Q17. Which of the following gene is not a constituent of T-DNA in Agrobacterium tumefaciens?
1. Octopine synthase
2. Isopentyl transferase
3. Virulence Gene G
4. Indoleacetamide hydrolase

Which Gene is NOT a Constituent of T-DNA in Agrobacterium tumefaciens?

Agrobacterium tumefaciens is a gram-negative soil bacterium that is well-known for its ability to transfer genes to plant cells. This is accomplished through the Ti (Tumor-inducing) plasmid, which contains a specific region called T-DNA (Transfer DNA).

The question asks which gene is not a part of T-DNA. The correct answer is:

✅ 3. Virulence Gene G

Let’s explore why Virulence Gene G is not a part of T-DNA and what the roles of the other genes are.

Understanding T-DNA and Its Role in Genetic Transformation

The T-DNA region of the Ti plasmid is the part that gets transferred from the bacterium into the plant genome. This transferred DNA integrates into the plant chromosome and causes the formation of crown gall tumors by producing plant hormones and other compounds.

However, the process of transferring T-DNA is mediated by virulence (Vir) genes, which are not themselves transferred to the plant. These Vir genes are located outside the T-DNA region and assist in the processing and transfer of T-DNA into the host plant.

Detailed Explanation of Each Option

1. Octopine Synthase (Present in T-DNA ✅)

• Function: Octopine synthase is an enzyme that catalyzes the production of octopine, a type of opine compound.
• Why is it in T-DNA? Agrobacterium uses octopine as a nutrient source, and its production benefits the bacteria by creating a selective advantage.
• Example: Opines like octopine and nopaline are commonly found in crown gall tumors caused by Agrobacterium infection.

2. Isopentyl Transferase (Present in T-DNA ✅)

• Function: This enzyme is involved in the biosynthesis of cytokinins, which are plant hormones that promote cell division.
• Why is it in T-DNA? The production of cytokinin leads to uncontrolled plant cell proliferation, resulting in tumor formation.
• Example: High levels of cytokinin are associated with rapid and excessive growth of infected plant cells.

3. Virulence Gene G (NOT Present in T-DNA ❌)

• Function: Virulence (Vir) genes, including VirG, help process and transfer T-DNA into the plant cell.
• Why is it NOT in T-DNA? Vir genes are located outside the T-DNA region on the Ti plasmid. They do not integrate into the plant genome but instead regulate the transfer of T-DNA.
• Example: The VirD2 and VirE2 proteins help guide the T-DNA into the plant nucleus.

👉 Virulence genes are essential for T-DNA transfer but are NOT transferred themselves!

4. Indoleacetamide Hydrolase (Present in T-DNA ✅)

• Function: This enzyme helps convert indoleacetamide into indoleacetic acid (IAA), which is a type of auxin, another plant hormone.
• Why is it in T-DNA? The production of auxin contributes to tumor formation by promoting excessive plant cell elongation and division.
• Example: Plants infected with Agrobacterium show high auxin levels in gall tissues, leading to their characteristic swollen appearance.

Summary: Why Virulence Gene G is NOT Part of T-DNA

How T-DNA Transfer Works in Plant Cells

1. Attachment to the Plant Cell

   • Agrobacterium attaches to wounded plant cells and releases chemical signals to activate virulence genes.

2. Processing of T-DNA

   • Virulence genes VirD1 and VirD2 cut the T-DNA region from the Ti plasmid.

3. Transfer of T-DNA

   • T-DNA is transported into the plant cell via a type IV secretion system.

4. Integration into the Plant Genome

   • The T-DNA gets integrated into the plant’s chromosome, leading to tumor formation and opine production.

Biotechnological Applications of T-DNA Transfer

• Genetic Engineering in Plants: T-DNA is used to introduce foreign genes into plants for herbicide resistance, pest resistance, and enhanced nutrition.
• Production of Recombinant Proteins: T-DNA can be engineered to express pharmaceutical proteins like edible vaccines in plants.
• Marker-Free Plant Transformation: Researchers use modified Ti plasmids without tumor-inducing genes for safe genetic transformation.

Conclusion

While Octopine Synthase, Isopentyl Transferase, and Indoleacetamide Hydrolase are part of the T-DNA region, Virulence Gene G is NOT because it is located outside T-DNA and only assists in the transfer process.

Understanding T-DNA and Agrobacterium-mediated gene transfer is essential for competitive exams like:
CSIR NET Life Science
IIT JAM Biotechnology
GATE Biotechnology
DBT BET JRF

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