Plant Tissue Culture

Plant Tissue Culture (PTC) is defined as the cultivation of undifferentiated mass of plant cells, tissues, or organs on artificial (nutrient) media under aseptic (sterile) and controlled environmental conditions (light, temperature, humidity) *in vitro* (outside the living organism, i.e., in glass/plastic vessels).

Key features:
- The technique is based on the principle of totipotency — the ability of a single cell or a group of cells to regenerate into a complete plant.
- Any plant part (explant) such as leaf, stem, root, embryo, apical meristem, etc., can be cultured.
- The cultured cells first form an undifferentiated mass called callus, which can then be induced to differentiate into a complete plant.

In summary: PTC is an *in vitro* technique that allows the growth and regeneration of whole plants from small plant parts under sterile, controlled conditions.

Plant Tissue Culture Media — Components:

The most widely used medium is Murashige and Skoog (MS) medium. Its components are:

1. Inorganic (Macro and Micro) Nutrients:
- *Macronutrients:* Required in large amounts — N, P, K, Ca, Mg, S (e.g., KNO₃, NH₄NO₃, MgSO₄, CaCl₂, KH₂PO₄).
- *Micronutrients:* Required in trace amounts — Fe, Mn, Zn, B, Cu, Mo, Co, I.
- These provide essential mineral elements for cell growth and metabolism.

2. Carbon Source:
- Usually sucrose (2–3%) is added as the primary carbon and energy source since cultured cells/tissues cannot photosynthesize efficiently.

3. Plant Growth Regulators (Hormones):
- Auxins (e.g., IAA, NAA, 2,4-D): Promote root formation and callus induction.
- Cytokinins (e.g., BAP, kinetin): Promote shoot differentiation and cell division.
- The ratio of auxin to cytokinin determines whether roots or shoots are formed:
- High auxin : Low cytokinin → Root formation
- Low auxin : High cytokinin → Shoot formation
- Equal ratio → Callus formation
- Gibberellins and Abscisic acid are also used in specific cultures.

4. Vitamins:
- Thiamine (B₁), Pyridoxine (B₆), Nicotinic acid, Myo-inositol — required for normal cell metabolism.

5. Organic Supplements:
- Amino acids (e.g., glutamine, asparagine), casein hydrolysate, yeast extract — provide nitrogen and growth factors.

6. Gelling Agents:
- Agar (0.6–1.0%) is used to solidify the medium for solid/semi-solid cultures.
- Liquid media (without agar) are used for suspension cultures.

7. Antibiotics:
- Added to prevent microbial (bacterial/fungal) contamination, e.g., ampicillin, kanamycin.

8. pH:
- The pH of the medium is adjusted to 5.7–5.8 before autoclaving. Incorrect pH affects nutrient solubility and agar solidification.

9. Water:
- Distilled or double-distilled water is used to prepare the medium to avoid contamination with ions.

General Steps of Plant Tissue Culture:

Step 1: Selection of Explant
- Choose a suitable plant part (explant) such as leaf, stem, root tip, apical meristem, embryo, etc., from a healthy donor plant.

Step 2: Surface Sterilization (Decontamination)
- The explant is surface-sterilized to remove microorganisms:
- Wash with detergent and running water.
- Treat with 70% ethanol for 30–60 seconds.
- Treat with sodium hypochlorite (1–2%) or mercuric chloride (0.1%) for 5–15 minutes.
- Rinse 3–4 times with sterile distilled water inside the laminar air flow cabinet.

Step 3: Preparation of Nutrient Medium
- Prepare the appropriate nutrient medium (e.g., MS medium) with required hormones, vitamins, carbon source, and gelling agent.
- Adjust pH to 5.7–5.8.
- Dispense into culture vessels and autoclave at 121°C, 15 psi for 15–20 minutes for sterilization.

Step 4: Inoculation
- Under aseptic conditions (inside laminar air flow hood), place the sterilized explant onto the sterile nutrient medium.

Step 5: Incubation / Culture
- Culture vessels are kept in a growth chamber at controlled conditions:
- Temperature: 25 ± 2°C
- Light: 16 h light / 8 h dark photoperiod
- Humidity: ~60–70%

Step 6: Callus Formation
- The explant dedifferentiates to form an unorganized, undifferentiated mass of cells called callus.

Step 7: Organogenesis / Somatic Embryogenesis
- By adjusting hormone ratios, callus is induced to differentiate into shoots and roots (organogenesis) or somatic embryos (somatic embryogenesis).

Step 8: Hardening and Transfer (Acclimatization)
- Regenerated plantlets are transferred to soil in pots and gradually acclimatized to greenhouse and then field conditions.
- This process is called hardening.

Applications of Plant Tissue Culture:

1. Micropropagation:
- Rapid, large-scale clonal propagation of elite/superior plants.
- Millions of genetically identical plants (clones) can be produced in a short time from a small amount of plant material.
- Used for ornamental plants, banana, potato, orchids, etc.

2. Production of Virus-Free Plants:
- Apical/axillary meristems are free from viruses (as viruses spread through vascular tissue which is absent in meristems).
- Meristem culture produces virus-free plants, e.g., potato, sugarcane, carnation.

3. Synthetic Seed Production:
- Somatic embryos are encapsulated in a protective coating (sodium alginate) to form synthetic/artificial seeds.
- These can be stored and transported like true seeds.

4. Haploid Plant Production (Anther/Pollen Culture):
- Culturing anthers or pollen grains produces haploid plants.
- When treated with colchicine, they become homozygous diploids, useful in plant breeding.

5. Triploid Plant Production:
- Endosperm culture produces triploid plants used for seedless fruit production (e.g., seedless watermelon).

6. Somatic Hybridization:
- Fusion of protoplasts from two different species produces somatic hybrids (e.g., Pomato — potato + tomato).
- Overcomes sexual incompatibility barriers.

7. Production of Secondary Metabolites:
- Plant cells in culture can produce commercially important secondary metabolites:
- Taxol (anticancer) from *Taxus* sp.
- Shikonin (dye/antimicrobial) from *Lithospermum erythrorhizon*.
- Azadirachtin (biopesticide) from *Azadirachta indica*.

8. Germplasm Conservation:
- Endangered or rare plant species can be conserved *in vitro* (cryopreservation at –196°C in liquid nitrogen).

9. Genetic Transformation:
- Tissue culture is used as a tool for introducing foreign genes into plants (transgenic plant production).

10. Somaclonal Variation:
- Variations arising in tissue culture can be exploited to develop new varieties with improved traits (disease resistance, yield, etc.).

Development of Somatic Hybrids:

Somatic hybridization is the fusion of protoplasts (cells without cell walls) from two different plant species to produce a hybrid called a somatic hybrid.

Steps involved:

Step 1: Isolation of Protoplasts
- Protoplasts are isolated from the leaf mesophyll or other tissues of both parent plants.
- The cell wall is removed enzymatically using cellulase and pectinase enzymes in an osmotically stabilized solution (mannitol/sorbitol).
- This yields naked protoplasts.

Step 2: Fusion of Protoplasts
Protoplast fusion can be achieved by:
- Chemical method: Using polyethylene glycol (PEG) — PEG causes membrane destabilization and promotes fusion.
- Physical method (Electrofusion): Brief electric pulses cause membrane fusion.
- Spontaneous fusion can also occur during isolation.

Step 3: Selection of Hybrid Cells (Cybrid/Hybrid)
- After fusion, a mixture of unfused protoplasts, homokaryon fusions, and heterokaryon fusions is obtained.
- Hybrid cells (heterokaryons — containing nuclei from both parents) are selected using:
- Complementation methods
- Drug resistance markers
- Fluorescent labeling

Step 4: Culture and Regeneration
- Selected hybrid protoplasts are cultured on appropriate nutrient medium.
- They regenerate cell walls, undergo cell division, form callus, and are then induced to regenerate into complete somatic hybrid plants.

Example: *Pomato* — somatic hybrid of potato (*Solanum tuberosum*) and tomato (*Lycopersicon esculentum*).

Significance: Somatic hybridization allows combination of genomes of sexually incompatible species, enabling transfer of useful traits like disease resistance.

Somaclonal Variations:

Definition: Somaclonal variations are the genetic variations that arise in plants regenerated through tissue culture (somatic cells). These variations are not present in the original donor plant and appear spontaneously during the *in vitro* culture process.

Causes of Somaclonal Variations:
- Chromosomal rearrangements (deletions, duplications, inversions, translocations)
- Changes in chromosome number (polyploidy, aneuploidy)
- Point mutations
- Activation of transposable elements (transposons)
- Epigenetic changes (DNA methylation)
- Long duration of callus culture increases the frequency of variation.

Characteristics:
- These variations are heritable and can be passed to the next generation.
- They may be beneficial (improved traits) or detrimental.
- The frequency increases with prolonged callus culture.

Applications:
- Somaclonal variations can be exploited in crop improvement to develop new varieties with:
- Disease resistance
- Improved yield
- Stress tolerance
- Novel biochemical traits
- Example: Somaclonal variants of sugarcane with resistance to *Fiji* disease have been developed.

Note: To minimize somaclonal variation (when clonal fidelity is desired), the duration of callus culture should be kept short and direct organogenesis should be preferred over callus-mediated regeneration.

Explant — Definition:

An explant is any part of a plant (cell, tissue, or organ) that is excised (cut out) from the donor plant and used to initiate a culture *in vitro* on a nutrient medium. The explant serves as the starting material for plant tissue culture.

Five Most Commonly Used Explants:

| S.No. | Explant | Use |
|-------|---------|-----|
| 1. | Shoot Apical Meristem / Axillary Bud | Micropropagation, virus-free plant production |
| 2. | Leaf Segments / Leaf Disc | Callus induction, transformation studies |
| 3. | Embryo (Zygotic/Somatic) | Embryo rescue, somatic embryogenesis |
| 4. | Hypocotyl | Callus culture, regeneration studies |
| 5. | Cotyledon | Shoot organogenesis, transformation |

Other commonly used explants include: root tips, anthers (for haploid production), pollen grains, nodal segments, and protoplasts.

Key point: The choice of explant depends on the objective of the culture, the plant species, and the regeneration potential of the tissue.

Somatic Embryogenesis:

Definition: Somatic embryogenesis is the process by which somatic (non-reproductive) cells of a plant are induced to develop into embryo-like structures called somatic embryos (or embryoids) that morphologically and developmentally resemble zygotic embryos.

Process:
1. Explant (e.g., leaf, hypocotyl, cotyledon) is cultured on MS medium with high concentration of auxin (e.g., 2,4-D).
2. Cells dedifferentiate to form callus.
3. When transferred to medium with reduced or no auxin, embryogenic cells in the callus develop into somatic embryos.
4. Somatic embryos pass through the same developmental stages as zygotic embryos: globular → heart → torpedo → cotyledonary stage.
5. These embryos can be germinated to produce complete plantlets.

Application in Synthetic Seed Development:

Synthetic seeds (artificial seeds) are somatic embryos encapsulated in a protective coating to mimic natural seeds.

Method of Synthetic Seed Production:
1. Somatic embryos are produced *in vitro* through somatic embryogenesis.
2. The embryos are encapsulated in a hydrogel matrix — most commonly sodium alginate (2–3%) mixed with nutrients, hormones, and protective agents.
3. The embryo-alginate mixture is dropped into a calcium chloride (CaCl₂) solution, which causes ionic gelation, forming bead-like capsules.
4. These capsules (synthetic seeds) can be stored, transported, and planted directly in soil.

Advantages of Synthetic Seeds:
- Mass production of elite/superior genotypes.
- Easy storage and transport.
- Can include nutrients, pesticides, and biocontrol agents in the capsule.
- Useful for plants that produce recalcitrant seeds (seeds that cannot be stored for long).
- Useful for propagating sterile or seedless plants.
- Cost-effective alternative to micropropagation for large-scale use.

Role of pH in Plant Tissue Culture Nutrient Media:

Optimal pH: The pH of plant tissue culture media is generally adjusted to 5.7–5.8 before autoclaving.

Importance of pH:

1. Nutrient Solubility and Availability:
- pH directly affects the solubility of inorganic salts and nutrients in the medium.
- At incorrect pH, certain nutrients may precipitate out of solution and become unavailable to the plant cells.
- For example, iron (Fe) precipitates at high pH, making it unavailable.

2. Agar Solidification:
- pH affects the gelling property of agar.
- Low pH (acidic, below 5.0): Results in poor solidification — the medium remains soft or liquid, making it unsuitable for culture.
- High pH (alkaline, above 6.5): Results in very hard/firm medium, which makes it difficult for explants to be placed and for roots to penetrate.

3. Hormone Activity:
- pH influences the ionization and activity of plant growth regulators (auxins, cytokinins) in the medium.

4. Uptake of Nutrients:
- pH affects the transport of ions across the cell membrane. Optimal pH ensures efficient nutrient uptake by cultured cells.

5. Microbial Contamination:
- Extreme pH values may promote or inhibit microbial growth.

Summary Table:

| pH Condition | Effect |
|---|---|
| Below 5.0 (too acidic) | Poor agar solidification, altered nutrient solubility |
| 5.7–5.8 (optimal) | Good solidification, optimal nutrient availability |
| Above 6.5 (too alkaline) | Very hard medium, nutrient precipitation |

Conclusion: Maintaining the correct pH (5.7–5.8) is critical for proper nutrient availability, agar solidification, and overall success of plant tissue culture.

Somatic Hybridization — Method:

Somatic hybridization involves the fusion of protoplasts from two different plant species to produce a somatic hybrid.

Step-by-Step Method:

Step 1: Isolation of Protoplasts
- Leaf tissue (or other tissue) from both parent plants is treated with a mixture of cellulase and pectinase enzymes in an osmotically balanced solution (0.5–0.8 M mannitol or sorbitol).
- This removes the cell wall and releases intact protoplasts.
- Protoplasts are purified by filtration and centrifugation.

Step 2: Protoplast Fusion
Fusion is induced by:
- Chemical Fusion: Polyethylene glycol (PEG) (MW 1500–6000, 15–50% concentration) is added to a mixture of protoplasts from both parents. PEG destabilizes membranes and promotes fusion. Calcium ions (Ca²⁺) at high pH enhance the process.
- Electrofusion: Brief, high-intensity electric pulses (electroporation) cause temporary membrane breakdown and fusion.

Step 3: Selection of Heterokaryons
- After fusion, the mixture contains:
- Unfused protoplasts of parent A
- Unfused protoplasts of parent B
- Homofusion products (A+A or B+B)
- Heterokaryons (A+B) — the desired somatic hybrids
- Selection methods:
- Complementation of auxotrophic mutants
- Drug/antibiotic resistance markers
- Fluorescent dye labeling (one parent labeled with one color, other with another; fused cells show both colors)

Step 4: Culture and Regeneration
- Selected heterokaryons are cultured on nutrient medium.
- They regenerate cell walls → undergo mitosis → form callus → differentiate into shoots and roots → complete somatic hybrid plants.

Step 5: Verification
- Hybrid nature is confirmed by cytological, biochemical, and molecular methods (isozyme analysis, RFLP, PCR).

Advantages of Somatic Hybridization:

1. Overcomes Sexual Incompatibility: Allows hybridization between species that cannot be crossed sexually (wide hybridization).
2. Combines Entire Genomes: Both nuclear and cytoplasmic genomes can be combined.
3. Cybridization: Fusion of a normal protoplast with an enucleated protoplast allows transfer of only cytoplasmic genes (mitochondria, chloroplasts) — useful for transferring cytoplasmic male sterility.
4. Novel Combinations: Creates new gene combinations not possible through conventional breeding.
5. Disease Resistance Transfer: Resistance genes from wild species can be transferred to crop plants.
6. Example: *Pomato* (potato + tomato), *Arabidopsis* + *Brassica* hybrids.

Somaclonal Variations:

Definition: Somaclonal variations are the heritable genetic and epigenetic changes that arise in plants regenerated through tissue culture. These variations are not present in the original donor plant and occur spontaneously during the *in vitro* culture process.

Causes:
- Chromosomal rearrangements (deletions, duplications, inversions, translocations)
- Changes in ploidy level (polyploidy, aneuploidy)
- Point mutations (base substitutions)
- Activation of transposable elements
- DNA methylation changes (epigenetic)
- Prolonged callus culture increases the frequency of variation.

Role of Somaclonal Variations in Crop Improvement:

1. Source of New Genetic Variability:
- Somaclonal variations generate novel genetic diversity that can be screened for useful traits, supplementing conventional breeding.

2. Disease Resistance:
- Somaclonal variants with resistance to fungal, bacterial, and viral diseases have been identified.
- Example: Sugarcane variants resistant to *Fiji* disease and eyespot disease have been developed through somaclonal variation.

3. Improved Yield and Quality:
- Variants with higher yield, better nutritional quality, or altered biochemical composition can be selected.
- Example: Tomato somaclones with improved fruit quality.

4. Stress Tolerance:
- Variants tolerant to drought, salinity, heat, and cold stress can be selected from somaclonal populations.

5. Herbicide and Pest Resistance:
- Somaclonal variants resistant to specific herbicides or pests have been identified in several crops.

6. Reduced Breeding Time:
- Somaclonal variation can produce useful variants faster than conventional mutagenesis and breeding programs.

Limitations:
- Most somaclonal variations are random and unpredictable.
- Many variations are deleterious.
- Requires extensive screening to identify useful variants.

Conclusion: Although somaclonal variations are generally considered undesirable when clonal fidelity is required (e.g., micropropagation), they can be deliberately exploited as a tool for crop improvement by selecting variants with superior agronomic traits.

Correct Answer: (d) All of the above

Justification: Based on the principle of totipotency, any living plant cell has the potential to regenerate into a complete plant. Shoot apical meristem, embryo, and leaf segments all contain living, competent cells that can be cultured *in vitro* on appropriate nutrient media to regenerate complete plants. All three are routinely used as explants in plant tissue culture.

Correct Answer: (c) Apical meristem

Justification: Viruses spread through the vascular system (phloem) of plants. The apical meristem (shoot tip) is a rapidly dividing region that lacks differentiated vascular tissue. Since viruses have not yet colonized this region, culturing the apical meristem produces virus-free plants. Leaf segments, seeds, and stem cuttings may already be infected with viruses.

Correct Answer: (a) Cybridization

Justification: Cybridization (cytoplasmic hybridization) is the process in which the nuclear genome of one parent is combined with the cytoplasmic genome (mitochondria, chloroplasts) of the other parent. This is achieved by fusing a normal protoplast with an enucleated protoplast (cytoplast). It is used to transfer cytoplasmic traits like cytoplasmic male sterility (CMS).

Correct Answer: (c) Antibiotics

Justification: The standard Murashige and Skoog (MS) medium formulation contains inorganic nutrients (macro and microelements), carbon source (sucrose), organic nutrients (vitamins, amino acids), and plant growth regulators. Antibiotics are not an essential component of the standard MS medium formulation — they are added optionally only when there is a risk of microbial contamination or in specific transformation experiments. All other options (inorganic nutrients, carbon source, organic nutrients) are essential components.

Correct Answer: (c) Interfere with solidification of the medium and results in poor solidification.

Justification: A decrease in pH (acidic conditions, below 5.0) primarily interferes with the gelling property of agar, resulting in poor or incomplete solidification of the medium. It is the high pH (alkaline conditions) that increases the hardness of the medium. While pH does affect solubility of some salts, the most direct and characteristic effect of decreased pH is poor agar solidification. Therefore, option (c) is the most accurate and specific answer.

Correct Answer: (a) Plants regenerated through tissue culture

Justification: Somaclonal variations arise specifically during *in vitro* tissue culture due to chromosomal rearrangements, mutations, and epigenetic changes that occur during callus formation and prolonged culture. These variations are characteristic of plants regenerated through tissue culture. Plants generated through seeds or sexual reproduction undergo normal meiosis and fertilization and do not show somaclonal variation.

Correct Answer: (a), (b), and (c) — effectively: All of the above (but since 'All of the above' is not an option, the question implies each is correct individually)

Note: Since the options do not include 'All of the above,' each of options (a), (b), and (c) is individually correct:
- (a) Virus-free plants — produced by meristem culture.
- (b) Somatic hybrid plants — produced by protoplast fusion.
- (c) Synthetic seeds — produced by encapsulation of somatic embryos.

If a single best answer is required: (d) None of the above is incorrect. All three — (a), (b), and (c) — are valid applications of *in vitro* tissue culture. The most comprehensive correct answer encompasses all three options (a), (b), and (c).

Correct Answer: (d) Both assertion and reason are false.

Justification:
- Assertion is FALSE: Somatic (synthetic) seeds are not encapsulated by a seed coat. They are encapsulated in a hydrogel matrix (typically sodium alginate) which acts as an artificial seed coat. The term 'seed coat' refers to the testa of natural seeds derived from the integuments of the ovule.
- Reason is FALSE (partially): While it is true that seed coats provide protection, the reason as stated is misleading. The seed coat in natural seeds protects against mechanical damage, pathogens, and regulates water uptake — it does not simply 'prevent water desiccation' (in fact, seed coats in many species are permeable to water for germination). More importantly, since the assertion itself is false, the reason cannot explain it.
- Therefore, both assertion and reason are false.

Correct Answer: (c) Assertion is true but reason is false.

Justification:
- Assertion is TRUE: Virus-free plants can indeed be produced by culturing the apical or axillary meristems of virus-infected plants. This is a well-established technique in plant tissue culture (meristem culture).
- Reason is FALSE: The reason given is incorrect. Viruses do not require vascular bundles to replicate — viruses replicate inside any living host cell using the host's cellular machinery (ribosomes, enzymes). The correct reason is that apical/axillary meristems are rapidly dividing cells that are ahead of the virus infection front — viruses spread through the vascular system (phloem) and have not yet reached the actively dividing meristematic cells. The meristem is virus-free because viral movement through plasmodesmata cannot keep pace with the rapid cell division at the meristem tip.
- Therefore, the assertion is true but the reason given is false — option (c) is correct.