Model Answer
0 min readIntroduction
Crop diseases pose a significant threat to global food security, necessitating the development of disease-resistant varieties. Plant breeders employ various strategies to achieve this, broadly categorized under vertical and horizontal resistance. The concept of disease resistance is crucial, especially given the increased focus on sustainable agriculture and reducing reliance on chemical pesticides. Understanding these different resistance mechanisms is vital for effective crop improvement programs and mitigating yield losses due to diseases. The recent outbreak of Wheat Blast in several parts of India underscores the importance of robust disease resistance in crops.
Understanding Disease Resistance in Crops
Disease resistance in plants refers to the ability of a plant to withstand or prevent infection by a pathogen. This resistance can be conferred through various genetic mechanisms, broadly classified as vertical and horizontal resistance.
Vertical (or Genetic) Resistance
Vertical resistance, also known as genetic or major gene resistance, is characterized by a gene-for-gene interaction between the plant and the pathogen. A single, major resistance gene in the plant provides immunity or a high level of resistance to a specific, compatible race or biotype of the pathogen. This type of resistance is often durable initially but is prone to breakdown as the pathogen evolves to overcome the resistance gene.
- Characteristics: High level of resistance to specific pathogen races.
- Mechanism: Single gene control, often involving a hypersensitive response (HR).
- Advantages: Relatively simple to incorporate through conventional breeding.
- Disadvantages: Susceptibility to pathogen races that overcome the resistance gene; prone to breakdown with pathogen evolution.
- Example: The introduction of the Yr1 gene for resistance to wheat stem rust in wheat varieties. Initially highly effective, this resistance has been overcome by evolving rust races.
Horizontal (or Polygenic) Resistance
Horizontal resistance, also known as polygenic or durable resistance, is conferred by multiple genes, each contributing a small degree of resistance. It doesn't provide complete immunity but reduces disease severity and slows down pathogen spread. This type of resistance is generally more durable as it requires multiple mutations in the pathogen to overcome it.
- Characteristics: Reduced disease severity rather than complete immunity; durable resistance.
- Mechanism: Multiple gene control, affecting various plant processes (e.g., growth rate, leaf morphology).
- Advantages: More durable resistance, less prone to breakdown.
- Disadvantages: Requires more complex breeding strategies; difficult to quantify resistance.
- Example: Resistance to late blight in potatoes, which is controlled by numerous genes.
Comparison: Vertical vs. Horizontal Resistance
| Feature | Vertical Resistance | Horizontal Resistance |
|---|---|---|
| Gene Action | Single gene | Multiple genes |
| Resistance Level | High (immunity) | Low (reduced severity) |
| Durability | Low | High |
| Pathogen Interaction | Gene-for-gene | Complex, non-specific |
Implications for Crop Breeding
Modern breeding strategies often combine both vertical and horizontal resistance genes to achieve durable and broad-spectrum resistance. Pyramiding vertical resistance genes (stacking multiple major genes) can delay the breakdown of resistance, while incorporating horizontal resistance contributes to long-term durability. Genome editing techniques offer new avenues for precisely introducing or modifying resistance genes.
Case Study: Rice Blast Resistance
Rice blast, caused by the fungus Magnaporthe oryzae, is a devastating disease. Initially, rice varieties were bred with vertical resistance genes like Pi-b. However, the pathogen quickly evolved to overcome these genes. Modern breeding programs now focus on incorporating multiple Pi genes (pyramiding) along with horizontal resistance factors to enhance durability. The International Rice Research Institute (IRRI) has played a key role in developing blast-resistant rice varieties.
Conclusion
In conclusion, vertical and horizontal resistance represent distinct approaches to combating crop diseases. While vertical resistance offers initial, high-level protection, its susceptibility to pathogen evolution necessitates a shift towards durable horizontal resistance. Integrating both types of resistance, alongside modern breeding techniques, is crucial for ensuring sustainable food production and minimizing crop losses due to diseases. Continued research and innovation are vital to stay ahead of evolving pathogens and safeguard global food security.
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.