Somatic Embryogenesis | Biotechnology

The below mentioned article provides a study note on somatic embryogenesis.

Somatic embryogenesis is the process in which a single cell or a small group of cells follow a developmental pathway that leads to reproducible regeneration of non-zygotic embryos which are capable of producing a complete plant. These non-zygotic embryos may originate directly from other organs or parthenogenetic embryos (without fertiliza­tion) or androgenetic embryos (from the male gametophyte).

In general somatic embryos are those which are formed from the somatic tissue in cultural i.e., in vitro condition. Embryos formed in cultures have been variously designated as accessory embryos, adventive embryos, embryoids and supernumerary embryos.

The initiation and development of embryos from somatic tissues in plant culture was first recognised by Steward et.al. (1958) and Reinert (1958-1959) in Daucus carota.

Kohlenbach (1978) classified the embryos in following manner:

I. Zygotic Embryos: These are formed by fertilized egg or the zygote.

II. Non-zygotic Embryos: These are formed by cells other than zygote.

(a) Somatic Embryos: These are formed by sporophytic cells (except zygote), directly arising from other embryos or organs which are termed as adventive embryos.

(b) Parthenogenetic Embryos: These are formed by unfertilized egg.

(c) Androgenetic Embryos: These are formed by the male gametophyte i.e., micro­spore or pollen grains.

Somatic embryos should closely resemble their bipolar structure as in the case of zygotic embryos. There should be appropriate root, shoot and cotyledonary development.

Sharp et. al. (1980) described mainly two routes for somatic embryogenesis:

1. Direct Embryogenesis:

The embryos initiate directly from the explant without callus formation and here some cells which are called as ‘Pre-embryonic determined cells’ (PEDC) initiates embryonic development, only those cells need to be released. Such cells are found mostly in embryonic tissues, certain tissues of young in vitro grown plants, hypocotyl, nucellus, embryo-sac, etc.

2. Indirect Embryogenesis:

Here, the embryos are developed through cell proliferation i.e., callus formation. The cells from which embryos arise are called as ‘Induced embryogenic determined cells’ (IEDC). Here growth regulators with specific cul­tural conditions are required for initiation of callus and then redetermination of those cells into the embryo development.

Somatic embryos arise from single cells located within clusters of meristematic cells either in callus mass or in suspension. Such cells develop into pro-embryos with polarity following a pattern that tends to mimic the general pattern associated with the development of in vivo embryos in the ovule. Pro-embryo initials may be single cells or multicellular groups. When the conditions are suitable these embryos germinate to produce plantlets (Fig. 18.4).

Medium and Growth Regulators Requirement:

Somatic embryogenesis encompasses various stages from callus initiation to embryo development, maturation and subsequent plantlet formation. For many species different types of media are required for the whole process.

The presence of auxin in the media is generally essential for embryo initiation, but lowering the auxin content further in the medium is helpful for embryogenic clump formation which ultimately develops into mature embryo.

In general it has been observed that the auxin or auxin in combination with cytokinin in different ratios appear essential for the onset of growth of somatic embryos. Sometimes the medium lacking auxin but addition of low levels of cytokinin in combination with ABA may prove beneficial for somatic embryogenesis.

Sometimes the addition of charcoal into the medium has been proved to be useful for somatic embryo development. The germination of somatic embryos does not require any growth regu­lators, application of GA₃ or Zeatin may be helpful for shoot and root development.

Difference between Zygotic Embryogenesis and Somatic Embryogenesis:

In angiosperms, the ovule contains a haploid egg cell or ovum which is a female gamete gets fertilised by the male gamete resulting in the formation of unicellular zygote. This zygote gives rise to multicellular embryo which is known as zygotic embryo and the process must be called as zygotic embryogenesis.

Embryos may be formed from the unfertilized egg or any other cell of female gametophyte or the sporophytic tissue then these are called as non-zygotic embryos. In in vivo there is no embryo development from any somatic cell of the plant, i.e., the somatic cell does not have the embryogenic poten­tial in general.

But due to the totipotency nature of plant cell the somatic cell having the complete set of genome may be induced in cultures to form the organised bipolar structures bearing cotyledons resembling the zygotic embryo.

This phenomenon is called in vitro somatic embryogenesis. The term ’embryoids’ are used to distinguish these somatic embryos from the zygotic embryos. Any sporophytic cell or gametophytic cell may undergo somatic embryogenesis due to some changes occurring at molecular level i.e., due to induction of some genes, protein production and high metabolic activity.

The stages of developmental process of both somatic embryo an zygotic embryo resem­ble each other morphologically. In both the cases the single somatic cell (sporophytic or gametophytic) or the zygotic cell undergoes divisional phases to produce the globular stage, then the heart or torpedo stage at the end of which the cotyledons or first leaves begin to develop.

In dicots two cotyledons and in monocot only one cotyledon emerges. After this stage the developmental process differs in zygotic and somatic embryo (Fig. 18.5).

The zygotic embryos undergo a prolonged maturation phase and many storage proteins are being synthesized which is primarily regulated by abscisic acid. Then a dor­mancy period is required for further germination.

But the somatic embryos can grow and differentiate to produce the root and shoot apices with leafy development without any dormancy period. Another important morphological or developmental difference is that the somatic embryos are devoid of suspensor and endosperm.

Unlike zygotic embryos or seed embryos, the somatic or adventive embryos are devoid of an endosperm as a food reserve. Fluid drilling devices are harnessed to sow the naked embryos directly into the soil, or the embryos are encapsulated in plastic strips or pellets together with a little nutrient, aimed at using embryos for large scale planting.

Applications of Somatic Embryogenesis:

(i) Large Scale Propagation Compared to Zygotic Embryos:

Induction of somatic embryogenesis forms the ultimate goal in free cell suspension cultures relying on the totipotency of the cell and could reasonably be exploited for micro-propagation.

Each cell of the suspension cultures can be induced to produce somatic embryos which can be maintained in an arrested state of development by cold storage or using mitotic inhibitors until the time of sowing. Somatic embryo­genesis is highly desirable and holds out promise for rapid multiplication in a shorter time, with a shoot-root axis.

(ii) More Useful than Organogenesis:

The mass production of adventitious embryos in cell culture is still regarded by many as the ideal propagation system. The adventitious embryo is a bipolar structure that develops directly into a complete plantlet and there is no need for a separate rooting phase as with shoot culture.

(iii) Useful for Mutagenic Studies and Mutant Production:

The somatic embryos generally arise from single cells, so it may be advantageous for mutagenic studies. Also the plantlets arising from such somatic embryos are more homogeneous in nature, so the mutant gene expression can be studied well.

(iv) Useful for Genetic Manipulation Technique:

In plant biotechnological appli­cation, during foreign gene transfer if the transformed cell gives rise to plantlet via somatic embryogenesis then there is least possibility of chimera formation. So for transgenic plant production this method of multiplication system is very much useful.

(v) Useful for Pathogen-Free Plant Production:

Plants derived from this kind of somatic embryos may be free from viral or other pathogens. So it may be an alternative approach of disease free plant production.

(vi) A Good Source of Protoplast Culture:

Embryogenic cultures are specially valu­able in providing a source of regenerable protoplasts in the graminaceous and coniferous plants. Protoplasts from these cultures were induced to divide to form a cell mass from which the embryoids, even plantlets are regenerated on a suit­able nutrient medium.

(vii) Conservation of Genetic Resources:

Somatic embryos which originate from sin­gle cells and subsequently regenerate mostly genetically uniform plants are good materials for genetic resource conservation. Embryogenic cultures as well as somatic embryos remain viable upon storage at ambient temperature, cold storage or cryostorage.