📈 Host Plant Resistance — Concepts and Types
Host Plant Resistance — Concepts and Types.
Host plant resistance (HPR) is the ability of a plant to reduce the effect of a pathogen through its genetic makeup. Deploying resistant varieties is the most economical and environmentally friendly component of IPDM, requiring no additional input costs from the farmer.
Basic Concepts
Disease Triangle Revisited
Disease occurs when a susceptible host, a virulent pathogen, and a favorable environment coincide. HPR works by modifying the host component — making the plant less suitable for pathogen colonization and reproduction.
Resistance vs. Tolerance vs. Immunity
| Term | Definition |
|---|---|
| Immunity | Complete absence of disease; the pathogen cannot infect at all |
| Resistance | Plant limits pathogen development; disease is reduced |
| Tolerance | Plant endures infection without significant yield loss |
| Susceptibility | Plant supports full pathogen development and suffers damage |
Types of Resistance
Vertical Resistance (Qualitative / Race-Specific)
- Controlled by single major genes (R-genes)
- Provides complete resistance against specific pathogen races
- Follows a gene-for-gene relationship
- Highly effective but can be overcome when new pathogen races evolve
- Example: Lr genes in wheat against specific races of leaf rust (Puccinia triticina)
Horizontal Resistance (Quantitative / Non-Race-Specific)
- Controlled by many minor genes (polygenic)
- Provides partial but broad-spectrum resistance against all races of a pathogen
- More durable because pathogens must accumulate multiple mutations to overcome it
- Harder to breed for because of polygenic inheritance
- Example: Slow-rusting resistance in wheat varieties like CIMMYT's Lr34/Yr18 complex
Comparison
| Feature | Vertical | Horizontal |
|---|---|---|
| Genetic basis | Oligogenic (1-3 genes) | Polygenic (many genes) |
| Effectiveness | Complete (hypersensitive) | Partial (reduced infection rate) |
| Race specificity | Yes | No |
| Durability | Often short-lived | Long-lasting |
| Breeding ease | Simpler (backcross) | Complex (recurrent selection) |
Mechanisms of Resistance
Pre-existing (Passive) Defenses
- Structural barriers — thick cuticle, waxy coating, cell wall lignification
- Chemical barriers — preformed antimicrobial compounds (phytoanticipins) such as saponins and phenolics
Induced (Active) Defenses
- Hypersensitive Response (HR) — rapid localized cell death at the infection site, restricting pathogen spread
- Phytoalexin production — synthesis of antimicrobial compounds (e.g., pisatin in pea, phaseollin in bean)
- Pathogenesis-Related (PR) proteins — chitinases, glucanases, and other enzymes that degrade pathogen structures
- Cell wall reinforcement — callose deposition and lignification around infection sites
HPR in IPDM
Resistant varieties form the backbone of IPDM because they provide continuous, cost-free protection. However, reliance on a single resistance gene can lead to boom-and-bust cycles. IPDM programs therefore recommend gene pyramiding, multiline varieties, and combining HPR with cultural and biological methods.
Summary Cheat Sheet
Resistance Types
| Type | Nature | Durability |
|---|---|---|
| Vertical | Race-specific, major gene | Often lower |
| Horizontal | Quantitative, polygenic | Usually higher |
Quick Recall
- Immunity means no infection; resistance means reduced disease.
- Tolerance allows yield protection despite infection.
- HPR is low-input and foundational for IPDM.
Exam Traps
- Vertical resistance can fail quickly when new races emerge.
- Tolerance is not equivalent to immunity.
- Overdependence on a single gene causes boom-and-bust cycles.
References
2 sources • [1] [2]
References
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