Question

Gene for Gene Interaction is best observed in __________.

• A. Biotrophic Pathogens-Host Pathosystem
• B. Hemibiotrophic Pathogens-Host Pathosystem
• C. Necrotrophic Pathogens-Host Pathosystem
• D. Saprophytes

Hint:

H. H. Flor established the gene-for-gene hypothesis while working with flax rust (Melampsora lini), which is an obligate biotrophic fungus.
In this system, a single dominant $R$ gene in the host and a complementary dominant $Avr$ gene in the pathogen are required to trigger an incompatible (resistant) reaction.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The gene-for-gene hypothesis, formulated by Harold Henry Flor in 1942, describes the genetic interaction between plants and pathogens.
It states that for each gene conditioning resistance in the host, there is a complementary gene conditioning avirulence in the pathogen.

Step 2: Detailed Explanation:
Let us analyze the relationship between pathogen lifestyles and genetic interactions:
Biotrophic pathogens are obligate parasites that must keep their host cells alive to obtain nutrients, developing specialized feeding structures like haustoria within living host tissues.
Because of this intimate, long-term cellular relationship, biotrophs rely on maintaining a subtle molecular balance with the host, avoiding detection by the host immune system.
The host's immune system detects specific pathogen effector proteins (encoded by avirulence, $Avr$ genes) using corresponding host receptor proteins (encoded by resistance, $R$ genes).
This specific, highly complementary recognition is the classic foundation of the gene-for-gene model.
Hemibiotrophic pathogens start as biotrophs but eventually kill the host cells to complete their life cycle as necrotrophs, making their genetic interactions more complex and less strictly reliant on the simple $R-Avr$ gene-for-gene matching throughout their life cycle, which excludes Option (B).
Necrotrophic pathogens kill host cells using toxins and lytic enzymes to feed on dead tissue, often exploiting host sensitivity genes rather than traditional resistance-receptor genes, which excludes Option (C).
Saprophytes live on dead organic matter and do not interact with living host immune systems, which excludes Option (D).

Step 3: Final Answer:
The gene-for-gene interaction is best observed in biotrophic pathogens-host pathosystems.
This corresponds to Option (A).