Unit 1: Introduction; Callus and Suspension Culture - Subjective Questions

Plant tissue culture is the in vitro cultivation of plant cells, tissues, or organs on a nutrient medium under aseptic and controlled environmental conditions to regenerate whole plants or produce useful metabolites.

Historical Background:

• Gottlieb Haberlandt (1902): Considered the father of plant tissue culture; first proposed the concept of totipotency and attempted to culture isolated plant cells.
• White (1934): Successfully established continuous culture of tomato root tips.
• Gautheret and Nobecourt (1939): Established the first successful callus cultures of carrot and tobacco.
• Skoog and Miller (1957): Demonstrated that the ratio of auxin to cytokinin controls organogenesis (root vs. shoot formation).
• Murashige and Skoog (1962): Developed the widely used MS medium.
• Cocking (1960): Isolated protoplasts using enzymatic methods.

These milestones established tissue culture as a cornerstone of modern plant biotechnology.

Totipotency is the inherent capacity of a single plant cell to divide, differentiate, and regenerate into a complete, whole plant when provided with suitable nutrients and conditions.

Key points:

• The term was coined based on Haberlandt's hypothesis (1902).
• Every living plant cell contains the complete genetic information required to develop an entire organism.
• Totipotency is expressed when a differentiated cell is induced to dedifferentiate (revert to a meristematic state) and then redifferentiate into new organs or embryos.

Significance:

• Forms the theoretical basis of all tissue culture techniques.
• Enables micropropagation (clonal multiplication of plants).
• Allows regeneration of plants from single cells or protoplasts.
• Essential for genetic transformation and production of transgenic plants.
• Facilitates somatic embryogenesis and synthetic seed technology.

The basic techniques in plant tissue culture follow a systematic sequence:

1. Selection of explant: Choosing a healthy, disease-free plant part (e.g., leaf, node, meristem, root).
2. Sterilization: Surface sterilization of explant using agents like sodium hypochlorite, mercuric chloride (), or ethanol to eliminate microbial contamination.
3. Media preparation: Formulating a suitable nutrient medium (e.g., MS medium) containing macronutrients, micronutrients, vitamins, carbon source, and growth regulators.
4. Inoculation: Placing the sterilized explant onto/into the medium under aseptic conditions (laminar air flow chamber).
5. Incubation: Maintaining cultures under controlled temperature (), light, and humidity.
6. Subculturing: Transferring growing tissue to fresh medium periodically to maintain growth.
7. Regeneration: Inducing organogenesis or embryogenesis to form shoots, roots, or embryos.
8. Hardening/Acclimatization: Gradually adapting in vitro plantlets to ex vitro (soil) conditions.

Aseptic technique is maintained throughout to prevent contamination.

A plant tissue culture medium is a balanced formulation of nutrients and regulators. Its major components include:

1. Macronutrients (required in large amounts, > 30 mg/L):

• Nitrogen (as and ), Phosphorus, Potassium, Calcium, Magnesium, Sulphur.
• Essential for protein synthesis, nucleic acids, and structural functions.

2. Micronutrients (trace elements):

• Iron (Fe), Manganese (Mn), Zinc (Zn), Boron (B), Copper (Cu), Molybdenum (Mo), Cobalt (Co).
• Act as cofactors of enzymes and in electron transport.

3. Carbon source:

• Usually sucrose (2–3%); provides energy since in vitro tissues have limited photosynthesis.

4. Vitamins:

• Thiamine (B1), Nicotinic acid, Pyridoxine (B6), Myo-inositol; aid in metabolism.

5. Plant Growth Regulators (PGRs):

• Auxins (IAA, NAA, 2,4-D) promote cell division and root formation.
• Cytokinins (BAP, Kinetin) promote shoot formation.

6. Gelling agent:

• Agar (0.6–0.8%) to solidify the medium.

7. Other: Amino acids, pH adjusted to 5.6–5.8.

Media optimization is the systematic adjustment of medium components to achieve the best growth and morphogenetic response for a particular plant species or tissue.

Factors optimized:

• Type of basal medium: MS, B5, White's, Nitsch, etc., chosen based on species requirements.
• Growth regulator concentrations & ratios: The auxin:cytokinin ratio is critical:
  • High auxin : low cytokinin → root formation
  • Low auxin : high cytokinin → shoot formation
  • Balanced ratio → callus proliferation
• Carbon source and concentration: Type (sucrose, glucose) and amount affect growth.
• pH: Usually maintained at 5.6–5.8; affects nutrient availability and agar gelling.
• Gelling agent concentration: Affects water availability and diffusion.
• Vitamins and organic additives: Coconut water, casein hydrolysate may be added.

Importance:

• Maximizes growth rate and regeneration efficiency.
• Reduces problems like vitrification and browning.
• Species-specific optimization is essential as one formula does not suit all plants.
• Cost-effective and reproducible protocols depend on proper optimization.

These are three levels of in vitro cultivation differing in the type of explant used:

Summary:

• Cell culture deals with individual cells or protoplasts.
• Tissue culture deals with an unorganized mass of cells (callus).
• Organ culture maintains the structural organization of a specific organ.

Organogenesis is the in vitro formation of organs such as shoots, roots, or buds from cultured cells, tissues, or callus.

Types of Organogenesis:

1. Direct organogenesis: Organs develop directly from the explant without an intervening callus phase. Preferred as it maintains genetic stability.
2. Indirect organogenesis: Organs develop from a callus formed on the explant.

Pathways:

• Caulogenesis: Formation of shoots.
• Rhizogenesis: Formation of roots.

Role of Growth Regulators (Skoog & Miller concept, 1957):

The auxin-to-cytokinin ratio determines the type of organ formed:

• High cytokinin : low auxin → Shoot formation
• High auxin : low cytokinin → Root formation
• Intermediate/balanced ratio → Callus proliferation

Significance:

• Basis for micropropagation and mass clonal multiplication.
• Enables regeneration of complete plants from tissues.

Explant preparation and sterilization are critical to obtain contamination-free cultures.

Explant Selection & Preparation:

• Choose young, healthy, actively dividing tissues (meristems, nodes, leaf segments).
• Wash thoroughly under running tap water to remove dust and debris.
• Trim and cut into suitable sizes.

Surface Sterilization Techniques:

1. Detergent wash: Using Tween-20 or Teepol to remove surface dirt.
2. Ethanol treatment: Dip in 70% ethanol for 30–60 seconds (kills surface microbes, removes air films).
3. Chemical sterilants:
   • Sodium hypochlorite (NaOCl): 0.5–1.0% for 10–15 min.
   • Mercuric chloride (): 0.01–0.1% for 2–10 min (very effective but toxic).
   • Hydrogen peroxide, calcium hypochlorite may also be used.
4. Rinsing: Wash 3–4 times with sterile distilled water to remove residual sterilant.

Additional measures:

• Antibiotics/fungicides for internal contamination.
• All operations done in a laminar air flow (LAF) cabinet under aseptic conditions.
• Instruments sterilized by flaming/autoclaving.

Proper sterilization balances microbial elimination with minimal tissue damage.

Callus is an unorganized, undifferentiated, proliferating mass of parenchymatous cells formed from a plant explant cultured on a nutrient medium.

Initiation of Callus Culture:

1. Explant selection: Choose suitable tissue (leaf, stem, root, cotyledon).
2. Sterilization: Surface-sterilize the explant.
3. Inoculation: Place explant on solid medium (e.g., MS) supplemented with auxins (2,4-D, NAA) alone or with cytokinins.
4. Dedifferentiation: Cells at the cut surface revert to a meristematic state and begin dividing.
5. Callus proliferation: Continuous cell division forms a visible callus mass within 2–4 weeks.

Incubation conditions: Temperature , often in dark or diffused light.

Maintenance of Callus Culture:

• Subculturing: The callus must be transferred to fresh medium every 3–4 weeks, since nutrients deplete and toxic metabolites accumulate.
• Only healthy, actively growing portions are transferred.
• Growth regulators are maintained to keep the callus in an undifferentiated state.

Significance: Callus serves as a source for suspension cultures, organogenesis, somatic embryogenesis, and metabolite production.

Callus cultures are classified based on their appearance, texture, and morphogenetic potential:

1. Based on texture/consistency:

• Compact (hard) callus: Densely packed cells with little intercellular space; tough in texture.
• Friable (soft) callus: Loosely arranged cells that separate easily; ideal for establishing suspension cultures.

2. Based on morphogenetic ability:

• Organogenic callus: Capable of forming organs (shoots/roots).
• Embryogenic callus: Capable of forming somatic embryos.
• Non-morphogenic callus: Lacks regeneration capacity; used mainly for metabolite production.

3. Based on colour/pigmentation:

• Green callus: Contains chlorophyll (photosynthetic).
• White/creamy callus: Non-pigmented.
• Pigmented callus: Contains anthocyanins or other pigments.

4. Based on growth pattern:

• Fast-growing vs. slow-growing callus.

Significance: The type of callus determines its suitability for regeneration, suspension culture, or secondary metabolite production. Friable, embryogenic callus is most valued.

Suspension culture is the cultivation of free cells and small cell aggregates dispersed in a liquid medium kept in continuous agitation.

Initiation:

1. Friable callus (loosely packed) is transferred into liquid medium (same composition minus agar).
2. The culture is placed on an orbital/rotary shaker (80–150 rpm) for aeration and to prevent cell aggregation.
3. Cells separate and disperse, forming a fine suspension of single cells and small clumps.

Maintenance Methods:

1. Batch culture: Cells grown in a fixed volume of medium in a closed vessel; nutrients deplete over time, showing a sigmoid growth curve. Subculturing needed periodically.
2. Continuous culture: Fresh medium is added continuously and spent medium removed, maintaining cells in steady growth:
   • Open continuous culture: Cells and medium removed together.
   • Closed continuous culture: Only spent medium removed; cells retained.
3. Chemostat: Growth controlled by a limiting nutrient.
4. Turbidostat: Growth controlled by maintaining constant cell density.

Subculturing: Regular transfer (every 1–2 weeks) into fresh medium maintains viability.

Applications: Large-scale secondary metabolite production, biotransformation, and study of cell growth.

Both are methods for maintaining suspension cultures but differ in nutrient supply and growth dynamics:

Growth curve in batch culture follows a sigmoid pattern:

• Lag phase → Exponential (log) phase → Stationary phase → Decline phase

Conclusion: Batch culture is simpler and suited for research, whereas continuous culture is efficient for sustained, large-scale production.

In a batch suspension culture, cell growth follows a characteristic sigmoid (S-shaped) growth curve with distinct phases:

Phases of the Growth Curve:

1. Lag Phase:

   • Cells adapt to the new medium.
   • Little or no cell division; metabolic activity begins.

2. Exponential (Log) Phase:

   • Rapid and constant rate of cell division.
   • Cell number increases logarithmically.
   • Growth rate can be expressed as: where is the specific growth rate.

3. Linear/Deceleration Phase:

   • Growth rate begins to slow as nutrients deplete and metabolites accumulate.

4. Stationary Phase:

   • Cell division balances cell death.
   • Cell number remains constant due to nutrient exhaustion.

5. Decline (Death) Phase:

   • Cells die due to nutrient depletion and accumulation of toxic byproducts.

Importance:

• The late log/early stationary phase is ideal for subculturing.
• Understanding the curve helps optimize metabolite harvest timing.
• Subculturing before decline phase maintains viable cultures.

Organogenesis (organ formation in vitro) can occur via two pathways:

Direct organogenesis is preferred for clonal propagation because it maintains genetic fidelity.

Indirect organogenesis is useful for genetic transformation and generating variability but may introduce somaclonal variation.

Both are regulated by the auxin:cytokinin ratio in the medium.

Assessing cell growth and viability is essential for monitoring suspension cultures. Common methods include:

Growth Measurement Methods:

1. Cell number (Cell counting): Using a haemocytometer after treating with pectinase to separate cells.
2. Packed Cell Volume (PCV): Volume of cells after centrifugation, expressed as mL cells per mL culture.
3. Fresh weight & Dry weight: Cells filtered, weighed fresh, then dried and reweighed.
4. Mitotic index: Percentage of dividing cells.
5. Conductivity/medium changes: Indirect measure of growth via nutrient consumption.

Viability Testing Methods:

1. Vital staining:
   • Evans blue: Dead cells take up the stain; living cells exclude it.
   • Fluorescein diacetate (FDA): Living cells fluoresce green under UV.
   • Phenosafranine: Stains dead cells red.
2. TTC (Triphenyl tetrazolium chloride) test: Living cells reduce it to red formazan.
3. Cytoplasmic streaming observation under microscope indicates viability.

Importance: These measurements help determine the optimal harvesting time, subculturing schedule, and overall health of the culture.

MS medium (Murashige & Skoog, 1962) and White's medium (1934) are two classic tissue culture media differing mainly in salt concentration.

Why MS medium is most widely used:

• Contains high concentrations of essential nutrients, especially nitrogen, potassium, and phosphorus.
• Supports rapid growth and regeneration of a wide range of species.
• Well-balanced macro- and micronutrients meet the demands of most tissues.
• Reproducible and standardized, making protocols comparable across laboratories.
• Suitable for callus induction, shoot/root regeneration, and suspension cultures.

Hence, MS medium serves as the default basal medium in most plant tissue culture work.

Plant Growth Regulators (PGRs) are organic compounds that regulate growth and differentiation in tissue culture. The two most important classes are auxins and cytokinins.

Auxins:

• Examples: IAA, IBA, NAA, 2,4-D.
• Functions:
  • Promote cell division and elongation.
  • Induce callus formation (especially 2,4-D).
  • Stimulate root initiation (rhizogenesis).

Cytokinins:

• Examples: Kinetin, BAP, Zeatin, 2-iP.
• Functions:
  • Promote cell division (cytokinesis).
  • Induce shoot formation (caulogenesis).
  • Delay senescence and break apical dominance.

Auxin-Cytokinin Ratio (Skoog & Miller, 1957):

The morphogenetic response depends on the relative ratio rather than absolute amounts:

• High auxin : low cytokinin → Root formation
• Low auxin : high cytokinin → Shoot formation
• Balanced/intermediate ratio → Callus proliferation

Significance:

• This ratio is the key control point for organogenesis.
• Manipulating the ratio allows controlled regeneration of roots, shoots, or callus, which is fundamental to micropropagation and plant regeneration.

Media preparation is a critical, systematic process ensuring a nutrient-rich, sterile substrate for tissue growth.

Step-by-Step Procedure:

1. Preparation of stock solutions:

   • Prepare concentrated stocks of macronutrients, micronutrients, iron-EDTA, vitamins, and growth regulators separately.
   • Store stocks refrigerated to avoid repeated weighing errors.

2. Mixing components:

   • Add measured volumes of stock solutions to distilled water.
   • Add the carbon source (sucrose, 2–3%).
   • Add appropriate growth regulators.

3. pH adjustment:

   • Adjust pH to 5.6–5.8 using 0.1 N NaOH or HCl.
   • Correct pH ensures nutrient availability and proper agar solidification.

4. Addition of gelling agent (for solid medium):

   • Add agar (0.6–0.8%) and heat to dissolve.

5. Dispensing:

   • Pour the medium into culture vessels (test tubes, flasks, jars).

6. Sterilization:

   • Autoclave at 121°C, 15 psi pressure, for 15–20 minutes.
   • Heat-labile compounds (some vitamins, antibiotics, certain hormones) are filter-sterilized (0.22 µm membrane) and added after autoclaving.

7. Cooling & Storage:

   • Allow medium to cool and solidify in a sterile environment.
   • Store in a clean area; use within a few weeks.

Precautions: Maintain accurate weighing, correct pH, and strict asepsis throughout to ensure reproducible, contamination-free media.

Callus and suspension cultures are foundational techniques with wide-ranging applications:

Applications of Callus Culture:

1. Plant regeneration: Via organogenesis and somatic embryogenesis for micropropagation.
2. Somaclonal variation: Generating genetic variability for crop improvement.
3. Genetic transformation: Callus serves as target tissue for gene transfer (e.g., Agrobacterium).
4. Production of secondary metabolites: Alkaloids, pigments, and pharmaceuticals.
5. Study of cellular differentiation and morphogenesis.
6. Source for protoplast isolation.

Applications of Suspension Culture:

1. Large-scale secondary metabolite production: In bioreactors (e.g., shikonin, taxol precursors).
2. Biotransformation: Converting substrates into valuable products.
3. Production of single cells & protoplasts for fusion and transformation.
4. Mutant selection: Screening cells against selective agents (herbicides, salts).
5. Study of cell growth kinetics and metabolism.
6. Somatic embryogenesis for synthetic seed production.

Overall Significance:

• Enable clonal propagation, genetic improvement, and industrial production of valuable compounds.
• Provide model systems to study plant cell physiology and biochemistry.

Thus, callus and suspension cultures bridge basic research and commercial biotechnology.

The regeneration of whole plants from cultured tissues relies on two fundamental cellular processes: dedifferentiation and redifferentiation.

Dedifferentiation:

• The process by which mature, differentiated cells revert back to a meristematic (undifferentiated) state, regaining the capacity to divide.
• Occurs when an explant is placed on a medium with suitable growth regulators (auxins).
• Results in the formation of an unorganized callus.

Redifferentiation:

• The process by which dedifferentiated callus cells develop into new organized structures such as shoots, roots, or embryos.
• Controlled by manipulating the auxin:cytokinin ratio.

Sequence in plant regeneration:

1. Differentiated explant → placed on medium.
2. Dedifferentiation → callus formation (cells become meristematic).
3. Redifferentiation → organogenesis (shoots/roots) or somatic embryogenesis.
4. Complete plantlet develops.

Significance:

• Together they demonstrate totipotency of plant cells.
• Form the basis of micropropagation, somatic embryogenesis, and genetic transformation.
• Understanding these processes enables control over regeneration efficiency and clonal multiplication.

Note: Prolonged callus culture may cause somaclonal variation, affecting genetic fidelity during redifferentiation.