Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Gene for gene interaction can be usually observed in the biotrophic pathosystem.
Reason R: Biotrophs entirely dependent on their host plant to complete their life cycle.

In light of the above statements, choose the correct 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:

- Biotrophs = Depend on living tissue = Specific effector-receptor (gene-for-gene) interaction.
- Necrotrophs = Kill host tissue first = Produce broad-spectrum toxins/enzymes (less host-specific).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
H.H. Flor's Gene-for-Gene hypothesis states that for each gene conditioning resistance in the host, there is a complementary gene conditioning avirulence in the parasite.
This genetic relationship is characteristic of highly specialized biotrophic pathogens.
Detailed Explanation:
Let us analyze both statements:
Assertion A:
Gene-for-gene relationships are most commonly observed in biotrophic pathosystems.
Examples include rusts (Puccinia spp.), smuts, and powdery mildews.
In these systems, host resistance is absolute and highly specific, mediated by corresponding $R$ and $Avr$ genes.
Thus, Assertion A is true.
Reason R:
Biotrophs are obligate parasites that depend entirely on the host's living cells for nutrients and to complete their reproduction cycle.
Because they require living tissue, they cannot rely on broad-spectrum toxins that kill cells.
Instead, they must secrete highly specific effector proteins to suppress host immunity while maintaining host cell viability.
This survival strategy drives a continuous co-evolutionary arms race between the host's receptor genes ($R$-genes) and the pathogen's effector genes ($Avr$-genes), leading to gene-for-gene specificity.
Thus, Reason R is true and explains why these specific genetic interactions evolved in biotrophs.

Step 2: Final Answer:
Both A and R are true, and R is the correct explanation of A.
Therefore, the correct option is (A).