Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
Which one of the following organisms is responsible for crown gall disease in plants?
Options:
(A) Xanthomonas campestris
(B) Rhizobium etli
(C) Agrobacterium tumefaciens
(D) Erwinia stewartii
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.36)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Plant Biotechnology / Microbial Pathogenesis (Crown Gall Disease & *Agrobacterium*)
**2. Detailed Step-by-Step Analysis**:
- **Crown gall disease** is a neoplastic plant disease characterized by disorganized tumor-like swellings (galls) typically at the junction between the root and stem (the crown) of dicotyledonous plants.
- It is caused by the soil bacterium **Agrobacterium tumefaciens** (now reclassified as *Rhizobium radiobacter*).
- *A. tumefaciens* carries a large tumor-inducing plasmid (**Ti plasmid**). Upon sensing plant wound signals (phenolics such as acetosyringone), *vir* genes are induced, excised T-DNA is transferred through a Type IV secretion system into the host plant cell nucleus, and integrated stably into the plant nuclear genome.
- The T-DNA genes direct the host cell to overproduce auxins (*iaaM*, *iaaH*) and cytokinins (*ipt*), triggering uncontrolled cell proliferation (gall formation), while also synthesizing opines (such as octopine or nopaline) as bacterial nutrient sources.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: *Pseudomonas syringae* causes bacterial leaf spots, cankers, and halo blight, not crown gall.
- **Option (B) [INCORRECT]**: *Erwinia carotovora* (now *Pectobacterium*) causes soft rot in crops by producing pectolytic enzymes.
- **Option (C) [CORRECT]**: *Agrobacterium tumefaciens* is the causative agent of crown gall disease via T-DNA horizontal transfer.
- **Option (D) [INCORRECT]**: *Xanthomonas campestris* causes black rot and produces the exopolysaccharide xanthan gum.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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