somatic embryo genesis

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Somatic embryogenesisFrom Wikipedia, the free encyclopedia

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Somatic embryos are mainly producedin vitro and for laboratory purposes, using either solid or liquid

nutrient media which contain plant growth regulators (PGRs). The main PGRs used are auxins but can

contain cytokinin in a smaller amount.[1]Somatic embryogenesis is a process where a plant or embryo is

derived from a single somatic cellor group of somatic cells. This is in contrast tozygotic embryogenesis,

where ,in diploid species, twohaploid cells combine to form onediploid cell. Somatic embryos are

produced when somatic cells are restructured through a series of morphological and biochemical changes

in the embryogenic pathway.[2] Development of somatic embryos is not that different from zygotic embryos.

[3] Three examples of somatic embryogenesis from nature are ovules in Paeonia and on the leaves

ofAspleniumandKalanchoe. Shoots and roots are monopolar while somatic embryos are bipolar, allowing

them to form a whole plant without culturing on multiple media types. Somatic embryogenesis has served

as a model to understand the physiological and biochemical events that occur plant developmental

processes as well as a component to biotechnological advancement.[4] The first documentation of somatic

embryogenesis was by Steward et al in 1958 and Reinert in 1959 with carrot cell suspension cultures.[5][6]

Switchgrass somatic embryos

Contents

[hide]

1 Direct and indirect embryogenesis

2 Plant regeneration via somatic

embryogenesis

3 Factors influencing somatic embryogenesis

4 Uses of somatic embryogenesis

5 Problems associated with somatic

embryogenesis

6 Tracking and Fate Maps
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7 Angiosperms

8 Gymnosperms

9 See Also

10 References

11 External links

[edit]Direct and indirect embryogenesis

Somatic embryogenesis has been described to occur in two ways: directly or indirectly[7] Direct

embryogenesis occurs when embryos are started directly from explant tissue creating an identical clone

while indirectly occurs from unorganized tissue (callus).

[edit]Plant regeneration via somatic embryogenesis

Plant regeneration via somatic embryogenesis occurs in five steps: initiation of embryogenic cultures,

proliferation of embryogenic cultures, prematuration of somatic embryos, maturation of somatic embryos

and plant development on nonspecific media. Initiation and proliferation occur on a medium rich in auxin,

which induces differentiation of localized meristematic cells. The auxin typically used is 2,4-D. Once

transferred to a medium with low or no auxin, these cells can then develop into mature embryos.

Germination of the somatic embryo can only occur when it is mature enough to have functional root and

shoot apices.[8]

[edit]Factors influencing somatic embryogenesis

Factors and mechanisms controlling cell differentiation in somatic embryos are relatively ambiguous.

Certain compounds excreted by plant tissue cultures and found in culture media have been shown

necessary to coordinate cell division and morphological changes.[9] These compounds have been identified

by Chung et al.[10] as various polysaccharides, amino acids, growth regulators, vitamins, low molecular

weight compounds and polypeptides. Several signaling molecules known to influence or control the

formation of somatic embryos have been found and include extracellular proteins, arabinogalactan proteins

and lipochitooligosaccharides. Temperature and lighting can also affect the maturation of the somatic

embryo.

[edit]Uses of somatic embryogenesis

Plant transformations

Masspropagation[11]

[edit]Problems associated with somatic embryogenesis

High chance of mutations

Difficult method
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Loss of regenerative ability

High percentage of albino shoots during regeneration

Not possible with all plant species and must be optimized for each species and its use

[edit]Tracking and Fate Maps

Understanding the formation of a somatic embryo through establishment of morphological and molecular

markers is important for construction of a fate map. The fate map is the foundation in which to build further

research and experimentation. Two methods exist to construct a fate map: synchronous cell-

division and time-lapse tracking. The latter typically works more consistently because of cell-cycle-

altering chemicals and centrifuging involved in synchronous cell-division.[2]

[edit]Angiosperms

Embryo development in angiosperms is divided into several steps. The zygote is divided asymmetrically

forming a small apical cell and large basal cell. The organizational pattern is formed in the globular stage

and the embryo then transitions to the cotyledonary stage.[12]Embryo development differs in monocots and

dicots. Dicots pass through the globular, heart-shaped, and torpedo stages while monocots pass through

globular, scuetellar, and coleoptilar stages.[13]

Many culture systems induce and maintain somatic embryogenesis by continuous exposure to2 ,4 -

dichlorophenoxyacetic acid. Abscisic acid has been reported to induce somatic embryogenesis in

seedlings. Aftercallusformation, culturing on a low auxin or hormone free media will promote somatic

embryo growth and root formation. Inmonocots, embryogenic capability is usually restricted to tissues with

embryogenic or meristematic origin. Somatic cells of monocots differentiate quickly and then lose mitotic

and morphogenic capability. Differences of auxin sensitivity in embryogenic callus growth between different

genotypes of the same species show how variable auxin responses can be.[14]

Carrot Daucus carota was the first and most understood species with regard to developmental pathways

and molecular mechanisms.[2]Time-lapse tracking by Toonen et al (1994) showed that morphology of

competent cells can vary based on shape and cytoplasm density. Five types of cells were identified from

embryonic suspension: spherical cytoplasm-rich, spherical vacuolated, oval vacuolated, elongated

vacuolated, and irregular shaped cells. Each type of cell multiplied with certain geometric symmetry. They

developed into symmetrical, asymmetrical, and aberrantly-shaped cell clusters that eventually formed

embryos at different frequencies.[15]This indicates that organized growth polarity do not always exist in

somatic embryogenesis.[2]

[edit]Gymnosperms

Embryo development ingymnosperms occurs in three phases. Proembryogeny includes all stages prior

tosuspensorelongation. Early embryogeny includes all stages after suspensor elongation but before root

meristem development. Late embryogeny includes development of root and shoot meristems.[12] Time-lapse

tracking in Norway Spruce Picea abies revealed that neither single cytoplasmic-rich cells nor vacuolated
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cells developed into embryos. Proembryogenic masses (PEMs), an intermediate between unorganized

cells and an embryo composed of cytoplasmic-rich cells next to a vacuolated cell, are stimulated

with auxinand cytokinin. Gradual removal of auxin and cytokinin and introduction ofABA[disambiguation needed

] will allow an embryo to form.[2]Using somatic embryogenesis has been considered for mass production of

vegetatively propagated pine clones and cryopreservation ofgermplasm. However, the use of this

technology for reforestation and breeding of pine trees is in its infancy
http://en.wikipedia.org/wiki/Auxinhttp://en.wikipedia.org/wiki/Cytokininhttp://en.wikipedia.org/wiki/ABAhttp://en.wikipedia.org/wiki/ABAhttp://toolserver.org/~dispenser/cgi-bin/dab_solver.py?page=Somatic_embryogenesis&editintro=Template:Disambiguation_needed/editintro&client=Template:Dnhttp://en.wikipedia.org/wiki/Germplasmhttp://toolserver.org/~dispenser/cgi-bin/dab_solver.py?page=Somatic_embryogenesis&editintro=Template:Disambiguation_needed/editintro&client=Template:Dnhttp://en.wikipedia.org/wiki/Auxinhttp://en.wikipedia.org/wiki/Cytokininhttp://en.wikipedia.org/wiki/ABAhttp://toolserver.org/~dispenser/cgi-bin/dab_solver.py?page=Somatic_embryogenesis&editintro=Template:Disambiguation_needed/editintro&client=Template:Dnhttp://en.wikipedia.org/wiki/Germplasm

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