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Question #46602
The Question
During the establishment of the Rhizobium-legume symbiosis, successful nodulation requires precise molecular cross-talk. A researcher inoculates a host legume with a mutant strain of Rhizobium that produces structurally altered Nod factors. The plant initiates root hair curling and infection thread formation, but the developing nodule rapidly undergoes localized tissue necrosis and fails to fix nitrogen. Which of the following best explains this physiological shift?
Options
- A.
The mutation prevents the host from synthesizing leghaemoglobin, exposing the nitrogenase enzyme to oxygen and triggering oxidative necrosis of the nodule.
- B.
The altered Nod factors fail to maintain the symbiotic signaling pathway, causing the host's innate immune system to perceive the bacteria as a pathogen and mount a hypersensitive defense response.
- C.
The mutant bacteria act as complete parasites by absorbing all host photoassimilates without fixing nitrogen, leading to rapid starvation and subsequent death of the nodule tissue.
- D.
The altered Nod factors directly inhibit the host's nitrate reductase enzyme, causing a toxic accumulation of nitrites that destroys the nodule cells.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Evolutionary_Exception
Tackle problems where standard rules evolve or change based on specific environmental constraints.
Concepts Behind This Question
Symbiotic Nitrogen Fixation|Evolutionary Ecology of Parasitism|Rhizobium
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often view mutualistic symbiosis as a purely cooperative process, failing to realize it is an evolutionary tightrope walk.
The misconception is treating Rhizobium as inherently 'safe' to the plant.
In reality, without the exact molecular password (Nod factors), the plant defaults to its baseline physiological response to any bacterial invasion: Pattern-Triggered Immunity and localized necrosis.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Microbes in Human Welfare - Class 12 |
| Concept | Symbiotic Nitrogen Fixation |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |