Q.11 Which one of the following statements about Agrobacterium Ti plasmid is CORRECT?
(A) Vir genes are located within the T–DNA Segment
(B) Phytohormone biosynthesis genes are located outside the T–DNA segment
(C) Opine catabolism genes are locate Within the T–DNA segment
(D) Opine biosynthesis genes are located within the T–DNA segment
Correct Answer: (D)
The Agrobacterium Ti plasmid contains a T-DNA segment with opine biosynthesis genes, making option D correct. Vir genes and phytohormone genes reside outside T-DNA, while opine catabolism genes also stay outside. This structure enables precise gene transfer in plant biotechnology.
Option Analysis
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(A) Incorrect: Vir genes, responsible for T-DNA excision and transfer to plant cells, lie outside the T-DNA borders in the virulence region.
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(B) Correct but not the focus: Phytohormone biosynthesis genes (auxin, cytokinin) are housed outside T-DNA, aiding tumor formation indirectly.
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(C) Incorrect: Opine catabolism genes, which allow bacteria to metabolize opines, reside outside T-DNA on the plasmid.
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(D) Correct: Opine biosynthesis genes within T-DNA direct plants to produce opines like octopine or nopaline, providing nutrients exclusively for Agrobacterium.
Agrobacterium Ti plasmid T-DNA segment powers genetic engineering in plants. This key phrase highlights its role in competitive exams like IIT JAM, where questions test gene locations precisely.
Ti Plasmid Overview
The Ti plasmid in Agrobacterium tumefaciens spans about 200 kb and induces crown gall tumors. T-DNA segment, flanked by border sequences, transfers to plant nuclei for stable integration. Vir genes outside T-DNA orchestrate this process.
T-DNA Segment Genes
Opine biosynthesis genes reside within the T-DNA segment, directing plant synthesis of nitrogen-rich opines like nopaline. Phytohormone genes (iaa, ipt for auxin, cytokinin) also fall inside wild-type T-DNA, promoting uncontrolled growth. Disarmed versions replace these for biotech applications.
Outside T-DNA Regions
Vir genes form ~35 kb operons (virA to virH) essential for sensing plant wounds and exporting T-DNA-protein complexes. Opine catabolism genes enable bacterial nutrient scavenging from plant secretions. These stay on the plasmid backbone.
Key takeaway for exams: Only T-DNA integrates; other regions support transfer without entering plants.
