Q.40 Select the CORRECT combination of genetic components that are essential for the transfer of T-
DNA segment from Agrobacterium tumefaciens to plant cells.
(A) Border repeat sequences and oncogenes (B) Border repeat sequences and vir genes
(C) Opine biosynthetic genes and vir genes (D) Opine biosynthetic genes and oncogenes

T-DNA Transfer in Agrobacterium tumefaciens: Essential Genetic Components MCQ Solution

Agrobacterium tumefaciens transfers T-DNA from its Ti plasmid to plant cells via specific genetic elements, enabling natural genetic engineering. The correct combination involves border repeats for defining the transferable segment and vir genes for the transfer machinery.​

Option Analysis

• (A) Border repeat sequences and oncogenes: Incorrect. Oncogenes (like iaa and cyt genes) cause tumors but play no role in T-DNA excision or transfer process.​

• (B) Border repeat sequences and vir genes: Correct. ~25-bp border repeats mark T-DNA boundaries for VirD2-mediated nicking; vir genes (virA-G) encode T4SS proteins, chaperones (VirE2), and processing enzymes essential for T-strand export.​

• (C) Opine biosynthetic genes and vir genes: Incorrect. Opine genes on T-DNA enable bacterial nutrient scavenging post-integration but are neither processed nor required for transfer itself.​

• (D) Opine biosynthetic genes and oncogenes: Incorrect. Both reside on T-DNA for plant effects after transfer, bypassing the mechanics of DNA mobilization.​

Correct Answer

(B) Border repeat sequences and vir genes. This duo is indispensable: borders define what transfers, vir genes execute the process from sensing plant phenolics to nuclear import.​

T-DNA transfer from Agrobacterium tumefaciens to plant cells revolutionizes plant biotechnology as nature’s gene delivery system. This soil bacterium uses its Ti plasmid to inject T-DNA—bounded by border repeats—into dicot genomes, causing crown galls. Biotechnology harnesses this for stable transformation by pairing borders with vir genes while disarming tumor genes.​

Key Genetic Components

• Border repeat sequences: 25-bp imperfect direct repeats at T-region ends; right border initiates VirD1/D2 endonuclease nicking for single-stranded T-strand production.​

• Vir genes: Seven operons (virA–G) on Ti plasmid; VirA/G senses acetosyringone, VirB/D4 forms type IV secretion system (T4SS) pilus, VirE2 coats T-strand for nuclear targeting.​

• T-complex (VirD2-T-strand-VirE2) traverses plant cytoplasm, imports via host importins, integrates randomly.​

MCQ Strategy for Exams

Prioritize function: Transfer requires processing (borders + virD) and export (virB/E). Options lacking borders fail excision; those omitting vir genes halt export. (B) matches Ti plasmid essentials per molecular studies. Visualize: Phenolic induction → vir transcription → T-nick → T4SS injection.​

This mechanism underpins 80% of transgenic crops, aiding gene editing in biotech research and agriculture.​