Model Answer
0 min readIntroduction
Cybridization, also known as protoplast fusion, represents a significant advancement in plant breeding and biotechnology. It involves the fusion of somatic cells (cells other than gametes) from two different plants, resulting in a hybrid cell containing genomes from both parents. This technique bypasses sexual reproduction barriers, allowing for the creation of novel combinations of genetic material. Cybrids are particularly useful in transferring cytoplasmic traits, such as disease resistance or herbicide tolerance, while retaining desirable nuclear characteristics. The development of cybridization techniques has opened new avenues for crop improvement and understanding plant cellular processes.
What is Cybridization?
Cybridization is a technique in plant biotechnology where protoplasts (plant cells without cell walls) from two genetically distinct plants are fused to create a hybrid cell. This hybrid cell, the cybrid, contains a mixture of genomes – the nuclear genome from one parent and the cytoplasmic genome (including organelles like mitochondria and chloroplasts) from the other. It’s important to note that cybrids are generally sterile due to the incompatibility of the combined genomes during cell division.
An Economically Useful Cybrid: Tobacco Cybrids
Tobacco (Nicotiana tabacum) cybrids are a prime example of economically useful cybrids. They have been extensively used in research to study cytoplasmic male sterility (CMS) and to transfer disease resistance genes. Specifically, cybrids created by fusing protoplasts from disease-resistant wild Nicotiana species with cultivated tobacco varieties have shown enhanced resistance to viruses like Tobacco Mosaic Virus (TMV). This allows for the development of improved tobacco cultivars without compromising yield or quality.
Methods to Produce Cybrids
Several methods are employed to produce cybrids, broadly categorized into mechanical and chemical methods:
1. Mechanical Methods
- Protoplast Isolation: The first step involves isolating protoplasts from the desired plant tissues (leaves, roots, etc.) using enzymes like cellulase and pectinase to remove the cell walls.
- Protoplast Fusion: This can be achieved through:
- Polyethylene Glycol (PEG) Method: PEG induces protoplast fusion by altering the cell membrane permeability. PEG of high molecular weight (typically 4000-6000) is used at concentrations of 30-60%.
- Electrofusion: Protoplasts are aligned using an electric field and then subjected to a short pulse of high-voltage electricity, causing membrane fusion. This method is more efficient and avoids the toxicity associated with PEG.
- Microinjection: Directly injecting the cytoplasm of one protoplast into another using a micropipette. This is a labor-intensive method but offers precise control.
- Selection and Culture: Fused protoplasts (heterokaryons) are selected based on their hybrid nature (often using marker genes) and cultured in a suitable medium to allow cell division and callus formation.
2. Chemical Methods
- Dimethyl Sulfoxide (DMSO): DMSO can be used in conjunction with PEG to enhance protoplast fusion.
- Calcium Ions (Ca2+): High concentrations of Ca2+ can promote membrane fusion.
Table Summarizing Cybrid Production Methods
| Method | Principle | Advantages | Disadvantages |
|---|---|---|---|
| PEG Fusion | PEG alters membrane permeability | Simple, widely used | PEG toxicity, lower efficiency |
| Electrofusion | Electric field induces membrane fusion | High efficiency, no toxicity | Requires specialized equipment |
| Microinjection | Direct cytoplasm transfer | Precise control | Labor-intensive, low throughput |
Conclusion
Cybridization is a powerful tool in plant biotechnology, enabling the transfer of valuable cytoplasmic traits and facilitating the creation of novel genetic combinations. While cybrids are typically sterile, they serve as crucial intermediates in breeding programs and provide valuable insights into plant cellular biology. Continued advancements in protoplast fusion techniques, particularly electrofusion, promise to enhance the efficiency and applicability of cybridization in crop improvement and fundamental research.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.