Somatic Embryogenesis
Encyclopedia
Somatic embryos are mainly produced in vitro
and for laboratory purposes, using either solid or liquid nutrient media which contain plant growth regulators (PGR’s). The main PGRs used are auxins but can contain cytokinin
in a smaller amount. Somatic embryogenesis is a process where a plant or embryo is derived from a single somatic cell
or group of somatic cells. This is in contrast to zygotic embryogenesis
, where ,in diploid species, two haploid cells combine to form one diploid cell. Somatic embryos are produced when somatic cells are restructured through a series of morphological and biochemical changes in the embryogenic pathway. Development of somatic embryos is not that different from zygotic embryo
s. Three examples of somatic embryogenesis from nature are ovules in Paeonia and on the leaves of Asplenium
and Kalanchoe
. 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. The first documentation of somatic embryogenesis was by Steward et al in 1958 and Reinert in 1959 with carrot cell suspension cultures.
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.
Many culture systems induce and maintain somatic embryogenesis by continuous exposure to 2,4-dichlorophenoxyacetic acid
. Abscisic acid
has been reported to induce somatic embryogenesis in seedlings. After callus
formation, culturing on a low auxin or hormone free media will promote somatic embryo growth and root formation. In monocots, 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.
Carrot Daucus carota was the first and most understood species with regard to developmental pathways and molecular mechanisms. 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.This indicates that organized growth polarity do not always exist in somatic embryogenesis.
elongation. Early embryogeny includes all stages after suspensor elongation but before root meristem development. Late embryogeny includes development of root and shoot meristems.
Time-lapse tracking in Norway Spruce Picea abies revealed that neither single cytoplasmic-rich cells nor vacuolated 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 auxin
and cytokinin
. Gradual removal of auxin and cytokinin and introduction of ABA
will allow an embryo to form.Using somatic embryogenesis has been considered for mass production of vegetatively propagated pine clones and cryopreservation of germplasm
. However, the use of this technology for reforestation and breeding of pine trees is in its infancy.
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
and for laboratory purposes, using either solid or liquid nutrient media which contain plant growth regulators (PGR’s). The main PGRs used are auxins but can contain cytokinin
Cytokinin
Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...
in a smaller amount. Somatic embryogenesis is a process where a plant or embryo is derived from a single somatic cell
Somatic cell
A somatic cell is any biological cell forming the body of an organism; that is, in a multicellular organism, any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell...
or group of somatic cells. This is in contrast to zygotic embryogenesis
Plant embryogenesis
Plant embryogenesis is the process that produces a plant embryo from a fertilised ovule by asymmetric cell division and the differentiation of undifferentiated cells into tissues and organs. It occurs during seed development, when the single-celled zygote undergoes a programmed pattern of cell...
, where ,in diploid species, two haploid cells combine to form one diploid cell. Somatic embryos are produced when somatic cells are restructured through a series of morphological and biochemical changes in the embryogenic pathway. Development of somatic embryos is not that different from zygotic embryo
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
s. Three examples of somatic embryogenesis from nature are ovules in Paeonia and on the leaves of Asplenium
Asplenium
Asplenium is a genus of about 700 species of ferns, often treated as the only genus in the family Aspleniaceae, though other authors consider Hymenasplenium separate, based on molecular phylogenetic analysis of DNA sequences, a different chromosome count, and structural differences in the rhizomes...
and Kalanchoe
Kalanchoe
Kalanchoe , also written Kalanchöe or Kalanchoë, is a genus of about 125 species of tropical, succulent flowering plants in the Family Crassulaceae, mainly native to the Old World but with a few species now growing wild in the New World following introduction of the species.Most are shrubs or...
. 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. The first documentation of somatic embryogenesis was by Steward et al in 1958 and Reinert in 1959 with carrot cell suspension cultures.
Direct and indirect embryogenesis
Somatic embryogenesis has been described to occur in two ways: directly or indirectly Direct embryogenesis occurs when embryos are started directly from explant tissue creating an identical clone while indirectly occurs from unorganized tissue (callus).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.
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. These compounds have been identified by Chung et al. 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.Uses of somatic embryogenesis
- Plant transformationsTransformation (genetics)In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
- Mass propagationPlant propagationPlant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs and other plant parts. Plant propagation can also refer to the artificial or natural dispersal of plants.-Sexual propagation :...
Problems associated with somatic embryogenesis
- High chance of mutations
- Difficult method
- 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
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.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.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.Many culture systems induce and maintain somatic embryogenesis by continuous exposure to 2,4-dichlorophenoxyacetic acid
2,4-Dichlorophenoxyacetic acid
2,4-Dichlorophenoxyacetic acid is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America...
. Abscisic acid
Abscisic acid
Abscisic acid , also known as abscisin II and dormin, is a plant hormone. ABA functions in many plant developmental processes, including bud dormancy. It is degraded by the enzyme -abscisic acid 8'-hydroxylase.-Function:...
has been reported to induce somatic embryogenesis in seedlings. After callus
Callus (cell biology)
Plant callus is a mass of undifferentiated cells derived from plant tissue for use in biological research and biotechnology. In plant biology, callus cells are those cells that cover a plant wound. To induce callus formation, plant tissues are surface sterilized and then plated onto in vitro...
formation, culturing on a low auxin or hormone free media will promote somatic embryo growth and root formation. In monocots, 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.
Carrot Daucus carota was the first and most understood species with regard to developmental pathways and molecular mechanisms. 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.This indicates that organized growth polarity do not always exist in somatic embryogenesis.
Gymnosperms
Embryo development in gymnosperms occurs in three phases. Proembryogeny includes all stages prior to suspensorSuspensor
-Fungi:Suspensors are filamentous structural formations employed by Fungi in holding a zygospore between two strains of hyphae.-Plants:Suspensors are also found in plant zygotes in angiosperms in connecting the endosperm to an embryo. During embryo development in angiosperm seeds, normal...
elongation. Early embryogeny includes all stages after suspensor elongation but before root meristem development. Late embryogeny includes development of root and shoot meristems.
Time-lapse tracking in Norway Spruce Picea abies revealed that neither single cytoplasmic-rich cells nor vacuolated 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 auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
and cytokinin
Cytokinin
Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...
. Gradual removal of auxin and cytokinin and introduction of ABA
ABA
- Aviation :* AB Aerotransport, a former Scandinavian airline* ABA, the IATA airport code for Abakan Airport, in Russia- Broadcasting :* Alabama Broadcasters Association* Australian Broadcasting Authority...
will allow an embryo to form.Using somatic embryogenesis has been considered for mass production of vegetatively propagated pine clones and cryopreservation of germplasm
Germplasm
A germplasm is a collection of genetic resources for an organism. For plants, the germplasm may be stored as a seed collection or, for trees, in a nursery.-See also:*Germ plasm, the germ cell determining zone...
. However, the use of this technology for reforestation and breeding of pine trees is in its infancy.
See Also
- Plant embryogenesisPlant embryogenesisPlant embryogenesis is the process that produces a plant embryo from a fertilised ovule by asymmetric cell division and the differentiation of undifferentiated cells into tissues and organs. It occurs during seed development, when the single-celled zygote undergoes a programmed pattern of cell...
- Callus (cell biology)Callus (cell biology)Plant callus is a mass of undifferentiated cells derived from plant tissue for use in biological research and biotechnology. In plant biology, callus cells are those cells that cover a plant wound. To induce callus formation, plant tissues are surface sterilized and then plated onto in vitro...
- Plant tissue culturePlant tissue culturePlant tissue culture is a collection of techniques used to maintain or grow plant cells, tissues or organs under sterile conditions on a nutrient culture medium of known composition. Plant tissue culture is widely used to produce clones of a plant in a method known as micropropagation...
- Plant hormonePlant hormonePlant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...
- Embryo RescueEmbryo RescueEmbryo Rescue is one of the earliest and successful forms of in-vitro culture techniques that is used to assist in the development of plant embryos that might not survive to become viable plants. Embryo rescue plays an important role in modern plant breeding, allowing the development of many...
- HyperhydricityHyperhydricityHyperhydricity is a physiological malformation that results in excessive hydration, low lignification, impaired stomatal function and reduced mechanical strength of tissue culture-generated plants. The consequence is poor regeneration of such plants without intensive greenhouse acclimation for...
- Murashige and Skoog mediumMurashige and Skoog mediumMurashige and Skoog medium or is a plant growth medium used in the laboratories for cultivation of plant cell culture. MSO was invented by plant scientists Toshio Murashige and Folke K. Skoog in 1962 during Murashige's search for a new plant growth regulator...
External links
- http://www.biobasics.gc.ca/english/View.asp?x=799
- http://theagricos.com/tissue-culture/somatic-embryogenesis/
- http://passel.unl.edu/Image/siteImages/SomaticEmbryo13Steps.gif