In leguminous plants, both the rhizobium genes and the plant genes influence nodulation and
nitrogen fixation. Which one of the following functions is NOT encoded by the host plant
genes?
Options:
(A) Production of inducers that modify rhizobial cell wall
(B) Production of flavonoid inducers
(C) Establishment of contact between bacteria and legume
(D) Root hair curling
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.36)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Plant & Microbial Biotechnology (Rhizobium-Legume Symbiosis & Nodulation)
**2. Detailed Step-by-Step Analysis**:
- The symbiotic relationship between rhizobia (e.g., *Rhizobium leguminosarum*) and leguminous host plants requires reciprocal molecular signaling:
1. **Plant Host Role**: The legume root secretes chemical inducers—predominantly **flavonoids** (e.g., luteolin)—into the rhizosphere. The plant also produces lectins that mediate the initial contact and attachment of rhizobia to legume root hairs.
2. **Bacterial Role**: Flavonoids bind to the bacterial regulatory protein **NodD**, activating transcription of the bacterial *nodABC* operon. The enzymatic machinery (*nodA*, *nodB*, *nodC*) synthesizes and secretes lipochitooligosaccharide signaling molecules called **Nod factors**.
3. **Root Hair Curling**: Secreted bacterial Nod factors induce morphological remodeling of the host legume root hair, specifically triggering **root hair curling** (the "shepherd's crook" structure) and infection thread formation.
- Thus, root hair curling is triggered and driven by **bacterial Nod factors** (encoded by bacterial *nod* genes), not encoded by host plant genes.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Plant exudates induce modifications in rhizobia.
- **Option (B) [INCORRECT]**: Flavonoid inducers are synthesized by host plant genes via the phenylpropanoid pathway.
- **Option (C) [INCORRECT]**: Host lectins and attachment proteins establish contact between bacteria and legumes.
- **Option (D) [CORRECT]**: Root hair curling is directed by bacterial Nod factor signaling; the morphogenic stimulus is encoded by rhizobial genes.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **D**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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