Key Steps of Somatic Hybridization in Plant Biology
Somatic hybridization is the process of creating hybrid plants by fusing isolated somatic (plant body/vegetal cells other than the reproductive cells) protoplasts in a lab setting. The resulting heterokaryon is then developed into a hybrid plant. A new hybrid cell with traits from both parent plants can be created by fusing the protoplasts of two separate plant cells using the somatic hybridization process.
The improvement of domesticated plants has long been accomplished by sexual hybridization between closely related species. Unfortunately, the majority of the time sexual hybridization is restricted to cultivars within a species or, at best, to a small number of closely related wild species. Thus, the value of sexual hybridization for agricultural enhancement is constrained by species barriers.
Somatic Cell Fusion
Somatic cell fusion is the joining of two separate cells' protoplasts to create functional cell hybrids. In the hybrid cell, the cytoplasm and nuclei of both parents are combined. Sometimes a fused hybrid, also known as a cybrid or cytoplasmic hybrid, contains the nuclear genome of just one parent but the cytoplasmic genes (plastome) of both parents. These hybrids produced by the fusion of somatic cells are an excellent approach to getting around any species' barriers to sexual hybridization.
Somatic Hybridization Technique
Protoplast fusion, choosing hybrid cells, and identifying hybrid plants are the three components of somatic hybridization.
Fusion of Protoplasts
Plant cells without a cell wall are known as protoplasts; nevertheless, they do have a plasma membrane. Both spontaneous and induced fusion techniques can be used to accomplish protoplast fusion, which includes combining protoplasts with two distinct genomes.
When protoplasts are isolated from callus cultures, the spontaneous fusing of the protoplasts is seen. However, aside from going through a few divisions, spontaneous fusion products do not regenerate into complete plants and must be triggered by a variety of methods.
Methods of Induced Fusion: To fuse plant protoplasts from various origins, an appropriate substance (fusogen) is introduced. The various fusogens used include lysozyme, high pH/Ca++, NaNO3, artificial seawater, polyethylene glycol (PEG), electrofusion, and others.
Fusion Mechanism
The three primary stages of protoplast fusion are as follows:
Agglutination or Adhesion: Two or more protoplasts are brought together. Numerous treatments, such as PEG, high pH, and high Ca++ ions, might cause adhesion.
Plasma membranes of protoplasts that have been agglutinated by fusogen fuse at the point of adhesion.
Plasma membrane fusion at limited sites cytoplasmic bridges is created between the protoplasts as a result.
Rounding off of the fused protoplasts as a result of the expansion of cytoplasmic bridges results in the formation of spherical heterokaryon or homokaryon.
Selection of Hybrid Cells
Despite efforts to improve protoplast fusion efficiency, typically only 1 - 10% of the protoplasts in a treated population have undergone fusion. In order to choose the hybrid cells from this heterogeneous combination, procedures are developed. A few of them include:
Complementary Auxotroph: Auxotrophs are mutants that are unable to thrive in a minimal environment. When two protoplasts harbouring distinct recessive/deficient markers fuse together, complementation results in a fusion product with functional restoration. Since the parental cells cannot grow in the minimum media, the hybrids can be chosen because they can grow there, while the parental cells cannot.
Use of Metabolic Inhibitors: In this technique, parental cells are exposed to an irreversible biochemical inhibitor, such as iodoacetate or diethylpyrocarbonate and only hybrid cells are able to divide as a result.
Use of Visual Characteristics: The most efficient but the most tedious method to select products of protoplast fusion is to visually identify hybrid cells and mechanically isolate individual cells. This includes the use of morphologically distinct cells, fluorescent labelling, etc.
Identification of Hybrid Plants
Hybrids that have been developed must be verified as products of somatic fusion of two different protoplasts. Some ways of identification are listed below:
Morphology: When plant regeneration is achieved through protoplast fusion, the end products exhibit a variety of morphological characteristics. On them, hybrid verification can rely. Most of the time, the morphological traits of somatic or sexual hybrids fall somewhere in the middle between the two parents.
Isoenzyme Analysis: Isoenzymes are the different molecular forms of the enzyme that catalyse the same reaction. Isoenzyme electrophoretic banding patterns have been widely employed to confirm hybridity. Somatic hybrids may exhibit isoenzyme bands of specific enzymes belonging to either one of the parents, or both parents, at once.
Chromosomal Constitution: A quick and accurate way to confirm hybrid cells is by counting the number of chromosomes in the cells. Additionally, it reveals the ploidy condition of the cells.
Molecular Techniques: Species-specific restriction fragments of nuclear DNA coding for ribosomal RNA have been shown to verify somatic hybrids. PCR technology has been utilised for hybrid identification.
Advantages
Beyond the boundaries of sexual crossability, it gives us the chance to create hybrids between taxonomically dissimilar plant species. Additionally, it produces cells with novel genomic, nuclear, and cytoplasmic constitutions that would not otherwise be possible.
Limitations
Only a few somatic hybrid plants have been used as commercial varieties, including Brassica spp and the potato. This is due to the fact that similar to sexual breeding, the more genetically distant the parents being crossed, the more chromosomally unstable and sterile are their offspring, often producing material that is unsuitable for breeding.
Applications
Creation of brand new interspecific and intergeneric fusions between plants that are difficult or impossible to hybridise by traditional methods. It removes obstacles caused by sexual incompatibility.
Disease Resistance: Somatic hybridization has allowed the spread of disease resistance genes from one plant to numerous others. Tomatoes now have the ability to resist a number of illnesses, including TMV, the spotted wilt virus, insect pests, and cold tolerance.
Resistance to Abiotic Stress: Research on somatic hybridization for resistance to abiotic stress has been focused on the families Fabaceae, Brassicaceae, Poaceae, and Solanaceae and relates to cold and frost resistance.
Cytoplasmically encoded features such as some forms of male sterility and specific antibiotic and herbicide resistance traits are among the traits that are useful for agriculture. Resistance to antibiotics, herbicides as well as CMS has been introduced in so many cultivated species.
Summary
In somatic hybridization, the cytoplasm and nuclei of both parents are combined to form a hybrid cell which has the traits of both the fused parent cells. Somatic cells can be generated by somatic hybridization which consists of 3 stages: protoplast fusion, selection of hybrid cells, and identification of hybrid plants. Hybrid plants produced by somatic hybridization enable the construction of hybrids between taxonomically distant plant species beyond the limits of sexual crossability. These hybrid plants have better qualities/traits including disease and abiotic stress resistance. Despite the benefits, only a few commercial varieties are available as the progeny obtained are sterile and chromosomally unstable.
FAQs on Somatic Hybridization Explained: Process, Benefits & Uses
1. What is somatic hybridization in simple terms?
Somatic hybridization is a genetic technique used in laboratories to create a new hybrid plant by fusing the body cells (somatic cells) of two different plant species. This process allows scientists to combine desirable traits from plants that cannot be cross-bred using traditional methods.
2. What are the main steps involved in creating a somatic hybrid?
The process of somatic hybridization generally involves four key steps:
- Protoplast Isolation: The cell walls of the parent plant cells are first removed using enzymes, leaving behind the naked cells known as protoplasts.
- Protoplast Fusion: These protoplasts from two different species are then made to fuse together, often using a chemical like polyethylene glycol (PEG) or a mild electric shock.
- Hybrid Selection: After fusion, the successfully fused hybrid cells are carefully identified and separated from the cells that did not fuse.
- Culture and Regeneration: Finally, the selected hybrid cells are grown in a special nutrient medium where they develop a new cell wall, start dividing, and eventually regenerate into a whole new hybrid plant.
3. What are some real-world applications of somatic hybridization?
This technique is mainly used in agriculture for crop improvement. Some important applications include creating plants that have:
- Increased resistance to diseases, pests, or harsh environmental conditions like drought.
- Improved nutritional value or higher yield by combining the best traits from two different species.
- Unique characteristics by crossing plants that are too distantly related to breed naturally, creating new interspecific (between species) hybrids.
4. Is the pomato, a hybrid of potato and tomato, a successful example?
The pomato is a famous scientific example of somatic hybridization, created by fusing cells from a potato and a tomato plant. While it successfully demonstrated that the technique could work, the pomato was not a commercial success. The resulting plant had poor yield and quality, so it is used more as a proof-of-concept in textbooks than as a viable agricultural crop.
5. How is a somatic hybrid different from a cybrid?
The key difference is in their genetic makeup. A somatic hybrid contains the complete genetic material—both the nucleus and the cytoplasm—from both parent cells. In contrast, a cybrid, or cytoplasmic hybrid, contains the nucleus from only one parent but the cytoplasm from both. This makes cybrids useful for studying the specific effects of cytoplasmic genes.
6. Why do scientists use somatic hybridization when they can just cross-breed plants?
Traditional cross-breeding (sexual hybridization) is limited by natural reproductive barriers, which prevent distantly related species from producing offspring. Somatic hybridization bypasses these barriers completely by fusing cells directly in a lab. This allows for the creation of novel hybrids between species that could never be crossed naturally, opening up a wider range of genetic combinations for crop improvement.
7. What are the biggest challenges or limitations of this technique?
Despite its potential, somatic hybridization has significant drawbacks. The resulting hybrid plants often suffer from genetic instability, where chromosomes are lost or rearranged, leading to unpredictable traits. Many hybrids are also sterile or not viable. The process itself is technically complex, and there is no guarantee that the desired traits from both parents will be properly expressed in the new plant.
