The most thoroughly studied known bacteria- plant interactions is ,

Options

A. plant growth stimulation by phosphate -solubilising bacteria
B. cyanobacterial symbiosis with some aquatic ferns
C. gall formation on certain angiosperms by agrobacterium
D. nodulation of sesbania stems by nitrogen fixing bacteria

Answer

### Step-by-Step Solution:

1. **Understanding Bacteria-Plant Interactions**: Bacteria can interact with plants in various ways, including symbiosis, pathogenicity, and mutualism. Some bacteria can stimulate plant growth, while others can cause diseases.

2. **Analyzing the Options**:
- **Option A**: Plant growth stimulation by Phosphate solubilizing bacteria. This interaction is beneficial but not the most extensively studied.
- **Option B**: Cyanobacterial symbiosis with aquatic ferns. This is also an important interaction but lacks the depth of study compared to others.
- **Option C**: Gall formation on certain angiosperms by Agrobacterium. This interaction involves Agrobacterium tumefaciens, which is known for causing crown gall disease in dicot plants.

3. **Focusing on Agrobacterium**: Agrobacterium tumefaciens is a well-studied pathogen that transfers a piece of its DNA (T-DNA) into the plant cells, leading to tumor formation. This interaction is significant because:
- It transforms normal plant cells into tumor cells.
- The tumors produce substances that benefit the bacteria.
- The entry point for Agrobacterium is typically through wounds in the plant.

4. **Importance in Biotechnology**: The T-DNA of Agrobacterium is utilized as a vector in plant biotechnology. This has made the study of this interaction particularly important and beneficial for agricultural practices.

5. **Conclusion**: Based on the analysis, the most thoroughly studied known bacteria-plant interaction is gall formation on certain angiosperms by Agrobacterium.

### Final Answer:
The most thoroughly studied known bacteria-plant interaction is **gall formation on certain angiosperms by Agrobacterium**.

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