Q.Assertion (A): Agrobacterium tumefaciens is a pathogen of several monocot plants. Reason (R): It is able to deliver a piece of DNA known as 'T-DNA' to transform normal plant cells into a tumor. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

Concept: Agrobacterium tumefaciens as a natural genetic engineer and its host specificity.

Agrobacterium tumefaciens is a soil bacterium that causes crown gall disease by transferring T-DNA (transfer DNA) from its Ti plasmid into the host plant genome. This T-DNA integration transforms normal plant cells into tumor cells that proliferate uncontrollably and synthesize opines (nutrients for the bacterium).

The critical point: Agrobacterium naturally infects dicot plants (broadleaf plants like tobacco, tomato, sunflower), not monocots. Monocots (grasses, cereals) are generally resistant to Agrobacterium infection, though modern genetic engineering has developed methods to overcome this barrier in the laboratory. …

The question tests your understanding of Agrobacterium tumefaciens as a genetic engineering tool and its natural host range. This bacterium is famous in biotechnology because it naturally transfers DNA into plant cells—a mechanism scientists have hijacked for creating transgenic plants.

Let's examine each statement carefully.

Understanding Agrobacterium tumefaciens

Agrobacterium tumefaciens is a soil bacterium that causes crown gall disease in plants. The disease manifests as tumor-like growths (galls) at wound sites, typically near the soil line (the "crown" of the plant). What makes this bacterium remarkable is its natural ability to genetically engineer plants—it's essentially nature's own genetic engineer.

The bacterium carries a Ti plasmid (tumor-inducing plasmid) that contains a specific region called T-DNA (transfer DNA). When the bacterium infects a plant cell, it transfers this T-DNA into the plant's nucleus, where it integrates into the plant's chromosomes. The T-DNA carries genes that:

• Code for enzymes producing plant hormones (auxins and cytokinins), causing uncontrolled cell division (the tumor)
• Code for opine synthesis, creating unusual amino acids that only Agrobacterium can metabolize as food

Evaluating the Assertion

Assertion (A) claims: Agrobacterium tumefaciens is a pathogen of several monocot plants.

This is false. Agrobacterium tumefaciens naturally infects dicotyledonous (dicot) plants almost exclusively. Its natural host range includes:

• Grapevines
• Stone fruits (peaches, cherries)
• Roses and other ornamentals
• Tobacco, tomato, and other solanaceous plants

Monocots (grasses, cereals, palms, orchids) are naturally resistant to Agrobacterium infection. This was historically a major limitation in plant biotechnology because economically important crops like rice, wheat, maize, and sugarcane are all monocots.

Evaluating the Reason

Reason (R) states: It is able to deliver a piece of DNA known as 'T-DNA' to transform normal plant cells into a tumor.

This is true and accurately describes the mechanism of pathogenicity. The T-DNA transfer process involves:

1. Recognition: The bacterium detects phenolic compounds (like acetosyringone) released from wounded plant cells …