Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
Assertion (A): Bacterial plant pathogens need wound or natural opening to enter the host
Reason (R): Bacterial plant diseases are mostly caused by Gram negative group
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Plant-pathogenic bacteria cannot directly penetrate host cell walls, regardless of whether they are Gram-negative or Gram-positive. They must enter via wounds or stomata.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Bacterial pathogens lack specialized structures, such as appressoria or penetration pegs, to physically pierce the intact plant cuticle or cell wall.
As a result, they rely on existing openings to gain entry into host tissues.

Step 2: Detailed Explanation:
Let us evaluate both statements:
- Assertion (A) is true: Because plant-pathogenic bacteria cannot directly penetrate intact host plant surfaces, they must enter through physical wounds (e.g., from mechanical damage, pruning, or insect feeding) or natural openings (such as stomata, lenticels, or hydathodes).
- Reason (R) is true: The vast majority of economically important plant-pathogenic bacteria are Gram-negative, belonging to genera such as Pseudomonas, Xanthomonas, Ralstonia, Pectobacterium, and Agrobacterium.
Only a few, such as Clavibacter and Streptomyces, are Gram-positive.
Although both statements are biologically correct, the fact that most plant-pathogenic bacteria are Gram-negative does not explain why they lack active penetration mechanisms and must enter through wounds or natural openings.
The requirement for natural openings or wounds is due to the lack of mechanical penetration structures, which is true for both Gram-negative and Gram-positive plant-pathogenic bacteria.
Therefore, (R) is not the correct explanation of (A).

Step 3: Final Answer:
Hence, the correct option is (B).