Somatic hybridization | Agriculture Optional Notes for UPSC PDF Download

Introduction

Somatic hybridization is an innovative method that enables the fusion of two distinct plants, resulting in a new hybrid plant that inherits characteristics from both parent plants.
Traditionally, sexual hybridization was employed to create hybrid plants. However, this approach had limitations. It only allowed closely related species to be hybridized, and there were compatibility issues. Somatic hybridization overcomes these limitations. It involves the in vitro fusion of protoplasts to generate a hybrid cell, which is then cultured to develop into a hybrid plant.
Somatic hybridization comprises three key components: the fusion of protoplasts, the selection of hybrid cells, and the identification of hybrid plants.
We will delve into each of these aspects in detail:

Aspects of Somatic Hybridization

Protoplast Fusion

Protoplasts, which lack a cell wall, facilitate easy fusion without encountering compatibility issues when merging two genomes.
This fusion can be accomplished using three methods:

• Mechanical Fusion: Protoplasts are brought into contact by placing them on a concave slide, although this method can occasionally damage the protoplasts.
• Spontaneous Fusion: A natural process occurring during the enzymatic breakdown of cell walls, where surrounding protoplasts fuse without physical intervention, although this does not yield complete plants.
• Induced Fusion: Isolated protoplasts are fused with the aid of chemical fusogens such as NaNO3, PEG, polyvinyl alcohol, lysozyme, dextran, fatty acids, and electrofusion.
  The mechanism of induced fusion involves:
  • Agglutination/Adhesion: Fusogens like PEG and NaNO3 bring two protoplasts into close contact, causing them to adhere.
  • Plasma Membrane Fusion: The protoplast membranes fuse at the adhesion site, forming a cytoplasmic bridge. The rate of membrane fusion can be increased with high pH and Ca2+ concentration.
  • Formation of Heterokaryons: The fused protoplasts assume a spherical shape, forming either a homokaryon (with the same genetic material) or a heterokaryon (with different genetic material).

Selection of Hybrid Cells

Not all protoplasts fuse; typically, only 20-25% form heterokaryons, resulting in a mixture of homokaryons, heterokaryons, and unfused protoplasts.
Three selection methods are employed:

• Biochemical Method: Selection of fused cells from unfused ones using biochemical compounds:
  • Drug Sensitivity: One protoplast is resistant to antibiotics, and the other is not. After fusion, the hybrid protoplast will gain resistance, allowing selection when grown on an antibiotic-containing medium.
  • Auxotrophic Mutants: Hybrids can grow in a minimal medium, while parental cells cannot.
• Visual Method: Involves visually and mechanically selecting hybrid cells, either by growing cells on different media or using a pipette (e.g., Drummond pipette) for mechanical separation.
• Cytometric Method: Modern techniques like flow cytometry and fluorescent markers are used for efficient cell selection.

Identification of Hybrid Plants

After developing from hybrid cells, molecular evidence is needed for the identification of hybrid plants.
Several common approaches include:

• Morphology of Hybrid Plants: Hybrid plants typically exhibit intermediate characteristics of the parent plants, making them easily identifiable.
• Isoenzyme Analysis: Isoenzymes, which are different forms of the same enzymes catalyzing various reactions, can be analyzed electrophoretically to confirm hybridity.
• Symmetric and Asymmetric Hybrids: Symmetric hybrids have the same chromosome number as their parents and are sterile. Asymmetric hybrids, on the other hand, have abnormal chromosome numbers or ploidy levels.

Applications of Somatic Hybridization

• Disease Resistance:
  • Disease resistance genes from one plant have been transferred to various other plants using somatic hybridization.
  • For instance, somatic hybridization has enabled the creation of disease-resistant tomato hybrids, protecting them against pathogens like TMV and spotted wilt virus.
• Environmental Tolerance:
  • Somatic hybridization has been employed to develop plants with the ability to thrive in extreme environmental conditions, such as cold, heat, and saline environments.
• Quality Traits:
  • Desired characteristics, including high protein content, have been introduced into numerous plant varieties through somatic hybridization.
• Cytoplasmic Male Sterility:
  • Many traits in plant cells are controlled by the cytoplasm, and somatic hybridization offers a means to introduce such traits into hybrids.
  • Examples of these traits include resistance to antibiotics, herbicides, and the induction of male sterility.

Advantages of Somatic Hybridization

• Feasibility in Young Plants: Somatic hybridization is feasible in early, immature plants.
• Simplified Study of Cytoplasmic Genes: This technique facilitates the study of cytoplasmic genes and their functions.
• Unique Nuclear-Cytoplasmic Combination: Protoplast fusion results in a distinctive combination of nuclear and cytoplasmic factors within the hybrid cell.
• Generation of Novel Plants with Desirable Traits: It leads to the creation of innovative plants with favorable characteristics.

Disadvantages of Somatic Hybridization

• Limited Viability and Fertility: Plants produced through somatic hybridization are not always both viable and fertile.
• Genetic Instability due to Protoplast Fusion: Fusion of protoplasts can, at times, introduce genetic instability.
• Variable Seed Production: Although it allows fusion between distant plant genera, the production of viable seeds is not always guaranteed.
• Restricted Selection Methods: The available methods for selecting desired hybrids are limited.