Question

Which of the following statements is/are correct about Agrobacterium?
(A). Agrobacterium causes crown gall in apple
(B). Agrobacterium causes gall by multiplying within the gall
(C). Agrobacterium incites gall by transferring T-DNA
(D). Agrobacterium need wounds to incite gall
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (B) and (D) only.
• C. (A), (C) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Remember the genetic mechanism of Agrobacterium: it requires a physical wound to initiate infection, then transfers its T-DNA into the plant genome to drive host cell proliferation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Agrobacterium tumefaciens is a soil-borne, Gram-negative bacterium widely known as "nature's genetic engineer" due to its unique ability to transfer genetic material into plant cells.
This bacterium infects wounded plant tissue, leading to the formation of tumors or galls at the crown of the plant.

Step 2: Detailed Explanation:
Let us analyze each of the statements to determine their validity:
Statement (A) is correct: Agrobacterium tumefaciens has a broad host range among dicotyledonous plants, including fruit trees such as apple (Malus domestica), where it causes crown gall disease.
Statement (B) is incorrect: The bacterium does not cause the gall simply by multiplying inside the tissues.
Instead, it genetically transforms host plant cells, directing them to synthesize excess phytohormones (auxins and cytokinins) that stimulate uncontrolled cell division (hyperplasia) and cell enlargement (hypertrophy), which forms the gall.
The physical growth of the gall is driven by the plant's own deregulated cells, not bacterial multiplication.
Statement (C) is correct: The mechanism of gall induction relies on the transfer of a specific segment of the tumor-inducing (Ti) plasmid, known as the T-DNA (transfer DNA), into the plant cell nucleus, where it integrates into the host genome.
Statement (D) is correct: Agrobacterium cannot actively penetrate healthy, intact plant cell walls.
It requires a physical wound (such as from graft sites, pruning, or insect damage) to access host cells.
Phenolic compounds (such as acetosyringone) released by wounded cells act as chemoattractants that activate the virulence (vir) genes on the Ti plasmid, initiating T-DNA transfer.
Therefore, statements (A), (C), and (D) are correct, while (B) is incorrect.

Step 3: Final Answer:
Hence, the correct option is (C).