Ti Plasmid: Structure, Function and Applications in Biotechnology

Ti plasmid (tumour-inducing plasmid) is an extra-chromosomal DNA molecule found in Agrobacterium tumefaciens. This bacterium causes crown gall disease in plants, primarily through the transfer of specific DNA regions from the Ti plasmid into plant cells. Researchers have harnessed this natural gene transfer system to develop powerful tools for plant genetic engineering. In this guide, we will explore the ti plasmid full form, its function, its structure, and its significance in biotechnology. We will also compare the Ti plasmid with the Ri (root-inducing) plasmid, discuss its diverse applications, and provide some additional resources to help you understand this topic comprehensively.

Ti Plasmid Structure Overview

A simplified structure of the Ti plasmid typically includes:

1. Virulence (vir) Genes: Required for T-DNA processing and transfer.

2. Left and Right Borders (LB and RB): Flank the T-DNA region, crucial for precise excision and integration.

3. T-DNA Region: Contains genes that, in the wild-type plasmid, manipulate the plant’s hormone production, leading to tumour formation. In genetically modified plasmids, these genes are replaced with beneficial ones.

4. Opine Catabolism Genes: Help the bacterium utilise opines produced by infected plant cells.

5. Origin of Replication: Ensures plasmid replication in the bacterial host.

History and Background

Agrobacterium tumefaciens is a Gram-negative bacterium belonging to the class Alphaproteobacteria. While many members of this class (such as Caulobacter, Rhodobacter, and Rhizobium) are non-pathogenic or symbiotic, A. tumefaciens is pathogenic to plants. The discovery that A. tumefaciens transfers a region of the Ti plasmid (T-DNA) into plant cells to induce tumours (crown galls) paved the way for modern plant genetic engineering.

Key Features of the Ti Plasmid

1. Virulence Region

• The virulence (vir) region contains genes essential for transferring T-DNA into plant cells. These genes encode proteins that mediate the processing and transfer of T-DNA and help the bacterium infect host cells.

2. T-DNA (Transferred DNA) Region

• The T-DNA region is typically 15–20 kbp in length.

• It integrates into the plant genome, altering the plant’s hormonal balance and leading to tumour formation.

• In genetic engineering, this region can be modified to carry desirable genes instead of disease-causing segments.

3. Opine Catabolism

• Opines are derivatives of amino acids or sugar phosphates.

• Once the plant cells produce opines under the influence of T-DNA, the bacterium utilises them as a nutrient source.

• Common opine types include nopaline and octopine.

4. Origin of Replication

• This region ensures the plasmid can replicate within bacterial cells.

• Stable replication is crucial for maintaining the plasmid and the virulence factors needed for infection.

5. Size

• Ti plasmids can vary widely in size, generally ranging from about 100 kbp to as large as 2 Mbp.

6. Disarmed Ti Plasmid

• A disarmed Ti plasmid lacks the tumour-causing T-DNA region, making it non-pathogenic.

• These modified plasmids are used in biotechnology for transferring genes of interest without causing crown gall disease.

Ti Plasmid Function in Genetic Engineering

• Natural Gene Transfer: The Ti plasmid function in nature is to transfer T-DNA into plants, causing tumorous growths. Scientists exploit this transfer mechanism to introduce beneficial genes into crop plants.

• Creating Transgenic Plants: By replacing the tumour-inducing genes with target genes, the Ti plasmid becomes a vector for crop improvement, helping produce plants with enhanced traits such as pest resistance, drought tolerance, or improved nutritional content.

• “Nature’s Genetic Engineer”: Because of its unique ability to transfer DNA across kingdoms (from bacteria to plants), the Ti plasmid is often referred to as "nature’s genetic engineer."

Difference Between Ti Plasmid and Ri Plasmid

While the Ti plasmid is commonly associated with crown gall disease (tumour formation), the Ri plasmid (root-inducing plasmid) is linked to “hairy root” disease in plants. Here is a quick comparison:

Uses in Bioengineering

1. Production of Transgenic Plants

• Ti plasmid notes often highlight its role in transferring genes of interest, enabling the creation of genetically modified plants with desirable traits such as enhanced resistance to pests, increased yield, or improved nutritional value.

2. Cloning Vector

• By removing pathogenic genes, scientists have developed safe cloning vectors derived from the Ti plasmid.

• These vectors allow the introduction of foreign genes into plant cells for research and crop improvement.

3. Gene Function Studies

• Researchers use the Ti plasmid system to study the function of genes by selectively inserting or knocking out gene sequences in plant cells.

Additional Insights: Crown Gall Disease

• Crown gall disease is characterised by tumorous growths near the crown of the plant, where the stem meets the roots.

• The disease results from the expression of genes in the integrated T-DNA, which alter the plant’s hormonal pathways.

Quick Quiz (With Answers)

1. Which bacterium carries the Ti plasmid?
   Answer: Agrobacterium tumefaciens.

2. What does ‘T’ in T-DNA stand for?
   Answer: Transferred DNA.

3. What disease is caused by the Ti plasmid in plants?
   Answer: Crown gall disease.

4. Name a key difference between Ti plasmid and Ri plasmid.
   Answer: Ti plasmid induces tumour formation (crown gall), while Ri plasmid induces hairy root formation.

5. What is the main advantage of using a disarmed Ti plasmid in biotechnology?
   Answer: It can transfer genes of interest into the plant without causing tumour formation.

Related Topics

• Difference Between Plasmid DNA and Chromosomal DNA