Unit 1: Plant Tissue Culture
1. Historical Perspective and Media Composition
Plant tissue culture (PTC) is the in vitro cultivation of plant cells, tissues, or organs on a nutrient medium under aseptic conditions.
Historical Perspective
- Gottlieb Haberlandt (1902): Known as the father of tissue culture; he first attempted to culture isolated plant cells.
- Hanning (1904): Successfully cultured embryos from cruciferous plants.
- White (1934): Successfully established continuous root cultures of tomato.
- Murashige and Skoog (1962): Developed the MS medium, which remains the most widely used medium in PTC labs.
Composition of Media
A standard culture medium must provide all essential nutrients for plant growth. Major components include:
- Inorganic Nutrients: Macronutrients (N, P, K, Ca, Mg, S) and Micronutrients (Fe, Mn, Zn, B, Cu, Mo).
- Carbon Source: Usually sucrose (2-4%), providing energy as cells are often not fully autotrophic in vitro.
- Gelling Agent: Agar is commonly used to solidify the medium for supporting the explants.
- Vitamins and Amino Acids: Thiamine (B1) is essential; others like Myo-inositol are often added.
2. Role of Vitamins, Hormones, and Totipotency
Success in tissue culture relies heavily on the physiological state of the cells and the external chemical signals provided in the medium.
Totipotency
Definition: The inherent ability of a single living plant cell to differentiate and regenerate into a whole, complete plant.
Role of Plant Growth Regulators (Hormones)
The balance between different hormones determines the pathway of development:
- Auxins (e.g., NAA, 2,4-D): Primarily used for callus induction and root formation.
- Cytokinins (e.g., BAP, Kinetin): Used to promote cell division and shoot bud differentiation.
- Auxin/Cytokinin Ratio: High Auxin favors rooting; High Cytokinin favors shooting; intermediate concentrations promote callus growth.
3. Organogenesis and Embryogenesis
These are the two primary pathways for regenerating whole plants from tissue culture.
Organogenesis
The process of forming organs (roots or shoots) either directly from the explant or indirectly from a callus. It is controlled by the balance of plant growth regulators in the medium.
Embryogenesis (Somatic)
The development of embryos from somatic (non-germ) cells. These somatic embryos resemble zygotic embryos in their bipolar structure (having both a shoot and root pole).
4. Protoplast Isolation, Culture, and Fusion
Protoplasts are "naked" plant cells where the cell wall has been removed.
- Isolation: Achieved using cell-wall-degrading enzymes like cellulase and pectinase in an isotonic osmoticum.
- Culture: Protoplasts are grown in liquid or semi-solid media where they regenerate a cell wall before dividing.
- Fusion (Somatic Hybridization): Two different protoplasts can be fused using chemical agents (like PEG) or electric current (electrofusion) to create hybrid cells. This allows for the crossing of sexually incompatible species.
5. Micro-propagation, Androgenesis, and Haploid Production
Micro-propagation
The large-scale clonal propagation of plants under in vitro conditions. It allows for the rapid production of thousands of identical, disease-free plants from a single parent.
Androgenesis and Haploid Production
The production of haploid plants from male gametophytic cells (pollen grains or anthers).
- Significance: Haploids are useful in plant breeding to produce homozygous lines quickly after chromosome doubling.
6. Specialized Cultures and Virus Elimination
- Secondary Metabolite Production: Cell suspension cultures can be used as "biological factories" to produce high-value chemicals like drugs, dyes, and flavors.
- Virus Elimination: Viruses are often absent from the shoot apical meristem. Culturing the meristem (Meristem Culture) can produce virus-free plants from an infected stock.
- Triploids: Produced through endosperm culture, often resulting in seedless fruits.
7. Cryopreservation and Germplasm Conservation
Methods to preserve plant genetic resources for long periods.
- Cryopreservation: Storage of cells or tissues at ultra-low temperatures, usually in liquid nitrogen (-196°C). This stops all metabolic activity without killing the cells.
- Germplasm Conservation: PTC allows for the maintenance of large collections of genetic diversity in small spaces, protected from pests and climate change.
8. Exam Focus: Tips and FAQs
Exam Tip: Understand the difference between Direct and Indirect Organogenesis. Direct happens straight from the explant, whereas Indirect involves an intermediate Callus stage. This is a common point of confusion in exams.
Frequently Asked Questions
Q: Why is sucrose used in PTC media?
A: Explants are often heterotrophic and cannot perform enough photosynthesis to survive; sucrose provides the necessary carbon and energy source.
Q: What is the main advantage of somatic hybridization?
A: It allows breeders to combine traits from two different species that cannot be bred naturally due to sexual incompatibility.