A comprehensive guide to Tissue Culture, Hybridization, and Cryopreservation
Brief History: The concept of cell totipotency (the ability of a single cell to divide and produce all the differentiated cells in an organism) was first proposed by Gottlieb Haberlandt in 1902. Later, in the 1950s, Skoog and Miller discovered the vital role of plant growth regulators (auxins and cytokinins) in manipulating cell division and organogenesis.
Plant Tissue Culture relies on the plasticity and totipotency of plant cells. By providing a sterile nutrient medium (like MS Medium) containing specific carbon sources, vitamins, and hormones, isolated plant parts (explants) can regenerate into complete plants under controlled laboratory conditions.
Selection of healthy tissue (leaf, stem, root). It is washed with disinfectants (Sodium hypochlorite or Mercuric chloride) to eliminate surface microbes.
The sterile explant is placed on a nutrient agar medium. Unorganized cell division begins to form a mass of cells called a Callus.
Hormone Role: Equal ratio of Auxin & Cytokinin induces CallusThe callus is transferred to a medium formulated to promote the growth of multiple shoots.
Hormone Role: High Cytokinin : Low Auxin ratio promotes Shoot formationThe developed shoots are excised and transferred to a rooting medium.
Hormone Role: High Auxin : Low Cytokinin ratio promotes Root formationPlantlets are moved from sterile lab conditions (in vitro) to a greenhouse (in vivo) to adapt to natural environmental stress.
Mass multiplication of disease-free, genetically identical plants (clones) in a remarkably short time.
Brief History: In 1972, Peter Carlson produced the first interspecific somatic hybrid between *Nicotiana glauca* and *Nicotiana langsdorffii*.
Somatic hybridization involves fusing the protoplasts (plant cells without cell walls) of two different species to create a hybrid plant. This overcomes sexual incompatibility barriers.
Plant tissues are treated with cell wall degrading enzymes: Cellulase, Hemicellulase, and Pectinase. This releases naked protoplasts into an osmoticum (to prevent bursting).
Protoplasts from two species are mixed. Polyethylene Glycol (PEG) is added. PEG acts as a fusogen; it reduces the electrostatic repulsion between protoplast membranes, causing them to adhere and eventually fuse.
The mixture contains unfused protoplasts of both species, homokaryons (fused identical cells), and heterokaryons (the desired hybrid). Selection methods (like biochemical markers) are used to isolate the hybrids.
The hybrid protoplast regenerates a cell wall and divides to form a callus. The callus undergoes Somatic Embryogenesis. The distinct stages of the embryo are: Globular -> Heart-shaped -> Torpedo-shaped -> Cotyledonary stage.
Hormone Role: Auxin (like 2,4-D) initiates embryogenesis, removal of auxin progresses the embryo stages.Creation of a novel hybrid plant (e.g., the "Pomato"—a hybrid of potato and tomato) containing genetic material from both parents, even if they cannot naturally breed.
Brief History: Sipra Guha and S.C. Maheshwari (1964) were the first to produce haploid plants from the anthers of *Datura innoxia*.
Pollen culture is the in vitro cultivation of immature pollen grains to produce haploid plants. These plants contain only one set of chromosomes (n).
* Immediate expression of recessive mutations (since there is no dominant allele to mask them).
* Production of homozygous diploids (pure lines) in a single generation by treating haploids with Colchicine (which doubles the chromosomes).
Flower buds containing pollen at the uninucleate stage are selected and surface sterilized.
Anthers are excised or pollen is squeezed out and cultured on solid or liquid media.
The pollen grain divides to form a multicellular structure, bypassing normal male gametophyte development, directly forming an embryo or callus.
Haploid plantlets are generated and subsequently treated with colchicine to restore diploidy if pure lines are required.
Detailed Theory: Cryopreservation is the storage of biological materials (cells, tissues, embryos, seeds) at ultra-low temperatures, usually in Liquid Nitrogen (-196°C). At this temperature, all cellular metabolic activities and enzymatic processes are completely halted, allowing indefinite preservation of germplasm.
Cells are cold-acclimated to increase their tolerance to freezing.
Chemicals like DMSO (Dimethyl sulfoxide) or Glycerol are added. They prevent the formation of lethal intracellular ice crystals that would puncture cell membranes.
Samples are frozen (either slowly or ultra-rapidly) and plunged into liquid nitrogen for long-term storage.
Rapid thawing in a warm water bath (37°C) to prevent ice recrystallization, followed by washing to remove toxic cryoprotectants.
The viable cells are placed back onto tissue culture media to resume growth.
Click on the question to reveal the correct answer.