Organogenesis and Somatic embryogenesis    

The development of an adult organism from a single cell (zygote) is the result of the integration of cell division and cell differentiation. Isolated cells from differentiated tissues are generally non dividing and quiescent but to express totipotency, the differentiated cells first undergoes differentiation and then re-differentiation. The phenomenon of a mature cell reverting to a meristematic state and forming a dedifferentiated callus tissue is called dedifferentiation whereas the ability of differentiated cell to form a whole plant/plant organs is termed as re-differentiation.

Organogenesis

            Organogenesis is the development of adventitious organs or primordia (embryoid) from undifferentiated cell mass (callus) in tissue culture. It is controlled mostly by a balance between cytokinin and auxin. A relatively high ratio of auxin:cytokinin induces root formation in callus tissues whereas, a low ratio induces shoot formation. Caulogenesis is a type of organogenesis by which only adventitious shoot bud initiation takes place in the callus tissue.

            When it is applicable for root, it is known as rhizogenesis. Anomalous structures when develop during organogenesis is called organoids. The localizecd meristematic cells on a callus which give rise to shoots and/or roots is termed as meristemoids.

Non-zygotic embryogenesis

Somatic embryogeneisis

            Somatic embryogenesis is the process of a single cell or a group of cells intiating the developmental pathway that leads to reproducible regeneration of non-zygotic embryos capable of germinating to form complete plants. Under natural conditions this pathway is not normallsy followed, but from tissue cultures somatic embryogenesis occurs most frequently and an alternative to organogenesis for regeneration of whole plants. Adherence to this pattern of morphogenesis depends on co-ordinated behaviour of a cells or cells to establish polarity as a unit and therby initiate gene action sequenctially specific to emerging tissue regions. Non-zygotic embryos have been shown to be functionally equivalent to zygotic embryos.

The bipolar structure of the somatic embryo contains both shoot and root meristems. Generally induction of somatic embryogenesis in most species requires a high concentration of auxin, usually 2,4-D, in the culture.   As the embryos (both zygotic or non-zygotic) develop, they progress through the distinct structural steps of the globular, heart, torpedo, cotyledonary, and mature stages.

Induction Factors

Ø  Effects of gene expression

Ø  Effect on intercellular interactions

Ø  Role of Cytokinins

Ø  Miscellaneous factors: a) Genotype and explant characteristics, b) Medium Components, c) Culture environment d) Bacterial compounds

Synthetic seeds:

Synthetic seeds /Artificial seeds are the living seed like structure derived from Somatic embryo in vitro after encapsulation by a hydro gel. The preserved embryoids are termed as synthetic seeds. Such seeds are contaminated with microbes and desiccate quickly when subjected to field conditions. So, encapsulated in Calcium alginate.

Steps

Induction of SE

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Maturation of SE

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Encapsulation of SE

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Evaluation of Embryoid and plant conversion

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Planting

Procedure

Somatic embryos are mixed with 3 % sodium alginate solution and the mixture is added drop by drop into CaCl2 solution with a pipette. After 30 to 60 min, the drops are gelled completely. Those with a single embryo are collected. Nutrients and/or pesticides may be added to the alginate solution.

High costs of production and very low field germination are the main reasons why the artificial seeds are not widely used until now.