Somatic Hybridisation
In conventional plant breeding, new varieties are created by crossing two different plants through sexual reproduction. However, this method has a major limitation: it is often impossible to cross plants from entirely different species or genera due to natural sexual incompatibility. To overcome this barrier, scientists developed a technique called somatic hybridisation. It involves the fusion of somatic cells (regular body cells, not reproductive cells) from two different plant varieties to create a new hybrid plant that combines the desirable characteristics of both parents.
The Concept of Protoplast
To understand somatic hybridisation, one must first understand the structure of a plant cell. A typical plant cell is surrounded by a rigid cell wall. Inside this wall is the cell membrane (plasma membrane) enclosing the cytoplasm and nucleus. If scientists use specific enzymes (like cellulase and pectinase) to digest and completely remove the rigid cell wall, the remaining “naked” cell is called a protoplast. Because protoplasts lack a cell wall, they can easily be fused together.
Steps Involved in Somatic Hybridisation
The process of somatic hybridisation is carried out in a sterile laboratory environment and involves a logical sequence of steps:
- Isolation of Protoplasts: Single cells are isolated from the leaves or other tissues of two different plant varieties. These cells are treated with enzymes to dissolve their cell walls, releasing the naked protoplasts.
- Fusion of Protoplasts: The isolated protoplasts from the two different plants are brought together. Their fusion is artificially induced using a chemical agent like Polyethylene Glycol (PEG) or through a mild electrical pulse (electrofusion).
- Formation of a Somatic Hybrid: When the two protoplasts fuse, their cytoplasms and nuclei combine to form a single hybrid cell. This new cell is called a somatic hybrid.
- Cell Wall Regeneration and Culturing: The hybrid cell is placed in a special nutrient medium. It first regenerates its cell wall and then begins to divide continuously, forming a mass of unspecialised cells called a callus.
- Plant Regeneration: Using tissue culture techniques and specific plant growth hormones (like auxins and cytokinins), the callus is stimulated to develop roots and shoots, eventually growing into a complete, new hybrid plant.
The Example of the Pomato
A famous, real-life example of somatic hybridisation is the creation of the pomato. Scientists isolated the protoplast of a tomato plant and fused it with the protoplast of a potato plant. The resulting somatic hybrid was grown into a new plant combining the characteristics of both. The goal was to grow a plant that produced tomatoes above the ground and potatoes below the ground. While the scientific fusion was successful and the plant grew, it unfortunately did not possess the exact combination of traits required for successful commercial farming.
Significance in Agriculture
Despite challenges like those seen with the pomato, somatic hybridisation holds immense value for agricultural science:
- Overcoming Incompatibility: It allows breeders to bypass natural sexual barriers, enabling the transfer of beneficial genes (like disease resistance or drought tolerance) between distantly related or entirely different plant species.
- Creating Novel Plants: It opens up the possibility of creating entirely new, man-made plant varieties that do not exist in nature, expanding the genetic diversity available for future food security.
