Question

Arrange the following steps of root nodule formation in correct order :
A. Formation of bacteroids and differentiation into nitrogen fixing symbiosomes
B. Attachment of bacterial cells to root hairs
C. Plant root hair with attached Rhizobium undergoes curling
D. Bacterial penetration into root hair cell and cell to cell spread of rhizobia
E. Plant root releases flavonoids
Choose the correct answer from the options given below :

• A. \(C, D, E, B, A\)
• B. \(C, A, D, B, E\)
• C. \(E, B, C, D, A\)
• D. \(B, D, A, C, E\)

Hint:

Remember the sequence: \[ \text{Flavonoids} \rightarrow \text{Attachment} \rightarrow \text{Root hair curling} \rightarrow \text{Infection thread} \rightarrow \text{Bacteroids} \]
• Rhizobium fixes atmospheric nitrogen in legume root nodules.
• Nitrogenase enzyme carries out nitrogen fixation.

Answer 1

The Correct Option is C

Concept: Root nodule formation in legumes occurs due to symbiosis between: \[ \textit{Rhizobium} \text{ bacteria and leguminous plants} \] This interaction leads to biological nitrogen fixation. The process involves:
• Chemical signaling
• Bacterial attachment
• Infection thread formation
• Nodule development
• Differentiation into nitrogen-fixing bacteroids

Step 1: Release of flavonoids by plant roots. The process begins when plant roots release: \[ \text{Flavonoids} \] These compounds attract Rhizobium and induce nod gene expression. Thus, the first step is: \[ \boxed{E} \]

Step 2: Attachment of bacterial cells to root hairs. Rhizobial cells attach to root hairs: \[ \boxed{B} \] through specific molecular interactions.

Step 3: Root hair curling. After attachment: \[ \text{Root hairs curl around the bacteria} \] This traps the bacteria and initiates infection thread formation. Thus: \[ \boxed{C} \]

Step 4: Penetration and spread of rhizobia. Rhizobia penetrate the root hair through an: \[ \text{Infection thread} \] and spread from cell to cell. Thus: \[ \boxed{D} \]

Step 5: Formation of bacteroids and symbiosomes. Inside nodule cells:
• Rhizobia differentiate into bacteroids
• Nitrogen-fixing symbiosomes are formed Thus: \[ \boxed{A} \] Therefore, the correct sequence is: \[ \boxed{E \rightarrow B \rightarrow C \rightarrow D \rightarrow A} \] Hence, the correct answer is: \[ \boxed{(C)\ E, B, C, D, A} \]