Somatic embryogenesis

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Somatic embryogenesis is a process where a plant or embryo is derived from a single somatic cell or group of somatic cells. Somatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e. ordinary plant tissue. No endosperm or seed coat is formed around a somatic embryo. Applications of this process include: clonal propagation of genetically uniform plant material; elimination of viruses; provision of source tissue for genetic transformation; generation of whole plants from single cells called protoplasts; development of synthetic seed technology. Cells derived from competent source tissue are cultured to form an undifferentiated mass of cells called a callus. Plant growth regulators in the tissue culture medium can be manipulated to induce callus formation and subsequently changed to induce embryos to form from the callus. The ratio of different plant growth regulators required to induce callus or embryo formation varies with the type of plant.[1] 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.[2] 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.[3] The first documentation of somatic embryogenesis was by Steward et al. in 1958 and Reinert in 1959 with carrot cell suspension cultures.[4][5]

Switchgrass somatic embryos

Direct and indirect embryogenesis

Somatic embryogenesis has been described to occur in two ways: directly or indirectly[6] 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.[2]

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.[7] These compounds have been identified by Chung et al.[8] 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

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.[10]

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.[11] 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.[12]

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.[13]

Carrot Daucus carota was the first and most understood species with regard to developmental pathways and molecular mechanisms.[10] 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.[14] This indicates that organized growth polarity do not always exist in somatic embryogenesis.[10]

Gymnosperms

Embryo development in gymnosperms occurs in three phases. Proembryogeny includes all stages prior to suspensor elongation. Early embryogeny includes all stages after suspensor elongation but before root meristem development. Late embryogeny includes development of root and shoot meristems.[11] 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 abscisic acid (ABA) will allow an embryo to form.[10] 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.[15]

See also

References

  1. ↑ http://www.accessexcellence.org/LC/ST/st2bgplant.html Plant Tissue Culture
  2. ↑ 2.0 2.1 E.F. George et al (eds.), Plant Propagation by Tissue Culture 3rd Edition, 335-354.
  3. ↑ Quiroz-Figueroa, F. R., Rojas-Herrera, R., Galaz-Avalos, R. M., and Loyola- Vargas, V. M. 2006. Embryo production through somatic embryogenesis can be used to study cell differentiation in plants. Plant Cell Tiss. Org. Cult. 86: 285–301.
  4. ↑ Steward, F.C., Mapes, M.O., and Smlth, J. (1958). Growth and organized development of cultured cells. I. Growth and division of freely suspended cells. Am. J. Bot. 45, 693-703.
  5. ↑ Reinert J (1959) Uber die kontrolle der morphogenese und die induktion von adventivembryonen an gew- ebekulturen aus karotten. Planta 53:318–333
  6. ↑ Sharp et al. (1980). In: Horticultural Reviews,Vol. 2. (janick, J., ed.). AVI Publishing Co, Westport, Conn., USA, p. 268.
  7. ↑ Warren, G.S., Fowler, M.W. 1981. Physiological interactions during the initial stages of embryogenesis in cultures of Daucus carota L. New Phytol 87:481-486.
  8. ↑ Chung, W., Pedersen, H., Chin, C-K. 1992. Enhanced somatic embryo production by conditioned media in cell suspension cultures of Daucus carota. Biotechnol Lett 14:837-840.
  9. ↑ Jiménez V.M., Guevara E., Herrera J. and Bangerth F. 2001. Endogenous hormone levels in habituated nucellar Citrus callus during the initial stages of regeneration. Plant Cell Rep. 20: 92–100.
  10. ↑ 10.0 10.1 10.2 10.3 Yang, Xiyan and Zhang, Xianlong(2010) 'Regulation of Somatic embryogenesis in Higher Plants', Critical Reviews in Plant Sciences, 29: 1, 36 — 57
  11. ↑ 11.0 11.1 Von Arnold S, Sabala I, Bozhkov P, Dyachok J and Filonova L (2002) Developmental pathways of somatic embryogenesis. Plant Cell Tiss Org. Cult. 69: 233–249
  12. ↑ Jime ́nez VM, Thomas C (2005) Participation of plant hormones in determination and progression of somatic embryogenesis. in: Mujib A, Šamaj J (eds) Somatic embryogenesis. Springer, Berlin, pp 103–118.
  13. ↑ Fehér, Attila. Why somatic plant cells start to form embryos? In: Mujid, Abdul and Samaj, Josef. eds. Somatic Embryogenesis. Plant Cell Monographs, Springer; Berlin/Heidelberg, 2005, vol. 2, p. 85-101.
  14. ↑ Toonen, M. A. J., Hendriks, T., Schmidt, E. D. L., Verhoeven, H. A., van Kammen, A., and De Vries, S. C. 1994. Description of somatic-embryo- forming single cells in carrot suspension cultures employing video cell tracking. Planta 194: 565–572.
  15. ↑ Häggman H.; Vuosku J.; Sarjala T.; Jokela A.; Niemi K. Somatic Embryogenesis of pine species: from functional genomics to plantation forestry. Dig. Plant Cell Monogr. 2: 119–140; 2006.

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