Question

Which of the following is known to cause abnormal and unlimited proliferation of plant cells?

• A. Agrobacterium
• B. Rhizobium
• C. Xanthomonas
• D. Erwinia

Hint:

Think of Agrobacterium as nature's genetic engineer.
Its ability to insert T-DNA encoding auxin and cytokinin synthesis genes directly causes host cells to undergo uncontrolled division.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Tumorigenesis or gall formation in plants is often driven by hormonal imbalances, specifically the overproduction of auxins and cytokinins.
Certain plant-associated bacteria have evolved mechanisms to manipulate host cell division to create a nutritional niche for themselves.

Step 2: Detailed Explanation:
The genus Agrobacterium (specifically Agrobacterium tumefaciens) is a soil-borne bacterium famous for causing crown gall disease in a wide range of dicotyledonous plants.
The molecular mechanism involves the transfer of a specific segment of DNA, known as the T-DNA (Transfer DNA), from the bacterium's Ti (Tumor-inducing) plasmid into the nuclear genome of the host plant cell.
Once integrated, the genes within the T-DNA are expressed by the plant's own cellular machinery.
These genes encode enzymes for the biosynthesis of phytohormones, specifically indole-3-acetic acid (auxin) and trans-zeatin (cytokinin).
The local accumulation of these hormones stimulates rapid, uncontrolled cell division (hyperplasia) and cell enlargement (hypertrophy).
This abnormal and unlimited cellular proliferation results in the characteristic crown galls.
In contrast, Rhizobium induces controlled root nodules for nitrogen fixation.
Xanthomonas species typically cause necrotic lesions, blights, or vascular wilts.
Erwinia species are well-known soft-rot pathogens that produce pectolytic enzymes that break down cell walls.
Therefore, only Agrobacterium is known for causing unlimited, tumor-like cell proliferation.

Step 3: Final Answer:
Agrobacterium is the correct answer.