plant regeneration via organogenesis is a mono-polar structure.
Plant production through organogenesis can be achieved by two modes:
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(i) Emergence of adventitious organs directly from the explant.
(ii) Organogenesis through callus formation with de novo origin (Fig. 18.1).
Direct Adventitious Organ Formation:
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Every cell of plant is derived from the original zygote through mitotic divisions containing the complete genome. The formation of adventitious buds depends on the reactivation of genes concerned with embryonic phase of development. The addition of growth regulators like auxin and cytokinin in the medium is required to initiate shoot formation from different kinds of tissue explant.
Adventitious in vitro regeneration may give a much higher rate of shoot production than is possible by proliferating axillary shoots. This technique is much used for multiplication in micropropagation system (Fig. 18.2A.
In suitable medium supplemented with growth hormones the somatic tissues of higher plants are capable of regenerating adventitious buds/shoots. These buds are formed directly from a plant organ or any piece of tissue without forming any callus structure. This type of organogenesis is mostly found in herbaceous plants.
Promotion of bud formation by cytokinin occurs in several plant species, though the requirement of exogenous cytokinin and auxin in the process varies with the tissue system of different species. In some plant species, the adventitious buds are produced during vegetative reproduction.
Organogenesis through Callus Formation:
Sometimes plant regeneration from cultured explants (cotyledons, hypocotyl, stem, leaf, shoot apex, shoot, root, young inflorescence, embryos, etc.) involves the initiation of basal callus formation and then shoot bud differentiation (Fig. 18.2B). For different species different kinds of explants may be necessary for successful plant regeneration.
Though the explants with mitotically active cells i.e., meristems, shoot tips, axillary buds, immature leaf, immature embryos are good for callus initiation and also can be used successfully to initiate plantlets through organogenesis.
Two modes of cell culture are generally used for organogenic path:
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(i) The cultivation of cell clusters on a solid medium;
(ii) The cultivation of cell suspensions in liquid medium.
Medium and Growth Regulators Requirement:
The application of growth regulators are critical for morphogenesis, which varies greatly depending on the tissue type. Callus tissue is comprised of wide range of cell types, the meristematic cells are interspersed within the vacuolated cells. During sub-culturing the meristematic cells may be favoured by the medium composition and organized growth of meristematic tissue may lead to formation of meristemoids.
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In these structures the vascularization starts due to appearance of tracheidal cells in the callus which ultimately initiates the formation of shoots and roots. Lowering the auxin and increasing the cytokinin concentration is traditionally performed to induce shoot organogenesis from callus (Fig. 18.3 A).
Auxins alone or in combination with low concentration of cytokinin are important in the induction of root primordia (Fig. 18.3B). Organogenesis can be induced in either cell suspension or callus culture cells; transfer of cells from callus forming medium to regeneration medium and then continued sub-culturing helps in organ formation.
Organogenic differentiation is an outcome of the process of dedifferentiation followed by re-differentiation of cells. Dedifferentiation favours unorganized cell growth and the resultant developed callus has meristems randomly divided.
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Most of these meristems, if provided appropriate “in vitro” conditions, would re-differentiate shoot buds and roots. The whole plant regeneration from cultured cells may occur either through shoot-bud differentiation or somatic embryogenesis. All these events establish the totipotency of somatic cells.
