Which genes control resistance to specific pathogens?

Specific resistance in plants is governed by two interacting sets of genes—one in the host plant and one in the pathogen. This interaction is explained by the gene-for-gene hypothesis, proposed by H. H. Flor. It states that for each resistance (R) gene in the plant, there is a corresponding avirulence (Avr) gene in the pathogen.

Plant Resistance (R) Genes

The plant’s R genes produce proteins that help recognize the invading pathogen and trigger defense mechanisms. The most common type of R gene codes for NBS-LRR proteins (Nucleotide-Binding Site Leucine-Rich Repeat proteins).

• LRR Domain: Acts as the “detector” region that identifies specific pathogen effectors. It is highly variable and determines which pathogen the plant can recognize.
• NBS Domain: Functions as a molecular switch that activates plant defense once the effector is detected.
• How It Works: If a plant carries a specific R gene (for example, R₁) that matches the pathogen’s Avr₁ gene, the plant will resist infection. If this R₁ gene is absent, the plant becomes susceptible.

Pathogen Avirulence (Avr) Genes

In the pathogen, Avr genes produce effector proteins that help in infection by suppressing plant immunity. However, when the plant possesses the matching R gene, these effectors act as “avirulence” factors, as they expose the pathogen to the plant’s immune system. This recognition triggers an Effector-Triggered Immunity (ETI) response, preventing disease development.

In summary: A plant shows resistance when its R gene recognizes a corresponding Avr gene in the pathogen. It becomes susceptible if either the plant lacks the matching R gene or the pathogen’s Avr gene is missing or mutated.