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Somatic Embryogenesis and Artificial Seeds
An embryo derived from a somatic cell of a plant rather than the zygote is known as somatic embryo. The process of production of somatic embryo is known as somatic embryogenesis. Micro propagation refers to the technique by which different meristem such as root, shoot, somatic embryos are utilized to produce new plants from them under controlled environmental conditions or in vitro. The regeneration refers to development of organised structures like root or shoot from culture cells or tissues. Somatic embryos when used in micropropagation and differentiation leads to formation of a whole new plant in vitro.

Mechanism of development of somatic embryos:
The development of somatic embryo by somatic embryogenesis occurs from a single cell through micropropagation of meristematic cell also known as explant. This single cell undergoes rapid division and differentiation to form a cluster of cells. This gets isolated by breaking of cytoplasmic connections between different cells of the cell mass. The highly active mass of meristematic cells of somatic embryos undergoes rapid changes by differentiation. It develops through different phases like globular, round shaped, heart shaped, torpedo shaped and finally cotyledonary stages to form a somatic embryo.

A somatic embryo consists of a shoot plumule and a root radicle. Often it is seen that, in a developing somatic embryo the shoot plumule is seen propagating outwards and the radicle towards the center of callus or cell mass. Thus in most cases, the developing embryo develops only shoot and undergo shoot regeneration. In order to regenerate root, this has to be induced with growth factors which promote root regeneration. The somatic embryos which are produced by the following were seen to be associated with abnormal developmental features like cotyledons which are more in number, abnormally shaped. This problem can be prevented by the addition of abscisic acid in the culture medium used to propagate somatic embryos.

The development of somatic embryos goes through various processes like somatic embryo induction phase where the induction is initiated in auxin medium to produce a mass of cells. This is then transferred into developmental medium low in auxin concentration where the cell develops into cotyledons. The cotyledons so produced enter a somatic embryo conversion phase to form embryos. These are mostly subjected to a maturation phase so that the somatic embryo formed gets stable.
In certain cases, the micro propagation of plants is possible only from somatic embryos such as oil palm, date palms etc. It is also recommended to micro propagate plants affected with virus of plant body by somatic embryos only.

The production of somatic embryos gets affected by factors like:
Growth regulators:

The presence of certain growth regulators is seen to influence the growth of somatic embryos. In most cases, the growth medium is added with an auxin which promotes development of somatic embryos. The presence of auxins promotes hypermethylation of DNA leading to the totipotency of the cell. In certain plants the presence of auxin is known to trigger the development of cells so that they divide asymmetrically and the daughter cells produced by each division sticks together to form a clump of cells known as proembryogenic masses or embryogenic clumps. These can be differentiated and each cell can be developed to produce a somatic embryo. Totipotent cells, which have the ability to divide and differentiate, release some glycoproteins into the medium when they differentiate. This glycoprotein when isolated and added to medium of cell cluster was found to initiate differentiation leading to somatic embryogenesis. These glycoproteins produced are known as arabinogalactan proteins.

Several other factors are also known to influence the growth of somatic embryos like nitrogen source, genotype of explant used, high potassium levels, dissolved oxygen level and even the presence of cytokinin in certain species.

Production of artificial seed:
Somatic embryo can be used to produce artificial seeds. It consists of a bead of gel containing somatic embryo along with nutrients, growth regulators, pesticides, antibiotics, all the necessary requirements needed by the embryo to develop into a new plantlet. It is produce by mainly two systems

Desiccated system: This involves hardening of the somatic embryo in the maturation phase by adding polymer or treating them with abscisic acid. This is followed by drying or desiccation to produce a desiccated system.

Hydrated system: This involves coating the embryos in a gel with materials similar to sodium alginate. The corresponding process involves allowing sodium alginate to fall into a solution of calcium chloride. The drop before falling is inserted with embryos thus it falls into the solution forming a gel coat around the embryo.

The hydrated system is less stable and has to be planted soon. It also undergoes hydration when it comes in contact with atmosphere thus by this process it has made possible to produce seeds which can be transported , stored and even planted to produce plants when required.
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