what do you understand by micropropagation and write the methods involved with suitable diagrams
Answers
Multiplication of genetically identical copies of a cultivar by asexual reproduction is called clonal propagation. The in vivo clonal propagation is often difficult, expensive and even unsuccessful. Tissue culture method offers an alternative way of clonal propagation which is popularly known as micro-propagation.
(i) Multiplication by Meristematic Tissue of Axillary and Apical Shoots:
Axilly and apical shoots contain quiescent or active meristems depending on the physiological state of the plant. When these shoot tips are cultured on a basal medium containing no growth regulators, these typically develop into single seedling like shoots with strong apical dominance.
On the contrary, when the shoots of the same explant material are grown on culture media containing cytokinin, axillary shoots develop precociously which proliferate to form clusters of secondary and tertiary shoots. These clusters when subdivided and transferred onto fresh medium again these will form similar clusters.
This subdivision process may be continued indefinitely when provided with basic nutrients. About 5-10 multiplication rates on 4-8 weeks of micro-propagation cycle may ultimately lead to extremely impressive clonal propagation range of 0.1-3.0 x 106 within a year.
(ii) Multiplication by Adventitious Shoots:
Axillary buds are “preformed meristems” present at the leaf axils, whereas adventitious buds may arise from any plant structures and this regeneration is often dependent on the presence of organised plant tissue. The explants may be stems, internodes, leaf blades, cotyledons, root elongation zone, bulb, corms, tubers, rhizomes, etc.
If these explants are induced by using appropriate level of growth regulators in the medium they will form the meristematic zone which regenerates multiple shoots on a suitable culture medium. High number of adventitious shoots may arise only from a single epidermal cell.
Continuous propagation by adventitious shoot proliferation from bulbs and corms can be achieved by cultivating two vertically split piece of shoot bases. Clusters of shoots may develop from the abaxial surfaces of developing leave and scales. Trimming of the shoot apices is a good approach which has been found to ensure continuously productive cultures of different hybrid plants for indefinite period.
(iii) Multiplication by Adventitious Embryo Formation:
It is another useful approach followed for many important plant species. Adventitious embryos can arise directly from a group of cells within the original explants or from primary embryoids.
There are many plant species which develop embryos in vivo from diverse kind of explants e.g. the orchid leaf tips produce large number of embryoids whereas Citrus and Mangifera have shown to develop polyembryos from nucellar tissue.
These adventitious embryoids are diploid in nature and can be used as clonal material for micro-propagation. In in vitro similarly the adventitious embryos originated from different explants can be good materials for clonal propagation.
(iv) Multiplication through Callus Culture:
Direct plantlet formation from the explants in culture is more desirable in case of micro- propagation or clonal propagation. But shoot formation may occur through organogenesis or embryogenesis from the callus produced from the explant.
The limitation of this system is that the callus cells are not genetically stable so it cannot be called as a single clone and this process is more time consuming. Also the plant regeneration capacity may decline due to the genetic unstable condition.
In case of some plant species genetically stable calli have also been derived, in these types of calli slow growing meristematic zones are formed from the peripheral layer. The meristematic layers invariably comprise of diploid cells expressing totipotency. These types of calli can be subdivided into smaller parts and which may produce multiple shoots from each.
