Disease Resistance in Plants

Definition; Disease resistance in plants refers to their ability to prevent or minimize the establishment, growth, and spread of pathogens. Resistance can be partial or complete, depending on the genetic and physiological makeup of the plant and its interactions with the pathogen.

Host Specificity and Variability

1. Pathogen Specificity: Many pathogens infect specific host species or cultivars. For example, wheat rust pathogens typically infect wheat, while potato blight affects potatoes.
2. Variability in Resistance: Even within a single plant species, resistance levels can vary significantly among cultivars due to genetic diversity. This variability influences how effectively a pathogen can establish and cause disease.

Mechanisms of Resistance

Resistance operates through various mechanisms that hinder pathogen success:

1. Pre-Infection Barriers:
   • Physical Barriers: Cuticle, bark, or lignin deposition prevents pathogen entry.
   • Chemical Barriers: Production of antimicrobial compounds like phytoalexins.
2. Post-Infection Responses:
   • Activation of immune responses, such as hypersensitivity (localized cell death) or systemic acquired resistance.
3. Pathogen Recognition:
   • Resistance genes in plants detect specific molecules (effectors) produced by pathogens, triggering defense mechanisms.

Key Terms in Resistance

1. Disease Escape:
   • A situation where plants, though genetically susceptible, avoid disease due to external factors:
     • Unfavorable environmental conditions for the pathogen.
     • Non-susceptible growth stages of the plant.
2. Disease Endurance:
   • Plants sustain growth and yield despite being infected.
   • For example, infected crops like wheat might still produce acceptable yields under moderate disease pressure.
3. Hypersensitivity:
   • A localized defense response in which cells surrounding the infection site die rapidly, limiting pathogen spread.
   • Often seen as small, dark specks on leaves or stems.

Types of Resistance

1. Monogenic Resistance:
   • Controlled by a single gene, often providing high but specific resistance to certain pathogen races.
   • Example: Resistance to a specific rust race in wheat.
2. Polygenic Resistance:
   • Controlled by multiple genes, providing broad-spectrum but moderate resistance.
   • More durable against a range of pathogen races.
   • Example: Resistance to multiple fungal diseases in rice.

Vertical and Horizontal Resistance

• Concept Origin: Proposed by Vander Plank (1968).
• Vertical Resistance:
  • Effective against specific pathogen races (race-specific resistance).
  • Provides complete resistance but is often short-lived as pathogens evolve.
  • Example: Resistance to specific rust races in coffee.
• Horizontal Resistance:
  • Effective against all pathogen races (race-nonspecific resistance).
  • Partial but durable, as it does not easily break down.
  • Example: Broad resistance to multiple fungal pathogens in barley.

Gene-for-Gene Hypothesis

• Proposed by H. H. Flor in the flax-rust pathosystem.
• States that for every resistance gene (R) in the host, there is a corresponding avirulence gene (A) in the pathogen.
• Resistance occurs when the plant’s resistance gene recognizes the pathogen’s avirulence gene product, triggering a defense response.

Genetic Interactions:

• Four possible gene combinations between host (R/r) and pathogen (A/a):
  1. AR: Resistant reaction (incompatible).
  2. Ar: Susceptible reaction (compatible).
  3. aR: Susceptible reaction (pathogen lacks avirulence gene A).
  4. ar: Susceptible reaction (host lacks resistance gene R).

Mechanisms Behind Resistance

1. Host Genes:
   • Resistance genes in plants produce proteins (receptors) that detect pathogen effectors (avirulence gene products).
2. Pathogen Genes:
   • Pathogens produce effectors that manipulate host cells for infection.
   • Avirulence genes in pathogens encode effectors recognized by host resistance genes.
3. Hypersensitive Response (HR):
   • Upon recognizing the pathogen, plants trigger HR, leading to localized cell death and restricting pathogen spread.

Breakdown of Resistance

1. Pathogen Evolution:
   • New virulence genes may arise in the pathogen population via mutations, allowing them to bypass host resistance.
   • Example: Evolution of virulent races of Puccinia (wheat rust).
2. Plant Breeding Response:
   • Breeders introduce new resistance genes into crop varieties to counter evolving pathogens.
   • Example: Introducing new R genes in rice for resistance to bacterial blight.

Types of Resistance Genetics

1. Qualitative Resistance:
   • Monogenic, race-specific, often complete resistance.
   • Example: R genes in tomato against Verticillium wilt.
2. Quantitative Resistance:
   • Polygenic, race-nonspecific, partial but durable resistance.
   • Example: Resistance to downy mildew in pearl millet.

Horizontal vs. Vertical Resistance: