123. Disarmed Ti plasmid of Agrobacterium tumefaciens does not result in crown gall phenotype since it does
not possess:
(1) ipt and iaaH genes
(2) Vir D gene
(3) Vir A gene
(4) Vir G gene

Detailed Explanation:

Question:
Disarmed Ti plasmid of Agrobacterium tumefaciens does not result in crown gall phenotype since it does not possess:

✅ Correct Answer: (1) ipt and iaaH genes

Background: Ti Plasmid and Crown Gall Disease

Agrobacterium tumefaciens is a soil bacterium known for its natural genetic engineering capabilities. It causes crown gall disease in plants by transferring a portion of its DNA (T-DNA) into the plant genome using the Ti (tumor-inducing) plasmid.

What Happens in a Natural Ti Plasmid?

The T-DNA of a natural (wild-type) Ti plasmid contains oncogenes, primarily:

• ipt (isopentenyl transferase): involved in cytokinin biosynthesis

• iaaH and iaaM: involved in auxin biosynthesis

These genes disrupt plant hormone regulation, leading to uncontrolled cell division and tumor (gall) formation at the infection site.

Why Disarmed Ti Plasmids Don’t Cause Tumors:

Disarmed Ti plasmids are genetically modified versions of the natural Ti plasmid used in plant genetic engineering. In these plasmids:

• The tumor-inducing genes like ipt and iaaH are removed or inactivated.

• As a result, while the bacterium can still transfer T-DNA into the plant genome (via Vir genes), no tumor is formed because the plant’s hormone levels are not disrupted.

This makes disarmed Ti plasmids ideal vectors for transgenic plant development, allowing gene transfer without the side effect of tumor formation.

Explanation of Incorrect Options:

• (2) Vir D gene
  Part of the Vir (virulence) region, VirD is essential for processing the T-DNA but not responsible for tumor formation.

• (3) Vir A gene
  A sensor protein in the Vir region that helps detect plant signals, but again, not an oncogene.

• (4) Vir G gene
  A regulatory protein involved in activating other Vir genes. Not directly responsible for gall formation.

Conclusion:

The absence of ipt and iaaH genes in a disarmed Ti plasmid is what prevents crown gall tumors from forming. These genes are critical for hormone overproduction that triggers uncontrolled plant cell proliferation. By removing them, scientists created a safe and effective tool for plant transformation.

✅ Correct Answer: (1) ipt and iaaH genes