Embryogenesis and Theories on Embryogenesis

Embryogenesis

An embryo is defined as a plant in its initial stage of development.

Each embryo possesses two distinct poles, one to form a root and the other to shoot, and is the product of the fusion of gametes.

In some plant species, embryos are produced without the fusion of gametes and termed asexual embryogenesis or adventitious embryony.

In an intact plant, this embryogenesis may occur in sporophytic tissues like integuments and nucellar tissues or from unfertilized gametic cells.

Apart from the normal course of embryo formations viz., zygotic embryogenesis and adventitious embryony, instances of embryo formations from the tissue cultures in vitro were reported.

This phenomenon termed somatic embryogenesis was first observed by Steward and his co-workers (1958) in suspension cultures of carrots followed by Reinert (1959).

Since then, several reports of embryo formation have been published.

Somatic embryogenesis or embryogenesis in vitro produces embryo-like structures resembling the zygotic embryos in structure and morphogenetic potential.

Despite this resemblance, the ontogeny of an embryo-like structure from a somatic cell differs from that of the zygotic embryo, whose origin is from a single cell.

Embryoid is generally used to denote the embryo-like structure from cultured tissues.

These embryoids possess bipolarity, no vascular connection with the mother tissue, and originate from a single cell or a group of cells.

Theories on embryogenesis

Several theories have been proposed to explain the phenomenon of somatic embryogenesis, of which the following are considered important.

Cell isolation theory:

Steward and his co-workers proposed this theory in 1964.
According to them, the embryo-producing cells are isolated from the neighbouring cells in a cell mass.
The isolation of cells favours embryogenesis.
The isolation of cells may be induced by the constraints in the surrounding cells, due to the physical and physiological separation of cells.
In most cases, the connection of plasmodesmata was severed.
But this generally appears to be secondary to the induction process.

Differentiation theory:

This theory states that the embryos would not be produced from the differentiated cells of the explants.
The cells of explants have to undergo de-differentiation to form callus.
Then the cells of the callus will produce embryos.
In other words, de-differentiation in cells is a prerequisite for the production of somatic embryos in vitro.
That the embryos can be formed directly from the epidermal cells of the stem or hypocotyl indicates the possibility of embryo formation without de-differentiation.
The need for differentiation depends on the explant material used during primary culture.
Epidermal cells of the stem, hypocotyl and young embryos may begin embryo development without going through a callus stage, while cortical cells and cells of xylem and phloem explants require de-differentiation.
This theory was proposed by Halperin in 1970.

Intercellular communication and cytodifferentiation theory:

According to this theory, cytodifferentiation in cells due to intercellular communication induces embryo formation.
The cytodifferentiation is regulated by diffusion gradients of nutrients, endogenous plant growth regulators and gaseous factors like O2, CO2 and ethylene.
The changing microclimate in the culture environment affects intercellular communication and in turn cytodifferentiation.
This concept was proposed by Street (1973).

Explant physiology and culture environment theory:

This concept was developed by Street in 1976.
He is of the view that embryogenesis is a dependent phenomenon on the explant and the culture environment.
Explants like flower buds, young embryos and parts of young seedlings are most responsive to producing somatic embryos, but not those of mature plants.
Apart from the explant physiology, culture environment is also a factor influencing embryogenesis.
For example, highly embryogenic callus culture can be maintained non-embryogenic if the medium is supplemented with a high level of auxin and the same may be induced to produce embryos when transferred to auxin auxin-free medium.

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Pre-determination theory:

This was proposed by Tisserat et al. (1979).
It states that the embryo production potential is a pre-determined phenomenon in the cells and the in vitro culture provides the opportunity for embryogenesis.
In other words, embryogenesis from a cell is an inherent one which is facilitated to produce embryos by optimal culture environment.

Pre and induced embryogenic determined cell theory:

Though the embryogenesis is pre-determined there are instances of non-formation of embryos directly from the explants.
In these cases, an intervening callus stage comes between the primary explant and the embryos.
The cells in the Calli are induced to produce embryos by the manipulation of a medium with a relevant growth regulator.
Based on this, the above theory was proposed by Sharp and his co-workers.
According to this theory, there are two types of embryogenic cells:
pre-embryogenic determined cells (PEDC) and induced embryogenic determined cells (IEDC).

In pre-embryogenic determined embryogenic cells, embryogeny is determined before mitosis while in induced embryogenic determined cells the embryogeny is induced by providing a suitable mitogenic substance i.e., the embryogeny is induced in the cells of callus by the application of plant growth regulators.

Thus, in the callus, embryogenic precursor cells or embryogenic mother cells are formed which then develop into embryogenic cells.

Later these cells undergo polarised cell divisions typical of normal embryogenesis by forming globular, heart and torpedo-shaped embryos.

What happens during embryogenesis

In 1959, Reinert made the remarkable claim that following a succession of changes in the nutrient media, root-derived callus tissue of Daucas carota produced normal bipolar embryos.

The changes made or observed in the nutrient medium were as follows:

maintenance of callus in White’s medium with a high level of auxin (IAA at 10 mg/litre)
and subculturing of callus for several months on White’s basal medium with additives like vitamins, amino acids, amides and purines.

As a result of these manipulations, the calli showed small protuberances on the surface.

Histological sections of these calli showed centres of organised development.

These tissues with organised centres, on transfer to auxin, lack coconut milk containing medium-produced embryoids and from embryoids, whole plants.