Unit 4: Methods of Gene Transfer

This method utilizes the natural ability of the soil bacterium Agrobacterium tumefaciens to transfer a portion of its DNA into plant cells.

The Process

• Natural Mechanism: In nature, Agrobacterium causes crown gall disease by transferring T-DNA from its Ti plasmid into the host plant genome.
• Vector Engineering: For biotechnology, the "tumor-inducing" genes are removed (disarmed) and replaced with the gene of interest.
• Infection: Plant explants are co-cultivated with the engineered bacteria, allowing the T-DNA to integrate into the plant chromosomes.

Advantages

• Provides high-frequency stable integration of genes.
• Usually results in single-copy or low-copy number insertions.

Electroporation is a physical method used to introduce foreign DNA directly into plant cells (often protoplasts).

Mechanism

• High-voltage electrical pulses are applied to a suspension of cells and DNA.
• The electricity creates temporary pores in the cell membrane, allowing the DNA to enter by diffusion.
• Once the pulse stops, the pores reseal, trapping the DNA inside.

Microinjection is the process of physically injecting DNA directly into the nucleus or cytoplasm of an individual plant cell using a fine glass capillary.

Key Features

• Requires specialized equipment (micromanipulators) and significant technical skill.
• High precision but extremely low throughput as it is done cell-by-cell.
• Useful for cells that are difficult to transform by other methods, such as large embryos or single protoplasts.

Commonly known as the Gene Gun method or Biolistics, this technique involves "shooting" DNA into plant tissues.

The Process

• Microprojectiles: Small particles of gold or tungsten are coated with the DNA of interest.
• Acceleration: These particles are accelerated at high speeds (using helium gas or gunpowder) into target plant cells.
• Penetration: The particles penetrate the cell wall and membrane, releasing the DNA inside the cell where it can integrate into the genome.

Applications

• Effective for plants that are resistant to Agrobacterium infection (e.g., many monocots).
• Can be used on intact tissues like leaves, embryos, and callus.

After transformation, researchers must distinguish successfully transformed cells from those that did not take up the DNA.

Selectable Marker Genes

These allow only the transformed cells to grow on a specific medium.

• Antibiotic Resistance: (e.g., nptII for kanamycin resistance). Transformed cells survive while non-transformed ones die.
• Herbicide Resistance: (e.g., bar gene).

Reporter Genes

These produce a visible product that helps identify transformed tissues without killing them.

6. Exam Focus: Tips and FAQs

Common Pitfalls

• Mistake: Thinking Agrobacterium can infect all plants equally. Correction: While its host range is wide, many monocots (like rice and wheat) were historically difficult to transform this way.
• Mistake: Confusing selectable markers with reporter genes. Correction: Selectable markers are for survival (killing non-recombinants); reporters are for visual confirmation (monitoring gene expression).

Frequently Asked Questions

Q: Why are gold or tungsten used in microprojectile bombardment?
A: These metals are inert (do not react with cellular components) and high density, providing the mass needed to penetrate cell walls.

Q: What is "disarmed" T-DNA?
A: It is a Ti plasmid where the genes responsible for gall formation (auxin and cytokinin synthesis) have been removed, making the bacteria safe to use as a delivery vehicle.