Somatic embryogenesis

  • In somatic embryogenesis the embryo generated from somatic cell tissues or organs or directly from the tissue which is the opposite of zygotic or sexual embryogenesis.
  • Various terms for non zygotic embryo have been reported in literature such as adventive embryo ,parthenogenetic embryo androgenetic embryo.
  • However in general content somatic embryo are those which are formed from the somatic tissue in culture in vitro conditions.
  • In sexual embryogenesis the act of fertilization triggers the egg cell to develop into embryo.
  • However it is not the Monopoly of the egg to form an embryo.
  • Any cell of the gametophyte or sporophytic tissue around the embryo sac ovary give rise to an embryo.
  • Cells of the nucleus or inner integument of members of Rutaceae family(citrus) may develop into embrayos.
  • However the experience of a sexual embryogenesis is generally restricted to intravascular tissues.
  • Embryogenesis in plants culture is the development of embryo from somatic cell in addition to their information from unfertilized genetic cells and tissue typically associated with invivo sexual embryogenesis .
  • Somatic embryogenesis differs from organogenesis in the embryo a bipolar structure with a closed radicular end rather then a monopolar structure.
  • The embryo arise from a single cell and has no vascular connection with the maternal callus tissues or the cultured explant.
  • Induction of somatic embryogenesis require a single to induce bipolar structure capable of forming a complete plant file in organogenesis it requires to different hormonal signals to induce first a shoot organ than a root organ.
  • The initiation and the development of embryo form somatic tissue in plant culture was first recognised by in the year 1958 and reinhardt in 1958 1959 cultures of Daucus Carota.
  • In addition to the development of somatic embryo form sporophyte cell embryo have been obtained from generative cells.
  • The leguminoseal and many monocot of Gramineae family which are so important agronomically have proven difficult to grow in culture and regenerate somatic embryo.
  • Though there are reports of success in these species but manipulation is not so easy as with solanaceous crops.

Direct embryogenesis

  • The embryo initiate directly from the explant tissue in the absence of callus proliferation.
  • This occurs through pre embryonic determined cells are the cells are committed to embryonic it development and need only to be released.
  • Such cells are found in embryonic development and need only to be released.
  • Search cells are found in embryonic tissues certain tissues of young in-vitro grown plantlets like hypocotyl in Daucus Corota.

Indirect embryogenesis

  • Cell proliferation as example colours from explant takes place from which embryo are developed.
  • The cells from which embryo arise are called emberyogenically determined such and forms embryos which are included to do so also called as induced embryogenic determinants cells.
  • In majority of cases embryogenesis is the indirect method.
  • Here specific growth regulation concentration and cultural condition required for initiation of callus and then re determination of these cells into the embryogenic pattern of development.
  • The two process are actually the some and that somatic embryos arise from the continuation of special cells in the original explant.
  • Somatic embryos arise from single cell located within clusters of meristematic cells either in the callus mass or in suspension.
  • Such cell develop into pre embryos with polarity following a pattern that tends to mimic the general pattern associated with the development of embryo in the ovule.
  • When the conditions are suitable this embryo germinate to produce plantlets.
  • For some spaces any part of the plant body serves as an explained for embryogenesis whereas in some species only certain region of the plant body may respond in culture.
  • Floral or the reproductive tissue in general has proven to be an explain source of embryogenic material.
  • Somatic embryogenesis encompasses various stages from callus initiation to embryo development and maturation of subsequently plantlet formation.
  • Equally important is the sequence of media and specially the growth regulators.
  • For many species one media is used for initial callusing and for the maintenance of callus a second medium is used for somatic embryo maturation and third to allow their growth in to plants.
  • An elaborate sequence of media is essential for somatic embryogenesis is lacking or difficult.

Protocol for Somatic Embryogenesis in Carrot
Somatic embryos have been grown on a range of media from dilute White’s medium to very high salt.Factors affecting somatic embryogenesis

  • The addition of reduced nitrogen in the medium helps in both embryo in association and maturation.
  • The sources of reduced nitrogen seems to play a special role.
  • In the absence of iron, embryo development fails to pass from the globular to the heart shaped stage.
  • Growth regulators in the medium specially oxygen or oxygen in combination with cytokinin appear essential for the onset of growth and the induction of embryogenesis.
  • The auxin for the primary and secondary media may be same or different.
  • One auxin or several may be used in the primary medium invariably during embryogenesis of crop plants.
  • The effective concentration range for kinetin is 0.5 to 5.0 mm.
  • Cytokinin are important in somatic embryo maturation and specially Cotyledon development.
  • However they have proven useful in embryo maturation or in stimulation of routing and subsequent growth of plants in a number of cases.
  • The role of growth inhibitor aba in somatic embryogenesis has emerged when added at non inhibitory levels.
  • aba promotes somatic embryo development and maturation and at the same time in inhibit abnormal produce occasion and initiation of accessory embryos.
  • Their addition to culture media permit somatic maturation to produce under conditions when it normally wouldn’t occur.
  • The addition of activated charcoal to the medium has proved to the useful for somatic embryo development.
  • Charcoal media shows lower levels of phenylacetic acid and benzoic acid compounds which inhibit somatic embryogenesis.
  • Also it absorb 5 hydroxymethyl for the an inhibitor formed by sucrose degradation during autoclaving.
  • Environmental condition of light, temperature, density of embryogenic cell in medium are important.
  • Regarding culture vessel the position of embryos and the physical state of medium have little effect.
  • Somatic embryogenesis as a means of propagation is seldom used because:-
  • There is a high probability of mutation arising.
  • The method is usually rather difficult.
  • The chance of losing regenerative capacity become greater with repeated subculture.
  • Induction of embryogenesis is often very difficult or impossible with many plant species.

Practical application of somatic embryogenesis:

clonal propagation:

  • Somatic embryogenesis has the potential application in plant improvement.
  • Since both the growth of embryogenic cell and subsequent development of somatic embryo can be carried out in a liquid medium it is possible to combine somatic embryogenesis with engineering technology to create large scale mechanised or automated culture system.

Source of regenerable protoplast system:

  • Embryogenic callus, suspension culture and somatic embryos have been employed as sources of protoplast isolation for range of species.
  • Cells or tissues in this system have demonstrated the potentially to regenerate in culture and therefore protoplast that are capable of forming whole plant.

Genetic transformation:

  • In seed embryogenesis zygotic embryo are seated deep inside the nuclear tissues.
  • They live in a protected environment besides being genetically heterogeneous.
  • Somatic embryos remain virtually unprotected and more or less give rise to genetically uniform plants.

Conservation of genetic resources:

  • Embryogenic culture as well as embryos remain viable upon storage and ambient temperature , cold storage or cryostage.
  • Therefore somatic embryogenesis has a great importance in plant germplasm conservation.

2 9