Unit 2: Micropropagation; Somatic Hybridization - Subjective Questions

Micropropagation is the technique of rapid in vitro multiplication of plants using small pieces of tissue (explants) under aseptic and controlled environmental conditions, exploiting the property of totipotency of plant cells.

Significance:

• Produces a large number of genetically identical plants (clones) in a short time.
• Enables multiplication of plants that are difficult to propagate by conventional methods.
• Facilitates production of disease-free (pathogen-free) plants through meristem culture.
• Allows year-round production independent of season.
• Requires very little starting material and space.
• Useful for conservation of rare and endangered species (germplasm conservation).
• Commercially important for horticulture, floriculture, and forestry.

Micropropagation is generally carried out in the following five stages (Stage 0 to Stage IV):

Stage 0 – Selection and preparation of the mother plant:

• Selection of healthy, disease-free stock (mother) plant.
• Maintaining the plant under hygienic conditions to reduce contamination.

Stage I – Initiation of culture (Establishment):

• The explant (shoot tip, node, meristem) is surface sterilized using agents like sodium hypochlorite, mercuric chloride, or ethanol.
• Inoculated onto a suitable nutrient medium (e.g., MS medium) to establish an aseptic culture.

Stage II – Multiplication (Shoot proliferation):

• Repeated subculturing on medium containing high cytokinin to auxin ratio.
• Produces multiple shoots/axillary buds; the main phase for large-scale multiplication.

Stage III – Rooting (Root induction):

• Individual shoots are transferred to a medium rich in auxins (e.g., IAA, IBA, NAA) to induce root formation.

Stage IV – Hardening / Acclimatization:

• Rooted plantlets are gradually transferred from in vitro to ex vitro conditions (greenhouse, then field).
• Humidity is gradually reduced to allow the plants to adapt to natural environmental conditions.

Somatic embryogenesis is the process by which embryos (somatic embryos) develop from somatic (vegetative) cells rather than from the fusion of gametes. These embryos are bipolar structures with both shoot and root meristems and are not attached to the parent tissue via vascular connections.

Two pathways:

Stages of somatic embryo development:

• Globular stage → Heart stage → Torpedo stage → Cotyledonary stage.

Applications: synthetic seed production, large-scale propagation, and genetic transformation.

Embryo culture is the in vitro aseptic isolation and growth of an immature or mature embryo on a nutrient medium, with the objective of obtaining a viable plant.

Types:

• Mature embryo culture: Embryos from ripe seeds are cultured, mainly to overcome seed dormancy.
• Immature embryo culture (embryo rescue): Embryos are excised before abortion and cultured on nutrient-rich media.

Technique:

• Sterilize the seed/ovule surface.
• Aseptically excise the embryo under a dissecting microscope.
• Inoculate on a suitable medium (with sugars, vitamins, amino acids, and growth regulators).
• Incubate under controlled light and temperature.

Importance:

• Overcomes seed dormancy and germination barriers.
• Recovers hybrids from incompatible crosses.
• Shortens the breeding cycle.
• Helps in seed viability testing.
• Facilitates production of haploids and rare hybrids.

Embryo rescue is a specialized form of embryo culture in which immature or non-viable hybrid embryos that would normally abort are excised and cultured in vitro to obtain viable plants.

Barriers overcome:

1. Post-zygotic barriers:

• In wide/interspecific crosses, the endosperm often fails to develop properly, leading to embryo starvation and abortion.
• Embryo rescue removes the embryo before abortion and supplies nutrients artificially.

2. Hybrid embryo abortion:

• Genetic incompatibility between parents can cause embryo degeneration; rescue prevents loss.

Methods of embryo rescue:

• Embryo culture: direct culture of the excised embryo.
• Ovule culture: culturing the whole ovule containing the embryo.
• Ovary culture: culturing the entire ovary.

Applications:

• Production of interspecific and intergeneric hybrids (e.g., Triticale).
• Recovery of hybrids in crops like cotton, tomato, and lily.
• Support for haploid production and wide hybridization breeding programs.

Micropropagation has wide-ranging applications:

1. Rapid clonal multiplication:

• Mass production of genetically uniform, elite plants (e.g., banana, orchids, sugarcane).

2. Production of disease-free plants:

• Meristem culture yields virus-free plants (e.g., potato, strawberry).

3. Germplasm conservation:

• Storage of rare, endangered, and elite genotypes in vitro and via cryopreservation.

4. Production of secondary metabolites:

• Large-scale culture for pharmaceuticals, dyes, and flavors.

5. Horticulture and floriculture:

• Commercial multiplication of ornamentals like orchids, gerbera, chrysanthemum.

6. Forestry:

• Clonal propagation of elite forest trees (eucalyptus, teak) for afforestation.

7. Genetic transformation support:

• Provides regeneration systems for producing transgenic plants.

8. Year-round, space-efficient production independent of climate.

Somatic hybridization is the technique of producing hybrid plants by the fusion of protoplasts of two different somatic cells, followed by regeneration of a somatic hybrid (parasexual hybrid), without the involvement of sexual reproduction.

Basic steps:

1. Protoplast isolation from both parents.
2. Protoplast fusion (chemical/electrical).
3. Selection of fused hybrid cells.
4. Regeneration of hybrid plant.

Significance:

• Overcomes sexual incompatibility barriers between species/genera.
• Enables gene transfer between distantly related plants.
• Combines cytoplasmic traits (e.g., cytoplasmic male sterility, disease resistance).
• Produces novel cybrids and hybrids not possible by conventional breeding.
• Useful for transferring organelle-encoded traits (chloroplast, mitochondria).
• Facilitates production of wide/interspecific hybrids (e.g., Pomato from potato + tomato).

A protoplast is a plant cell whose cell wall has been removed, leaving only the plasma membrane-bound cell. Isolation methods are:

1. Mechanical method:

• The tissue is cut/chopped and cells are subjected to plasmolysis; protoplasts are released by mechanical rupturing.
• Disadvantages: low yield, cell damage, tedious.

2. Enzymatic method (most widely used):

• Uses cell-wall-degrading enzymes to release protoplasts.
• Enzymes used:
  • Cellulase – degrades cellulose.
  • Hemicellulase – degrades hemicellulose.
  • Pectinase (macerozyme) – degrades the middle lamella (pectin).

Two enzymatic approaches:

• Sequential (two-step) method: pectinase first to separate cells, then cellulase to remove walls.
• Simultaneous (one-step) method: all enzymes applied together.

Osmotic protection:

• Mannitol or sorbitol is added to maintain osmotic balance and prevent bursting.

Purification:

• Protoplasts are purified by filtration and centrifugation and checked for viability.

Protoplast fusion is the process of merging two isolated protoplasts to form a single hybrid cell. Methods include:

1. Spontaneous fusion:

• Occurs naturally during isolation when adjacent protoplasts fuse via plasmodesmata, forming multinucleate protoplasts.

2. Induced fusion:

(a) Chemical fusion (chemofusion):

• PEG (Polyethylene glycol) method: PEG causes agglutination and tight adhesion of membranes, leading to fusion. Widely used; high frequency.
• Sodium nitrate (NaNO₃) method: an early method; low fusion frequency.
• High pH–high calcium () method: high pH with high ions induces fusion.

(b) Electrical fusion (electrofusion):

• Protoplasts are aligned in a chamber by an alternating current (AC) field (dielectrophoresis) to form 'pearl chains'.
• A brief high-voltage direct current (DC) pulse causes reversible membrane breakdown and fusion.
• Advantages: precise, controllable, non-toxic, high fusion frequency.

Products: homokaryons, heterokaryons, and finally hybrid cells after nuclear fusion.

After fusion, the mixture contains parental protoplasts, homokaryons, and heterokaryons (hybrids). Selection of true hybrid cells is essential.

Selection methods:

1. Complementation selection:

• Genetic complementation: Uses mutant parents (e.g., two auxotrophic/albino mutants). Only fused hybrid cells complement each other's deficiency and grow on selective medium.
• Metabolic/drug complementation: Uses parents resistant/sensitive to different drugs so only hybrids survive selective media.

2. Visual/mechanical selection:

• Heterokaryons are identified under microscope using morphological differences (e.g., one parent green chloroplasts + other colorless vacuolated) and picked individually with a micromanipulator.

3. Use of fluorescent markers:

• Parental protoplasts are labeled with different fluorochromes (e.g., FITC and rhodamine). Hybrids show both fluorescences and are sorted using Fluorescence-Activated Cell Sorting (FACS).

4. Auto-selection based on differential growth:

• When only hybrids can divide/grow under given conditions.

Selected hybrids are then cultured to regenerate whole hybrid plants.

Both are products of somatic hybridization but differ in nuclear/genetic contribution.

Note: Asymmetric hybridization is more useful in crop improvement because it allows transfer of only desired traits without unwanted genes.

Cybrids (cytoplasmic hybrids) are cells/plants that contain the nucleus of only one parent but the cytoplasm (organelles - chloroplasts and mitochondria) from both parents, or a mixture of cytoplasms.

Production of cybrids:

• Fusion of a normal protoplast with an enucleated protoplast (cytoplast), OR
• Fusion where one parent's nucleus is inactivated (by X-ray/gamma irradiation) while the other's cytoplasm is inactivated (by iodoacetate treatment).
• After fusion, only one nucleus survives, but cytoplasmic organelles from both parents mix, giving a cybrid.

Importance:

• Transfer of cytoplasmically inherited traits:
  • Cytoplasmic Male Sterility (CMS) – valuable for hybrid seed production.
  • Herbicide resistance (chloroplast-encoded).
  • Disease/antibiotic resistance (organelle-encoded).
• Study of nuclear-cytoplasmic interactions.
• Rapid transfer of organelle traits without altering nuclear genome.
• Overcomes limitations of sexual transfer of cytoplasmic genes.

Plant growth regulators (PGRs) control morphogenesis in vitro. The auxin : cytokinin ratio (Skoog & Miller concept) is critical.

Auxins:

• Examples: IAA, IBA, NAA, 2,4-D.
• Promote cell division, cell elongation, callus formation, and root initiation.
• 2,4-D is a potent callus inducer.

Cytokinins:

• Examples: BAP (BA), Kinetin, Zeatin, 2iP.
• Promote cell division, shoot multiplication, and axillary bud proliferation.
• Delay senescence.

Auxin : Cytokinin ratio effects:

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

Thus, by manipulating PGR concentrations at different stages (shoot multiplication vs. rooting), the desired developmental pathway is controlled during micropropagation.

Meristem culture is the in vitro culture of the apical meristem (the dome of dividing cells at the shoot tip, usually 0.1–0.5 mm, often with 1–2 leaf primordia).

Principle of virus elimination:

• The apical meristem region is largely free of viruses because:
  • Viruses move through vascular tissue, which is not yet differentiated in the meristem.
  • Rapid cell division in the meristem outpaces viral replication.
  • High metabolic activity and possible antiviral substances limit virus movement.

Procedure:

1. Excise a very small meristem tip under a microscope.
2. Surface sterilize and inoculate on nutrient medium.
3. Regenerate into a complete plant.
4. Index/test regenerated plants (ELISA, PCR) to confirm virus-free status.

Enhancement techniques:

• Combined with thermotherapy (heat treatment) and chemotherapy to increase virus-free recovery.

Applications: virus-free potato, banana, sugarcane, strawberry, and orchids.

After isolation and purification, protoplasts are cultured to regenerate whole plants.

Steps:

1. Culture methods:

• Liquid culture: protoplasts suspended in liquid medium.
• Agar/agarose plating: protoplasts embedded in solid medium.
• Feeder layer / nurse culture: actively dividing cells support low-density protoplasts.
• Droplet / microdrop culture for low densities.

2. Cell wall regeneration:

• Within 2–4 days, protoplasts synthesize a new cell wall and become intact cells.

3. Cell division and colony formation:

• Cells undergo repeated divisions to form microcalli, then visible callus.

4. Morphogenesis:

• Callus is transferred to regeneration media.
• Organogenesis (shoot/root formation) or somatic embryogenesis occurs by adjusting the auxin:cytokinin ratio.

5. Plantlet development and hardening:

• Complete plantlets are formed, rooted, and acclimatized to field conditions.

Osmotic support (mannitol/sorbitol) is gradually reduced as the cell wall reforms.

Conclusion: Somatic hybridization extends the range of gene combinations beyond what sexual hybridization allows, especially for cytoplasmic traits and wide crosses.

Somaclonal variation refers to the genetic and epigenetic variation observed among plants regenerated from cultured somatic cells, tissues, or protoplasts (somaclones).

Causes:

• Chromosomal changes (aneuploidy, polyploidy, translocations).
• Point mutations and gene amplification.
• Activation of transposable elements.
• Epigenetic changes (DNA methylation).
• Prolonged callus/suspension culture and presence of 2,4-D.

Relevance to micropropagation:

• Disadvantage: Undesirable in clonal propagation where genetic uniformity is required; hence direct methods and shoot-tip culture are preferred to minimize variation.

Relevance to crop improvement (advantage):

• Source of novel useful variants without hybridization.
• Selection of variants for disease resistance, salt/drought tolerance, herbicide resistance, and improved quality.
• Faster generation of variability than conventional breeding.

Examples: disease-resistant sugarcane and improved potato lines developed via somaclonal variation.

Synthetic seeds (artificial seeds) are encapsulated somatic embryos or other propagules (shoot buds, cell aggregates) that function like true seeds and can be sown/stored.

Production:

1. Production of high-quality somatic embryos via somatic embryogenesis.
2. Encapsulation of embryos:
   • Hydrated (gel-encapsulated) seeds: embryos coated with sodium alginate, then dropped into calcium chloride () solution to form firm calcium-alginate beads.
   • Desiccated seeds: embryos coated with polyethylene oxide and dried.
3. The capsule may contain nutrients, growth regulators, and protectants (an 'artificial endosperm').

Advantages:

• Enables clonal propagation through seed-like handling.
• Easy storage, transport, and mechanical sowing.
• Useful for plants with no/poor seeds or those that are heterozygous.
• Preserves genetic uniformity of elite genotypes.
• Cost-effective large-scale delivery of tissue-cultured plants.

Limitations: low conversion frequency, need for aseptic handling, and quality control of somatic embryos.

Somatic hybridization contributes significantly to crop improvement:

1. Overcoming incompatibility barriers:

• Enables hybrids between sexually incompatible species/genera.

2. Production of novel hybrids:

• Pomato (potato Solanum tuberosum + tomato S. lycopersicum).
• Arabidobrassica (Arabidopsis + Brassica).

3. Transfer of disease resistance:

• Transfer of resistance genes from wild species to cultivated crops (e.g., disease-resistant potato, Brassica).

4. Transfer of cytoplasmic traits via cybrids:

• Cytoplasmic Male Sterility (CMS) for hybrid seed production.
• Herbicide/antibiotic resistance (organelle-encoded).

5. Improvement of quality and stress tolerance:

• Transfer of salt/cold tolerance and improved nutritional traits.

6. Combining organelle genomes:

• Creation of novel nuclear-cytoplasmic combinations for research and breeding.

Significance: It broadens the gene pool accessible to breeders beyond sexual limits, especially for wild-relative and cytoplasmic traits.

Sequence of events during fusion:

1. Agglutination (adhesion):

• Membranes of two or more protoplasts come into close contact, induced by PEG, high /high pH, or an AC field in electrofusion.

2. Membrane fusion at points of contact:

• Plasma membranes fuse at localized points, forming cytoplasmic bridges.

3. Formation of a single fused protoplast (cytoplasmic fusion):

• The two protoplasts merge into one, initially with two separate nuclei (heterokaryon).

4. Nuclear fusion (karyogamy):

• The two nuclei fuse to give a true hybrid cell (synkaryon) containing both parental genomes.

Fusion products:

• Homokaryon: fusion of protoplasts from the same parent (like + like).
• Heterokaryon: fusion of protoplasts from different parents, with both nuclei intact — this is the desired product.
• Hybrid cell (synkaryon): heterokaryon whose nuclei have fused → develops into the somatic hybrid.
• Cybrid: cytoplasms mix but only one nucleus persists.

After fusion, hybrid cells are selected, cultured, and regenerated into whole somatic hybrid plants.