Embryos are of the following types:

Zygotic Embryos

Non-zygotic Embryos

Zygotic embryos are formed by the zygote or the fertilized egg. Non-zygotic embryos can further be divided into:

Somatic embryos: These are formed by the sporophytic cells in an in-vitro scenario. Embryos that directly emerge from other organs or embryos are referred to as adventive embryos.

Androgenetic Embryos: Formed by the male gametophytes.

Parthenocarpic Embryos: Formed by the unfertilized egg.

Somatic embryogenesis is a significant biotechnological tool which demonstrates numerous advantages in the areas of clonal propagation, genetic transformation, etc. When applied, these advantages are particularly beneficial.

In 1958, Stewart was the first to introduce embryos in carrots through suspension culture.

Table of Contents

What Is Somatic Embryogenesis? – A Definition

Process Of Somatic Embryogenesis

Types Of Somatic Embryogenesis

Advantages of Somatic Embryogenesis

Factors Affecting Somatic Embryogenesis

Somatic Embryogenesis Stages – Steps of Somatic Embryogenesis

Difference Between Organogenesis And Somatic Embryogenesis

Difference Between Somatic Embryos and Zygotic Embryos

What is Somatic Embryogenesis?

– Somatic Embryogenesis is a type of asexual reproduction in plants, where embryos are formed from somatic (non-reproductive) cells. It is a method of cloning plants from a single cell or tissue.

Somatic embryogenesis is an artificial process in which somatic cells differentiate into somatic embryos. These somatic embryos are formed from the cells of the plants, which normally do not participate in embryo development. Additionally, a seed coat or endosperm is not formed around the somatic embryo.

In the process, one cell or a cluster of cells initiates the developmental route, resulting in reproducible regeneration of non-zygotic embryos, which can germinate to form an entire plant.

The formation of an undifferentiated cluster of cells known as a callus can be induced from potential source tissues by culturing them in a tissue culture medium. Plant growth regulators can be added to this medium to further induce callus formation and modify it to induce embryo formation.

Process of Somatic Embryogenesis

The somatic embryogenesis procedure is a three-step process that induces embryogenesis, facilitates embryo development, and promotes embryo maturation.

The principle of somatic embryogenesis is rooted in the concept of totipotency of plant cells, and it demonstrates two aspects of plant embryogenesis:

The process of fertilization can be substituted by an internal mechanism.

Apart from the fertilized egg cells, other types of cells of the plant have the potential to form an embryo.

Somatic embryogenesis bypasses the process of fertilization, allowing for faster large-scale propagation of plants. Furthermore, it is a useful tool for genetic transformation of plants, and can be used to cryo-store embryos and germplasm.

Somatic Embryogenesis: Induction

Cells reactivate to differentiate and develop embryos, which occur through two processes: direct somatic embryogenesis and indirect somatic embryogenesis.

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Direct Somatic Embryogenesis

The explants of somatic embryogenesis involve the direct development of embryos from the cells of the explants, such as the cells of immature embryos, without an intermediary stage like the formation of callus. The explants are seen to entail PEDCs (pre-embryogenic determined cells).

Indirect Somatic Embryogenesis

It includes the formation of somatic embryos by reiterating numerous cycles of cell divisions. It includes intermediary steps of growth of the callus, and hence the process includes multiple steps.

The cells which do not carry the pre-embryogenic determined cells are caused to differentiate for the formation of the embryo through the application of various treatments. These cells then transform into IEDs (induced embryogenic pre-determined cells).

Types of Somatic Embryogenesis

Somatic embryogenesis is of two types:

Direct Somatic Embryogenesis

Embryos can be formed directly from explants without the formation of callus, when Pre-induced Embryogenic Determined Cells (PEDCs) are present.

Indirect Somatic Embryogenesis

The callus from the explants occurs from where the embryo develops as a result of Induced Embryogenic Determined Cells (IEDCs).

Advantages of Somatic Embryogenesis

1. Allows for rapid multiplication of elite genotypes
2. Produces uniform plants with desired characteristics
3. Can be used to produce haploid plants
4. Can be used to produce transgenic plants
5. Can be used to produce plants with enhanced stress tolerance
6. Can be used to produce plants with improved nutritional value

In comparison with zygotic embryogenesis, somatic embryogenesis has the following benefits:

A massive amount of embryos are obtained

The regulation of the development and environmental stage of somatic embryos can be achieved.

This process of embryogenesis can be monitored easily.

The importance of somatic embryogenesis is:

Production of Artificial Seeds

Higher Rate of Propagation

Apt in Suspension Culture

Labour Savings

Factors Affecting Somatic Embryogenesis

The factors influencing the process of somatic embryogenesis are:

Characteristics of an Explant

Despite the importance of the stage of development of explants for the initiation of embryogenic callus, variations of explants can also be used. Furthermore, juvenile explants tend to generate more somatic embryos than older explants, and different explant tissues from the same mother plant can produce embryogenic callus at varying frequencies.

Explants of immature zygotic embryos are typically the most suitable choice for somatic embryogenesis in the majority of plant species, as the desired species of plants to be induced for embryogenesis determines the selection.

Growth Regulators

Cytokinins: These have been consistently used in the primary medium at the time of embryogenesis of crop plants. They are essential for accelerating the process of maturation of somatic embryos and cotyledon development.

Auxins are essential for the initiation of growth and the induction of embryogenesis in all plants. They play a key role in the initial step of this process - the induction step. Excessive levels of auxins can inhibit embryogenesis in citrus plant explants.

Abscisic acid facilitates the development and maturation of somatic embryos, while also inhibiting the unusual proliferation and the initiation of accessory embryos at inhibitory levels.

Genotype

The process of embryogenesis is also impacted by the genotypic variation observed in different plants; research suggests that this can be due to the endogenous levels of hormones.

Sources of Nitrogen

Nitrogen forms present in the medium have a significant impact on the process of embryogenesis in plants. The presence of NO–3 as the sole source of nitrogen has an especially marked effect on somatic embryogenesis. Somatic embryo development is dependent upon the nitrogen form in the medium.

Polyamines

Experts observe that the concentration of polyamines is higher in polyembryonates than in monoembryonates, and it is said to have an effect on the process.

Electrical Stimulation

Electrical stimuli seem to aid in the development of the structured embryo by impacting cell polarity through changes to the microtubules and prompting the initial asymmetric division.

Somatic Embryogenesis Stages: Steps of Somatic Embryogenesis

The process of somatic embryogenesis occurs in the following stages:

Induction

The necessity of exogenous auxin to induce somatic embryogenesis is based on the nature of the explants, and for this process of induction, auxins, specifically 2,4-D, are typically essential. The concentration of the auxins should be proportional to the explants used.

Development

Once the process of cell division and cell proliferation is reinitiated in the presence of auxins, embryogenic cells are released into an auxin-free medium. These cells are grouped together and are referred to as the PEMs (Pro Embryonic Mass of Cells).

Maturation Process

The standard of somatic embryos in terms of conversion into plants or germinability is decreased due to normal-looking somatic embryos that are actually incomplete in their development. Unlike seed embryos, somatic embryos do not go through the final stage of embryogenesis known as embryo maturation, which involves the accumulation of reserve food substances and proteins that provide desiccation tolerance to the embryos. During this stage, the size of the embryos does not increase.

Difference between Organogenesis and Somatic embryogenesis

Organogenesis is the formation of organs from germ layers, whereas somatic embryogenesis is the formation of embryos from somatic cells.

While both organogenesis and embryogenesis are essential for the development of an entity, their processes differ. Embryogenesis involves the formation of an embryo from the zygote due to syngamy, while organogenesis involves the development of organs and tissues from the three germ layers of the embryo. The main distinction between the two processes is the formation of organs and embryos.

Table of Key Differences between Organogenesis and Somatic Embryogenesis

Organogenesis	Somatic Embryogenesis
Involves formation of organs from a single tissue	Involves formation of embryos from somatic cells
Occurs in plants and animals	Occurs in plants
Involves production of a few large organs	Involves production of numerous embryos

Organogenesis	Somatic Embryogenesis
Process of	Process of
Organ formation	Embryo formation

| What is it? |

| Formation of an Embryo from Somatic Cells | Development of Organs from Embryonic Cells |

| Is it found in nature? |

| More or less natural | No, artificially |

| Where can the process be seen? |

Animals	Plants

| The Outcome of the Process |

| An entire plantlet with shoot and root is formed | A somatic embryo is formed |

| Its Association with Maternal Tissue |

| Roots and shoots have a strong association with them | The somatic embryos are not in vascular association with their maternal callus |

Difference between Somatic Embryos and Zygotic Embryos

Somatic embryos are embryos that are formed from somatic cells, while zygotic embryos are embryos that are formed from zygotes. Somatic embryos are artificially created in a laboratory setting, while zygotic embryos are formed naturally in the body. Somatic embryos are used in plant biotechnology, while zygotic embryos are used in animal biotechnology.

Tabulated below are some important differences between somatic embryos and zygotic embryos. Read on to discover more:

Somatic Embryo	Zygotic Embryo
Embryo produced from somatic cell of an adult plant	Embryo produced from a zygote, or fertilized egg

| What are stalactites and stalagmites? |

| From Gametic Cells | By the Union of Sporophytic Cells |

Vascular Association of Embryos with the Cultured Explant

| Absent | Present |

| Distinct Suspensor |

Somatic embryos may not present as a well-developed structure, even if they are seen, as it may not be operational like in the case of seed embryos.

| Rate of Propagation |

| High | Low |

| Secondary Embryogenesis |

Usually Seen	Not Seen
Clouds	Unicorns

Fields of Application of Somatic Embryogenesis

Today, the process of somatic embryogenesis has many applications in different fields of science, including:

In Vitro Selection

In Vitro Conservation

Large-Scale Propagation

Genetic Transformation