Q.55 In classical model to explain the plant-pathogen interaction, the host will not develop the disease
upon the pathogen attack when
(A) The resistance gene (R) is non-functional (B) The avirulence gene (Avr) is non-functional
(C) Both R and Avr are non-functional             (D) Both R and Avr are functional

Correct Answer: (D) Both R and Avr are functional

In the classical gene-for-gene model proposed by H.H. Flor, disease resistance occurs when the plant’s dominant resistance gene (R) specifically recognizes the pathogen’s corresponding avirulence gene product (Avr), triggering hypersensitive response (HR) and localized cell death to halt infection.

Option Analysis

(A) The resistance gene (R) is non-functional: Incorrect. Without functional R, the plant cannot detect Avr even if present, resulting in compatible interaction (disease develops).

(B) The avirulence gene (Avr) is non-functional: Incorrect. Non-functional Avr evades R detection, allowing pathogen colonization despite functional plant R gene (disease develops).

(C) Both R and Avr are non-functional: Incorrect. Double absence creates compatible interaction; no recognition occurs regardless of pathogen presence.

(D) Both R and Avr are functional: Correct. R-Avr gene-for-gene recognition activates defense signaling, preventing disease establishment.

In classical model to explain plant-pathogen interaction, the host will not develop disease upon pathogen attack when both resistance gene (R) and avirulence gene (Avr) are functional, triggering specific recognition and defense activation.​

Gene-for-Gene Hypothesis

H.H. Flor (1940s flax-Melampsora) established:

Molecular Mechanism

• Avr encodes effectors (e.g., Avr9 protein).

• R proteins (NBS-LRR class) detect effectors directly/indirectly.

• MAPK cascade → HR → pathogen containment.​

Applications

• Marker-assisted breeding stacks multiple R genes.

• Pathogen surveillance monitors Avr evolution.

• GMO strategies engineer broad-spectrum R loci.​