In plant pathology, which of the following bacteria is known to cause crown gall disease, a tumour like growth at the crown region of many dicot plants?

Introduction / Context:
Crown gall disease is a well known plant disease that affects many woody and herbaceous dicot plants, especially at the crown region where stem and root meet. The disease is characterized by tumour like galls. It is caused by a specific soil bacterium that has become an important tool in plant genetic engineering because of its ability to transfer DNA into plant cells. Many botany and biotechnology questions ask you to identify this bacterium by name. This question focuses on that causative agent.

Given Data / Assumptions:
• The disease mentioned is crown gall, a tumour like disease of plants.• The options list Bacillus thuringiensis, Agrobacterium tumefaciens, Pseudomonas fluorescens, none of these, and Rhizobium leguminosarum.• We assume standard plant pathology and microbiology definitions.• We focus on the specific bacterium historically associated with crown gall disease.

Concept / Approach:
Agrobacterium tumefaciens is a Gram negative soil bacterium that causes crown gall disease in many dicot plants by transferring a segment of its plasmid DNA, called T DNA, into the host plant genome. This T DNA carries genes that stimulate cell division and produce unusual metabolites, leading to tumour formation. Bacillus thuringiensis is known for its insecticidal protein and is used as a biopesticide, not for causing crown gall. Pseudomonas fluorescens is typically beneficial to plants. Rhizobium species fix nitrogen in legume root nodules. Therefore, the correct answer is Agrobacterium tumefaciens.

Step-by-Step Solution:
Step 1: Identify that crown gall disease involves tumour like swellings, especially near the soil line of susceptible plants.Step 2: Recall that this disease is classically associated with Agrobacterium tumefaciens.Step 3: Remember that A. tumefaciens carries a Ti plasmid, which can transfer T DNA into plant cells.Step 4: Understand that expression of T DNA genes leads to uncontrolled cell division and gall formation.Step 5: Check the options and select Agrobacterium tumefaciens as the bacterium causing crown gall.

Verification / Alternative check:
This association is reinforced by the widespread use of Agrobacterium tumefaciens in plant genetic engineering. Researchers exploit its natural DNA transfer mechanism to introduce desirable genes into crop plants. Protocols for creating transgenic plants often begin with infection by engineered Agrobacterium. The same mechanism underlies the natural disease process of crown gall formation. None of the other listed bacteria have this characteristic gene transfer function or cause the typical crown gall symptoms, which confirms the identity of A. tumefaciens as the causative agent.

Why Other Options Are Wrong:
Bacillus thuringiensis produces Bt toxin that is lethal to certain insect larvae and is used as a biological insecticide; it does not cause crown gall tumours. Pseudomonas fluorescens is often used as a plant growth promoting rhizobacterium and in biocontrol of some plant diseases but is not known for causing crown gall. None of these is incorrect because there is indeed a well established bacterium associated with this disease. Rhizobium leguminosarum forms symbiotic nitrogen fixing nodules on legume roots and is considered beneficial rather than pathogenic in this context. Therefore these alternatives do not answer the question correctly.

Common Pitfalls:
Students sometimes confuse various plant associated bacteria and may mix up Agrobacterium with Rhizobium, since both inhabit the rhizosphere and can form associations with dicot roots. Another confusion is between beneficial biocontrol agents such as Bacillus thuringiensis or Pseudomonas fluorescens and pathogenic species. To avoid these mistakes, fix key associations in memory: Agrobacterium tumefaciens for crown gall and plant genetic engineering, Rhizobium for nitrogen fixation, and Bacillus thuringiensis for insect pest control.

Final Answer:
The bacterium that causes crown gall disease in plants is Agrobacterium tumefaciens, a soil bacterium that induces crown gall tumours.