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Somatic Embryogenesis - A somatic embryo (SE) is an embryo derived from a somatic cell, other than zygote, usually on culture in vitro, and the process is known as somatic embryogenesis. In contrast, embryos developing from zygotes are called zygotic embryos or often simply embryos, while those derived from pollen are known as pollen embryos or androgenetic embryos. By 1993, somatic embryogenesis was reported from over 100 species.
Developmental Pattern of SEs. SEs generally originate from single cells, which divide to form a group of meristematic cells. Usually, this multicellular group becomes isolated by breaking cytoplasmic connections with the other cells around it and subsequently by cutinization of the outer walls of this differentiating cell mass.
The cells of meristematic mass continue to divide to give rise to globular (round ball-shaped), heart-shaped, torpedo and cotyeledonary stages. In general, the essential features of SE development, especially after the globular stage, are comparable to those of zygotic embryos.

Somatic embryos are bipolar structures in that they have a radicle and a plumule. The radicular end is always oriented toward the centre of callus or cell mass, while the plumular end always sticks out from the cell mass. In contrast, a shoot bud is monopolar as it does not have a radicular end. In many SEs, radicle is suppressed so that they often do not produce roots; in such cases, roots have to be regenerated from the shoots produced by germinating SEs.
SEs often show abnormal developmental features, e.g., 3 or more cotyledons, bell shaped cotyledon, larger size etc.; these problems are often overcome by the presence of ABA or a suitable concentration of mannitol.
In some species, normal looking somatic embryos are formed, but they fail to germinate. Somatic embryogenesis is influenced by several factors, e.g.
(1) GRs
(2) nitrogen source,
(3) explant,
(4) genotype and
(5) others

Finally, transformed cells produce opines, such as octopine, and nopaline, chemicals formed from two amino acids. Arginine and alanine form octopine while arginine and glutamine produce nopaline. Octopine and nopaline are not found in normal plant cells.

There are two more or less distinct types of crown galls. The first type grows as a relatively unorganized callus on artificial medium and on host plants. Cells of this type produce octopine. The second type grows as a callus containing green islands. The islands show a variable amount of organization, inducing production of multiple shoots. Cells of this type produce nopaline.

The two types are associated with different strains of A. tumefaciens. There is also a related disease ("hairy root disease") in which infected tissues proliferate in root tissue and produce an opine. This disease is associated with the bacterium A. rhizogenes.

Formation of opines explains the ecological significance of tumor formation. Each strain of Agrobacterium synthesizes enzymes (permease, dehydrogenase) that allow it to metabolize the specific type of opine formed by the tumor it induces. Thus by stimulating the plant to form opines, the bacteria insure themselves a supply of nutrients specifically designed for them.

Growth of the infected tissue is important because it increases the amount of opine forming tissue. It is possible, too, that the physical characteristics of the tumors parenchymal cells with extensive intercellular spaces provide a good habitat for the bacteria.

Early studies of the mechanism of "transformation" demonstrated that tumorous tissue remained transformed even in the absence of infecting bacteria. (The bacteria could be removed with antibiotics such as penicillin.).
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