Model Answer
0 min readIntroduction
The Gene-for-Gene hypothesis, proposed by Albert H. Sturtevant and E. Collin Hair in 1940, is a cornerstone concept in plant pathology, explaining the interaction between plants and their pathogens. It posits that resistance to a pathogen is governed by a dominant gene in the plant (R gene) and a corresponding dominant gene in the pathogen (Avr gene). This hypothesis initially provided a framework for understanding the genetic basis of plant disease resistance and has significantly influenced breeding strategies for disease-resistant crops. While largely superseded by more complex models, it remains a foundational concept.
Understanding the Gene-for-Gene Hypothesis
The Gene-for-Gene hypothesis proposes a direct correlation between plant and pathogen genes. It’s a simplified model explaining how a plant’s resistance to a specific pathogen is genetically controlled.
Key Components
- R gene (Resistance Gene): A dominant gene present in the plant. When this gene is present, it confers resistance to a specific pathogen.
- Avr gene (Avirulence Gene): A dominant gene present in the pathogen. This gene encodes for a protein that allows the pathogen to infect a plant *only* when the plant lacks the corresponding R gene.
The Interaction
The interaction can be summarized as follows:
- R gene present, Avr gene present: No infection – the plant recognizes the pathogen and activates defense mechanisms.
- R gene absent, Avr gene present: Infection occurs – the pathogen can successfully infect the plant.
- R gene present, Avr gene absent: No infection – the plant is resistant, but the pathogen cannot infect because it lacks the corresponding Avr gene.
- R gene absent, Avr gene absent: Results are variable and unpredictable; may or may not lead to infection.
Significance in Plant Breeding
The Gene-for-Gene hypothesis has been instrumental in developing disease-resistant crop varieties:
- Pyramiding Resistance Genes: Breeders can identify and combine multiple R genes into a single plant, providing broad-spectrum resistance.
- Marker-Assisted Selection (MAS): Molecular markers linked to R genes can be used to select resistant plants during breeding programs, accelerating the process.
- Understanding Pathogen Evolution: The hypothesis highlights the pathogen's ability to evolve Avr genes to overcome resistance, prompting breeders to continuously introduce new R genes.
Limitations and Refinements
While a valuable starting point, the Gene-for-Gene hypothesis has limitations:
- Oversimplification: It doesn't account for the complexity of plant-pathogen interactions, which often involve multiple genes and signaling pathways.
- Race-Specific Resistance: Resistance conferred by R genes is often race-specific, meaning that a pathogen can evolve to overcome it.
- Recognition of pathogen effectors: Recent research shows that plants recognize pathogen effectors through complex signaling cascades, not just a simple gene-for-gene interaction.
Modern concepts, such as the “Effector-Triggered Immunity” model, provide a more nuanced understanding of plant-pathogen interactions, acknowledging the roles of pathogen effectors and plant immune receptors.
| Scenario | R gene | Avr gene | Outcome |
|---|---|---|---|
| 1 | Present | Present | Resistance |
| 2 | Absent | Present | Susceptibility |
| 3 | Present | Absent | Resistance |
| 4 | Absent | Absent | Variable |
Conclusion
The Gene-for-Gene hypothesis, despite its limitations, remains a pivotal concept in understanding the genetic basis of plant disease resistance. It provided a foundational understanding for breeding disease-resistant varieties and spurred significant advancements in plant pathology. While newer models have emerged, incorporating more complex signaling pathways and effector recognition, the hypothesis's core principles continue to inform breeding strategies and our understanding of the ongoing evolutionary arms race between plants and pathogens. Its legacy lies in establishing a framework for studying plant-pathogen interactions at a genetic level.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.