Unit 8: Protoplast fusion

Protoplast fusion is the somatic hybridization technique in which the naked cells of two plant species—stripped of their cell walls—are merged to combine whole genomes without sexual crossing. It emerged as a practical tool after Cocking (1960) enzymatically isolated intact protoplasts, and Kao and Michayluk (1974) established polyethylene glycol (PEG) as a reliable chemical fusogen. This unit deals with the induced fusion of isolated protoplasts using PEG to generate somatic hybrid or cybrid plants that bypass pre- and post-fertilization barriers.

• Protoplast: A plant cell whose cellulosic wall has been removed enzymatically, leaving a spherical cell bounded only by the plasma membrane; osmotically fragile and totipotent.
• Fusogen: An agent that promotes membrane coalescence; PEG is the dominant chemical fusogen, an alternative to electrofusion.
• Somatic hybrid: A regenerated plant containing the combined nuclear genomes of both parental protoplasts.
• Cybrid (cytoplasmic hybrid): A plant with the nucleus of one parent but mixed or recombined cytoplasmic organelles (chloroplasts, mitochondria) of both.
• Totipotency: The capacity of a single protoplast to regenerate into a complete plant, on which the entire method depends.
• Osmoticum: A non-metabolized solute (mannitol, sorbitol, 0.4–0.7 M) that balances internal pressure and prevents the wall-less protoplast from bursting.

II. Fusion of Protoplast by PEG Method for Hybrid Production

Chemically induced somatic hybridization using polyethylene glycol

The PEG method uses polyethylene glycol, with or without high pH and Ca²⁺, to make adjacent protoplast membranes adhere and coalesce, producing hybrid cells that are then cultured to regenerate whole plants.

A. Principle and Purpose of PEG-Mediated Fusion

The subsection establishes why PEG works and what somatic fusion achieves that sexual crossing cannot.

• Overcoming sexual incompatibility: Fusion combines genomes of species that cannot be crossed by pollination, e.g. the Solanum tuberosum (potato) + Solanum lycopersicum (tomato) "pomato" hybrid of Melchers (1978).
• Molecular action of PEG: PEG (typically MW 1500–6000, at 15–45% w/v) is a polymer that binds water and cations, dehydrating the membrane surface and neutralizing surface charge so that closely apposed membranes can contact and merge.
• Role of divalent cations: Ca²⁺ (as CaCl₂) bridges negatively charged phosphate groups of adjacent plasma membranes, stabilizing the agglutinated pairs before merger.
• High-pH, high-Ca²⁺ elution: Washing agglutinated protoplasts with an alkaline (pH 9–10.5), high-Ca²⁺ (≈50 mM) solution triggers the actual membrane fusion as PEG is removed.
• Non-specificity of fusion: PEG fuses membranes indiscriminately, producing homokaryons (same parent) and heterokaryons (different parents) alike; only heterokaryons yield hybrids, so selection is essential.

B. Isolation of Parental Protoplasts

The subsection covers preparing the two viable protoplast populations that fusion requires.

• Source tissue: Young mesophyll from surface-sterilized leaves or fast-growing suspension-culture cells provide uniform, dividing cells.
• Enzymatic wall digestion: A mixture removes the wall in one step or sequentially:
  • Cellulase (1–2%): hydrolyses cellulose microfibrils of the primary wall.
  • Macerozyme / pectinase (0.2–0.5%): dissolves the pectin middle lamella separating cells.
  • Hemicellulase: clears residual hemicellulose in some protocols.
• Osmotic protection: Enzymes are dissolved in 0.4–0.7 M mannitol or sorbitol so released protoplasts do not lyse.
• Purification: Protoplasts are filtered through 40–100 µm nylon mesh to remove debris, then floated on a sucrose cushion or pelleted by low-speed centrifugation (≈100 × g).
• Viability check: Fluorescein diacetate (FDA) staining—viable protoplasts fluoresce green; Evans blue stains dead cells—confirms a healthy population before fusion.

C. The PEG Fusion Procedure

The subsection sets out the operational sequence from mixing to fusion in numbered stages.

1. Mix parental protoplasts   -> equal densities (~1 x 10^5 /mL each)
2. Settle onto substrate      -> thin layer in dish / on cover slip
3. Add PEG solution           -> 25-45% PEG, 5-15 min, RT
4. Agglutination              -> protoplasts clump, membranes apposed
5. Elution wash               -> high pH (9-10.5), high Ca2+ buffer
6. Membrane fusion            -> heterokaryons form
7. Gradual dilution           -> replace with culture medium

• Agglutination step: PEG causes tight, reversible clustering; without the following wash, cells merely adhere rather than fuse.
• Fusion trigger: The alkaline high-Ca²⁺ elution is the decisive step—it destabilizes and reorganizes the contacting bilayers into a single continuous membrane.
• Gradual washing: PEG must be diluted slowly with culture medium; abrupt removal causes osmotic shock and bursts the fragile fusion products.
• Fusion frequency: Typically 1–10% of protoplasts form heterokaryons, which is why downstream selection cannot be skipped.

D. Fusion Products and Nuclear Behaviour

The subsection distinguishes the possible outcomes of merging two protoplasts.

1. Homokaryon: Fusion of two protoplasts of the same parent—no hybrid value; a background product to be selected against.
2. Heterokaryon: Fusion of protoplasts from different parents; the target product that may become a hybrid.

• Cytoplasmic fusion first: The two cytoplasms mix immediately, pooling both parents' chloroplasts and mitochondria in one common cytoplasm.
• Nuclear fusion (karyogamy): The two nuclei may fuse during the first mitosis to give a symmetric somatic hybrid carrying both full nuclear genomes.
• Cybrid formation: If one nucleus is lost or was inactivated, the product retains one nucleus with mixed cytoplasm—useful for transferring cytoplasmic traits such as cytoplasmic male sterility.
• Organelle segregation: Chloroplasts usually sort out to one parental type over successive divisions, whereas mitochondrial genomes frequently recombine.

E. Selection of Somatic Hybrids

The subsection addresses recovering the rare hybrid cells from the mixed population.

• Complementation selection: Two parents each carrying a different non-lethal defect (e.g. albino + chlorophyll-deficient) grow only when fused, since the hybrid restores the missing functions.
• Drug/auxotroph resistance markers: One parent resistant to a selection agent and the other requiring a nutrient allows only the complemented hybrid to divide on selective medium.
• Physical/visual selection: Combining a green (chloroplast-bearing) mesophyll protoplast with a colorless, fluorescent-dye-labelled cell lets heterokaryons be picked out under the microscope or by flow sorting.
• Micromanipulation: Individual heterokaryons identified by dual staining (e.g. one parent pre-labelled with FDA, the other with rhodamine) can be manually isolated by micropipette.

F. Regeneration of Hybrid Plants

The subsection follows the selected hybrid cell to a whole plant.

• Cell-wall regeneration: Within a few days in osmotically supported medium the protoplast lays down a new cellulose wall, a prerequisite for division.
• Callus formation: Sustained division produces a hybrid callus on medium balanced with auxin and cytokinin.
• Organogenesis / embryogenesis: Shifting the auxin-to-cytokinin ratio induces shoots and roots (or somatic embryos) from the callus.
• Confirmation of hybridity: Regenerants are verified by intermediate morphology, chromosome counting, isozyme banding, and molecular markers (RAPD, RFLP, SSR) plus organelle DNA analysis to distinguish hybrids from cybrids.
• Worked example — the pomato: Fusing potato and tomato mesophyll protoplasts with PEG, selecting on complementation medium, and regenerating gave plants combining both parental genomes; though not agronomically productive, it proved that PEG fusion can unite two full sexually incompatible genomes into one regenerated plant.

G. Applications and Limitations

The subsection weighs where PEG fusion is useful against its practical drawbacks.

• Applications:
  • Wide-cross hybrids: creates combinations impossible by pollination, e.g. cereal × legume attempts and Brassica interspecific hybrids.
  • Disease and stress resistance transfer: moves polygenic or cytoplasmically encoded resistance from wild relatives into crops.
  • Cybrid production: transfers cytoplasmic male sterility (CMS) and herbicide resistance encoded on chloroplast/mitochondrial DNA for hybrid-seed programmes.
• Limitations:
  • Chemical toxicity: high PEG concentrations damage membranes, lowering protoplast viability compared with electrofusion.
  • Low, non-selective fusion: random fusion produces mostly homokaryons and multi-cell aggregates, demanding heavy selection.
  • Genomic instability: chromosome elimination and abnormal segregation often yield aneuploid or sterile regenerants.
  • Regeneration bottleneck: many species, especially monocot cereals, resist regeneration from protoplasts, restricting the method's crop range.

H. Comparison of PEG Fusion with Electrofusion

The subsection contrasts the two main induced-fusion routes to place PEG in context.

1. PEG (chemical) fusion: uses a fusogen polymer; inexpensive and simple, needs no specialized equipment, but is toxic, batch-variable, and non-directional.
2. Electrofusion (physical): aligns protoplasts by dielectrophoresis then merges them with a brief DC pulse; gentler and more reproducible with higher viability, but requires a fusion chamber and pulse generator.

• Control of pairing: electrofusion aligns cells in pearl chains for near-one-to-one pairing, whereas PEG agglutinates cells randomly in clumps.
• Practical choice: PEG remains the standard teaching-lab and low-budget option; electrofusion is preferred where viability and defined pairing are critical.