What is Somatic Hybridization?

Somatic hybridization is a technique of fusing protoplasts from different plant species to create hybrid plants. It is different from conventional ways involving sexual hybridization because it does not need sexual reproduction. Instead, it combines traits from different plants without being limited by species barriers. In this article, we will cover somatic hybridization notes, steps, and its applications.

Somatic hybridization is a technique of creating a hybrid cell through the in vitro fusion of separate protoplasts, which can then grow into a hybrid plant. Sexual hybridization has long been the preferred strategy for enhancing the traits of domesticated plants. The major limitation of sexual hybridization is that it can only take place between closely related plant species. This limits the modifications that may be made to plants.

By fusing somatic cells to create a viable hybrid, somatic cell fusion can go beyond the species barriers for plant improvement that is faced in sexual hybridization. Somatic hybridization joins separate protoplasts together in a lab setting to produce a hybrid cell that will eventually grow into a hybrid plant.

Somatic hybridization involves three processes. They are as follows:

• Protoplast fusion
• Hybrid Cell Selection
• Identifying hybrid plants

Now lets’s discuss these processes in detail.

Since isolated protoplasts lack cell walls, in vitro fusion of these structures is simple. For protoplast fusion, there are no obstacles to incompatibility (at interspecific, inter-generic, or even at inter-kingdom levels). A protoplast fusion that includes combining protoplasts with two distinct genomes can be done naturally, mechanically, or artificially. These are explained below:

Spontaneous Fusion or Natural Fusion of Protoplasts

The process of cell fusion occurs naturally as in the process of egg fertilization. Some of the neighbouring protoplasts may combine to produce homokaryocytes when the cell walls are being broken down by enzymes (homokaryons). There may occasionally be a large number of nuclei in these joined cells (2-40). 

This is mostly due to the growth and subsequent joining of cell-to-cell connections known as plasmodesmata. It was discovered that protoplasts separated from dividing cultured cells frequently became homokaryons. However, spontaneously fused protoplasts cannot grow again into whole plants without passing through a few cell divisions.

Mechanical Fusion of Protoplasts

The protoplasts can be mechanically pressed together to fuse. For example, by gently trapping the protoplasts of Lilium and Trillium in an enzyme solution, they can be united. Although protoplasts may get injured as a result of mechanical fusing.

Induced Fusion of Protoplasts

By induction, newly isolated protoplasts can merge. Fusogenic refers to a group of fusion-inducing substances, such as NaN0₃, high pH/Ca²⁺, polyethylene glycol, polyvinyl alcohol, lysozyme, concanavalin A, dextran, dextran sulfate, fatty acids, and esters, electrofusion, among others. There are descriptions of certain fusogenic and how they are used in induced fusion.

Due to its numerous benefits, the polyethylene glycol (PEG) treatment approach is often utilized in protoplast fusion:

• It causes the development of high-frequency heterokaryons, which is repeatable.
• Low cell toxicity.
• Lessening of bi-nucleate heterokaryon production.
• As PEG-induced fusion is non-specific, it can be used for a variety of plants.

Electro-Fusion of Protoplasts

This technique uses an electrical field to facilitate protoplast fusion. Protoplasts are made to fuse when they are put in a culture jar equipped with microelectrodes and given an electrical shock. The electro-fusion technique is popular since it is easy, quick, and effective. Additionally, unlike when fusogenic materials are used, cells created by electro-fusion do not exhibit cytotoxic reactions (including PEG). The major drawback of this method is its need for expensive and specialized equipment.

Fusion Mechanism of Protoplasts

Three processes are involved in the fusing of protoplasts:

1. Agglutination/Adhesion: When two protoplasts are brought into proximity by fusogenic substances such as polyethylene glycol (PEG) and NaNO₃, they stick together.
2. Plasma Membrane Fusion: At the location of adhesion, the protoplast’s membrane fuses, creating a cytoplasmic bridge that connects the two protoplasts. The pace of membrane fusion can be accelerated by high pH and Ca²⁺ concentration.
3. Formation of Heterokaryons: A spherical homokaryon or heterokaryon is formed when the united protoplasts circle up.

A heterokaryon is produced by the fusion of just 20–25% of the protoplasts. The combination is made up of unfused protoplasts, heterokaryons, and homokaryons. From this varied mixture, methods are developed to choose the hybrid cells. Three choices are available for selection:

1. Biochemical technique: This technique separates the fused cells from the unfused cells using biological substances. There are two approaches.

• Drug sensitivity: In this procedure, one protoplast is antibiotic-resistant, preventing the other protoplast from growing in its presence. For instance, if protoplast 1 is resistant to actinomycin D but protoplast 2 is not, the fused protoplast will acquire the traits of both following unions. Protoplast 2 will not be able to develop, fused protoplasts will, and protoplast 1 produces little colonies that can be recognized and separated when the cells are cultivated on a medium containing the antibiotic.
• Mutants with auxotrophy: Mutants known as auxotrophs are unable to develop on a minimum media. The parental cells cannot grow in the minimum media, however, the hybrids can, allowing for cell selection.

2. Visual technique: Since the hybrid cells must be chosen physically and visually using this procedure, it is quite time-consuming. Using this technique, cells that develop on various mediums are combined and then visually separated. Another approach involves manually dividing the hybrid cells using a Drummond pipette. 
3. Cytometric technique: For simple cell selection, modern techniques like flow cytometry and fluorescent cells are used.

The initial somatic fusion of two distinct protoplasts must be confirmed as the source of any produced hybrids. The following list includes several methods of identification:

• Morphology: Plant regeneration is accomplished by protoplast fusion, and the resulting organisms display a range of morphological traits. Hybrid verification can rely on them. The physical characteristics of somatic or sexual hybrids often lie halfway between the two parents.
• Isoenzyme Analysis: The many molecular forms of the enzyme that catalyze the same reaction is known as isoenzymes. It is common practice to use isoenzyme electrophoretic banding patterns to establish hybridity. Somatic hybrids may display isoenzyme bands of certain enzymes that belong to both parents simultaneously and only one of the parents individually.
• Chromosomal Constitution: Counting the number of chromosomes in the cells is a rapid and accurate approach to determine whether they are hybrid. It also demonstrates the ploidy state of the cells.
• Molecular Techniques: Somatic hybrids can be confirmed using species-specific restriction pieces of nuclear DNA that code for ribosomal RNA. Hybrid identification has been carried out using the PCR technique.

Some of the applications of somatic hybridisation are:

• Crop Improvement: Somatic hybridization is used to develop crop varieties with desirable traits such as disease resistance, stress tolerance, and improved yield.
• Genetic Diversity Enhancement: By combining genetic material from different plant species, somatic hybridization contributes to the enrichment of genetic diversity in breeding programs.
• Creation of Novel Varieties: This technique enables the creation of novel plant varieties with unique combinations of traits not found in traditional breeding programs.
• Biotechnological Research: Somatic hybridization is instrumental in studying gene expression, functional genomics, and understanding the mechanisms of plant development and stress responses.
• Disease Resistance: Disease-resistance genes have been able to spread from one plant to many others due to somatic hybridization. The spotted wilt virus, TMV, insect pests, and cold tolerance can no longer harm tomatoes.

Some of the examples of somatic hybridisation are:

• Pomato: Created by merging tomato and potato protoplasts, resulting in a single plant that produces both tomatoes and potatoes.
• Wheat-Agropyron hybrids: Used to transfer genes for disease resistance and environmental adaptation from wild grasses to cultivated wheat.
• Citrus intergeneric hybrids: Generated by fusing protoplasts from different citrus species, producing hybrids with improved fruit quality and disease resistance.
• Brassica napus (rapeseed) hybrids: Utilized to introduce novel traits like herbicide resistance and increased oil content from related Brassica species.
• Banana breeding: Employed to combine traits for disease resistance and improved fruit quality from different banana varieties.

The advantages are:

• Somatic hybridization can be performed on young, immature plants.
• It is now simple to research cytoplasmic genes and how they work.
• Somatic hybridization was used to transfer genes into plants to provide environmental tolerance against cold and frost.
• Selective somatic hybrids with quality traits, such as the generation of high nicotine concentration, have been created.
• In the hybrid cell, the protoplast fusion results in a distinctive nuclear-cytoplasmic combination.
• It produces novel plants with desirable traits.
• An alternate technique for creating remote hybrids with desirable qualities considerably across species or genera, which cannot be achieved by the traditional approach of sexual hybridization, is somatic cell hybridization, parasexual hybridization, or protoplast fusion.

The limitations of somatic hybridisation are:

• The created plants are not always healthy and fruitful.
• The regenerated plants produced through somatic hybridization occasionally display diversity for a variety of causes, including soma clonal variations, chromosomal elimination, organelle segregation, etc.
• However, it does not necessarily result in the production of viable seeds, making the fusion of distant plant genera feasible.
• Genetic instability can occasionally result from protoplast fusion.
• The effective manifestation of a certain characteristic is not necessarily ensured by somatic hybridization.
• There are only a few selection criteria.

Somatic hybridization offers a revolutionary approach to plant breeding. It enables the creation of hybrid plants with desirable traits by fusing protoplasts from different species. Unlike traditional methods, somatic hybridization does not rely on sexual reproduction, thus overcoming species barriers. While it presents numerous advantages such as enhanced genetic variation and the creation of novel varieties, somatic hybridization also faces limitations such as genetic instability and the occasional production of unhealthy plants. Despite these challenges, somatic hybridization remains a promising tool for meeting the growing demands of agriculture and biotechnology in the future.

Also Read:

• Difference Between Cybrids and Hybrids
• Plant Breeding
• What is Triple Fusion?

Which Chemical is Used in Somatic Hybridization?

No specific chemical is universally used in somatic hybridization, but commonly, polyethylene glycol (PEG) is used to promote protoplast fusion.

What is the Difference Between Somatic Hybridization and Somaclone?

Somatic hybridization involves the fusion of protoplasts from different plant species to create hybrid plants, while somaclone refers to plants regenerated from a single somatic cell or tissue culture, often through genetic variation or mutation.

What is Somatic Hybridization in Fungi?

Somatic hybridization in fungi involves the fusion of fungal protoplasts to create hybrids with combined traits, potentially leading to improved characteristics such as increased productivity or disease resistance.

What is Pomato by Somatic Hybridization?

Pomato is a hybrid plant created through somatic hybridization between tomato and potato plants, resulting in a single plant that produces both tomatoes and potatoes.

What is Somatic Embryogenesis and Somatic Hybridization?

Somatic embryogenesis is a process where somatic cells are induced to form embryos, while somatic hybridization involves the fusion of cells from different plant species to create hybrid plants with combined traits, including those produced through somatic embryogenesis.

What is Somatic Hybridization Class 12?

Somatic hybridization is a biotechnological technique involving the fusion of protoplasts from different plant species to create hybrid plants with desirable traits, facilitating the transfer of genetic diversity and the development of improved crop varieties.