Model Answer
0 min readIntroduction
Crop diseases pose a significant threat to global food security, demanding continuous efforts in developing disease-resistant varieties. Plant breeders employ various strategies to achieve this, broadly categorized as vertical and horizontal resistance. Vertical resistance, also known as race-specific resistance, was the initial focus of breeding programs. However, the rapid evolution of pathogens often overcomes this, necessitating the development of horizontal resistance, offering broader, albeit less durable, protection. Understanding these distinctions is crucial for effective disease management in agriculture.
Understanding Disease Resistance in Crop Plants
Disease resistance in plants is the ability of a plant to prevent or reduce the severity of a disease caused by a pathogen. This resistance can be categorized into two main types: vertical and horizontal.
Vertical (Race-Specific) Resistance
Vertical resistance is conferred by a few major resistance (R) genes, each conferring resistance to a specific strain (race) of a pathogen. This is a gene-for-gene interaction where a specific R gene in the plant interacts with a corresponding avirulence (Avr) gene in the pathogen. If the interaction is successful, the plant activates defense mechanisms.
- Genetic Basis: Controlled by a small number of dominant genes (R genes).
- Specificity: Provides strong resistance to a narrow range of pathogen races.
- Durability: Susceptible to breakdown due to pathogen adaptation through mutations in Avr genes.
- Breeding: Relatively easy to incorporate into new varieties through simple dominant gene transfer.
- Example: The introduction of the ‘McIntosh’ apple variety, which had resistance to apple scab due to a specific R gene, initially provided excellent protection. However, the pathogen quickly evolved to overcome this resistance.
Horizontal (Broad-Spectrum) Resistance
Horizontal resistance, also called field resistance or general resistance, is a more durable form of resistance. It’s often polygenic, meaning it's controlled by many genes, each contributing a small amount to overall resistance. This resistance is less specific and provides protection against a wider range of pathogen strains.
- Genetic Basis: Controlled by multiple genes with small additive effects.
- Specificity: Provides resistance to a wider range of pathogen strains.
- Durability: More durable than vertical resistance because the pathogen needs to overcome multiple genetic barriers simultaneously.
- Breeding: More challenging to breed for as it involves complex quantitative traits.
- Example: Many traditional crop varieties exhibit horizontal resistance to diseases, accumulated over generations through natural selection and farmer practices. The resistance to rust in wheat varieties developed in India is an example of field resistance.
| Feature | Vertical Resistance | Horizontal Resistance |
|---|---|---|
| Genetic Control | Few major genes (R genes) | Many genes (polygenic) |
| Specificity | Race-specific | Broad-spectrum |
| Durability | Low | High |
| Breeding Complexity | Simple | Complex |
The Role of Pyramiding and Gene Editing
Modern breeding strategies often combine both vertical and horizontal resistance. Pyramiding involves stacking multiple R genes to delay the breakdown of vertical resistance. Furthermore, techniques like gene editing (e.g., CRISPR-Cas9) are being explored to enhance resistance genes and create more durable varieties. The Indian government’s efforts to promote biofortified crops often incorporate disease resistance genes.
Case Study: Rice Blast Resistance
Case Study: Rice Blast Resistance in India: Rice blast, caused by Magnaporthe oryzae, is a devastating disease in India. Initially, rice varieties with vertical resistance (based on Pi genes) were deployed. However, the pathogen quickly evolved to overcome these. Current breeding programs focus on pyramiding multiple Pi genes and incorporating horizontal resistance components, alongside improved agronomic practices, to manage the disease effectively. The All India Coordinated Rice Improvement Programme (AICRIP) plays a crucial role in this.
Conclusion
In conclusion, vertical and horizontal disease resistance represent distinct strategies for protecting crops from pathogens. While vertical resistance offers initial, strong protection, its susceptibility to breakdown necessitates the adoption of horizontal resistance for long-term durability. Integrating both approaches, alongside modern breeding technologies, is essential for ensuring sustainable and resilient agricultural systems. Continuous monitoring of pathogen evolution and adaptive breeding strategies are vital for safeguarding food security.
Answer Length
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