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Role of Virulence (Vir) Genes

 

Introduction:

  • Virulence (Vir) genes are located on the Ti (Tumor-inducing) plasmid of Agrobacterium tumefaciens and the Ri (Root-inducing) plasmid of Agrobacterium rhizogenes.

  • These genes play a crucial role in transferring T-DNA (Transfer DNA) from the bacterium into the plant genome, which is the basis for creating transgenic plants.


Location and Types of Virulence Genes:

  • Found outside the T-DNA region on the plasmid.

  • There are several vir gene clusters (VirA, VirB, VirC, VirD, VirE, VirG, etc.), each encoding proteins with specific functions.


Key Functions of Virulence Genes:

  1. VirA and VirG: Activation of the Vir Genes

    • VirA: A sensor protein that detects plant signals (like phenolic compounds) released from wounded plants.

    • VirG: Acts as a regulatory protein, activating other vir genes in response to VirA signals.

    • Together, they start the vir gene cascade, initiating the transfer process.

  2. VirB: Formation of the T-DNA Transfer Channel

    • Encodes proteins that form a type IV secretion system (T4SS).

    • Creates a channel or conduit through which T-DNA moves from the bacterium into the plant cell.

  3. VirD: Processing and Cutting T-DNA

    • VirD1 and VirD2 proteins recognize the border sequences of T-DNA.

    • VirD2 cuts the T-DNA and attaches to it, guiding it into the plant cell.

  4. VirE: Protection and Guidance of T-DNA

    • VirE2 binds to the T-DNA, forming a T-DNA-VirE2 complex.

    • Protects the T-DNA from degradation inside the plant cell and assists in guiding it to the nucleus.

  5. VirC: Enhances T-DNA Transfer Efficiency

    • Helps in the recognition of border sequences, ensuring accurate T-DNA processing and transfer.



Process of T-DNA Transfer Mediated by Vir Genes:

  1. Plant Wounding:

    • Agrobacterium detects plant wounds and releases VirA to sense plant-derived signals (phenolics).

  2. Vir Gene Activation:

    • VirA activates VirG, which then activates other vir genes.

  3. T-DNA Processing:

    • VirD proteins process and cut the T-DNA from the plasmid.

    • T-DNA binds to VirD2 and VirE2 for protection.

  4. T-DNA Transfer:

    • The VirB complex forms a channel through which the T-DNA complex is transferred into the plant cell.

  5. Integration into Plant Genome:

    • Inside the plant nucleus, the T-DNA integrates into the plant chromosome, expressing the inserted genes.


Importance in Genetic Engineering:

  • Gene Delivery Tool: Vir genes are essential for using Agrobacterium as a vector in genetic engineering.

  • Customizable System: By replacing the natural T-DNA with foreign genes, scientists can create transgenic plants with desired traits (e.g., pest resistance).


Conclusion:

Virulence genes play a central role in T-DNA transfer from Agrobacterium to plant cells. They ensure the accurate processing, protection, and integration of foreign DNA into the plant genome. This mechanism is the foundation of plant genetic engineering, enabling the development of transgenic crops with beneficial traits

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