Introduction:
Ti (Tumor-inducing) plasmids from Agrobacterium tumefaciens and Ri (Root-inducing) plasmids from Agrobacterium rhizogenes are commonly used as vectors in plant genetic engineering.
They are modified to deliver foreign genes into plant cells, enabling the development of transgenic plants with desirable traits.
1. Why Ti and Ri Plasmids are Used as Vectors:
These plasmids naturally transfer a portion of their DNA (T-DNA) into the plant genome.
Scientists exploit this natural mechanism to introduce specific genes instead of the tumor or root-inducing genes.
2. Structure of Ti and Ri Plasmids for Vector Use:
T-DNA Region:
The T-DNA is modified to carry the desired gene.
Tumor-inducing (Ti) or root-inducing (Ri) genes are removed and replaced with foreign genes.
Vir (Virulence) Genes:
Essential for transferring the T-DNA into the plant cell.
Usually retained because they facilitate the T-DNA transfer process.
Selectable Marker Genes:
Added to identify transformed cells (e.g., antibiotic resistance).
3. Steps in Using Ti and Ri Plasmids as Vectors:
Gene Insertion:
The desired gene is inserted into the T-DNA region of the plasmid.
Transformation of Agrobacterium:
The modified plasmid is introduced back into Agrobacterium cells.
Infection of Plant Tissues:
Plant tissues (e.g., leaves, stem, or callus) are infected with the transformed Agrobacterium.
Integration into Plant Genome:
The Vir genes help transfer the T-DNA, containing the foreign gene, into the plant genome.
Selection and Regeneration:
Transformed plant cells are selected using marker genes (e.g., antibiotic resistance).
The selected cells are regenerated into whole plants through tissue culture.
4. Applications of Ti and Ri Plasmid Vectors:
Development of Transgenic Crops:
Introducing genes for pest resistance, herbicide tolerance, and enhanced nutrition.
Example: Bt cotton with a gene for insect resistance.
Production of Pharmaceutical Proteins:
Plants engineered to produce vaccines or therapeutic proteins.
Study of Gene Function:
Used in research to understand the role of specific genes in plants.
Root Culture Studies (Ri Plasmid):
Ri plasmids are particularly useful for studying root development and producing valuable secondary metabolites in root cultures.
5. Advantages of Using Ti and Ri Plasmids as Vectors:
Efficient Gene Transfer: Natural ability to transfer DNA into plants.
Stable Integration: The foreign gene integrates into the plant genome and is passed on to the next generation.
Wide Host Range: Can be used with many different plant species.
Low Cost and Versatility: Easy to modify for different genes and traits.
6. Limitations:
Limited to Dicots: Agrobacterium naturally infects mainly dicotyledonous plants (e.g., tomatoes, potatoes). Monocots (e.g., wheat, rice) require special techniques.
Regulatory and Safety Concerns: Transgenic plants require extensive testing and regulatory approval.
Conclusion:
Ti and Ri plasmids are essential tools in plant biotechnology, acting as vectors for transferring foreign genes into plant genomes.
Their natural ability to integrate T-DNA into plants makes them ideal for developing transgenic crops with improved traits, contributing to advancements in agriculture and pharmaceutical production