Model Answer
0 min readIntroduction
Plant diseases pose a significant threat to global food security, causing substantial yield losses and economic hardship. While chemical control remains prevalent, there's a growing emphasis on sustainable and eco-friendly approaches. One such approach is harnessing the plant's innate defense mechanisms, particularly Systemic Acquired Resistance (SAR). SAR represents a broad-spectrum, long-lasting resistance to pathogens, triggered by localized infection. Understanding SAR and incorporating disease resistance into breeding programs is crucial for developing resilient and sustainable agricultural systems.
Systemic Acquired Resistance (SAR)
SAR is a genetically programmed defense response in plants. It's characterized by enhanced resistance to a broad spectrum of pathogens following localized infection. Unlike Induced Systemic Resistance (ISR), which is primarily mediated by beneficial microbes, SAR is triggered by pathogen-associated molecular patterns (PAMPs) or effectors.
Mechanism of SAR
- Initial Infection: A localized infection triggers the production of salicylic acid (SA).
- Signal Transduction: SA acts as a signaling molecule, initiating a complex signal transduction pathway. This pathway involves the production of reactive oxygen species (ROS) and the upregulation of defense-related genes.
- Systemic Signaling: The signal travels systemically throughout the plant via the phloem, preparing distal tissues for pathogen attack.
- Defense Response: Distal tissues exhibit enhanced resistance, often involving the accumulation of pathogenesis-related (PR) proteins like chitinases and β-1,3-glucanases, which degrade fungal cell walls.
Sources of Disease Resistance in Plants
1. Genetic Resistance
This is the most common and durable form of resistance. It's inherited and relies on specific resistance (R) genes. When an R gene product recognizes a corresponding avirulence (Avr) gene product from the pathogen, a hypersensitive response (HR) is triggered, leading to localized cell death and preventing pathogen spread.
Example: The wheat variety 'Norin 61' exhibits resistance to stem rust caused by Puccinia graminis due to the Lr13 gene. This gene recognizes a specific Avr gene in the rust pathogen.
2. Induced Resistance (Including SAR)
This involves activating the plant's own defense mechanisms in response to a stimulus. SAR is a key component of induced resistance. Other forms include ISR, triggered by beneficial microbes.
Advantages of Breeding for Disease Resistance in Plants
Breeding for disease resistance offers numerous advantages over relying solely on chemical control.
- Reduced Pesticide Use: Disease-resistant varieties require fewer pesticide applications, minimizing environmental impact and reducing production costs.
- Increased Yields: Reduced disease incidence translates directly to higher yields and improved food security.
- Improved Crop Quality: Disease can negatively impact crop quality (e.g., reduced nutritional value, altered appearance). Resistant varieties maintain quality.
- Sustainability: Genetic resistance is a sustainable solution, reducing reliance on external inputs.
- Reduced Post-Harvest Losses: Disease can cause significant losses during storage. Resistant varieties reduce these losses.
- Economic Benefits: Farmers benefit from lower input costs and higher yields.
Example: The development and widespread adoption of Bt cotton in India, while primarily insect-resistant, also demonstrated the potential for reduced pesticide use and increased yields through genetic modification. While Bt cotton focuses on insect resistance, the principle of incorporating resistance genes is applicable to disease resistance as well.
Challenges in Breeding for Disease Resistance
- Pathogen Evolution: Pathogens can evolve to overcome resistance genes (virulence). This necessitates continuous breeding efforts and the incorporation of multiple resistance genes (gene pyramiding).
- Linkage Drag: Resistance genes are often linked to undesirable traits. Breeders must carefully select for resistance while minimizing linkage drag.
- Complexity of Resistance: Many diseases are complex and involve multiple pathogen strains. Breeding for resistance to such diseases can be challenging.
Conclusion
In conclusion, Systemic Acquired Resistance (SAR) and breeding for disease resistance represent crucial strategies for ensuring sustainable and resilient agricultural systems. While genetic resistance offers long-term protection, continuous breeding efforts are necessary to stay ahead of evolving pathogens. Integrating induced resistance mechanisms like SAR alongside conventional breeding practices holds immense potential for developing robust and environmentally friendly crop varieties, thereby contributing to global food security and sustainable agriculture.
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.