Brassinosteroids (BRs), recognized by the Food and Agriculture Organization (FAO) and the International Society of Plant Physiology as the sixth major class of plant hormones, play a pivotal role in regulating plant growth, development, and stress responses. Unlike traditional plant growth regulators (PGRs) such as auxins, gibberellins, cytokinins, ethylene, and abscisic acid, BRs exhibit a unique ability to integrate multiple hormonal pathways, enhancing crop productivity, stress tolerance, and fruit quality. This paper explores the physiological functions, advantages, and practical applications of brassinosteroids in agriculture, with a focus on 28-homobrassinolide (28-HBL), the most bioactive BR variant.
1. Introduction
Plants rely on phytohormones—small organic molecules that regulate growth, development, and environmental responses. Five classical hormones (auxins, gibberellins, cytokinins, ethylene, and abscisic acid) have long been studied, but brassinosteroids (BRs) have emerged as critical modulators of:
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Cell division & elongation
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Photosynthesis efficiency
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Stress resistance (drought, salinity, cold, pests)
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Fruit setting & yield enhancement
Discovered in the 1970s, BRs are now essential in precision farming, particularly 28-homobrassinolide (28-HBL), which outperforms other analogs (e.g., 24-epibrassinolide) in bioactivity.
2. Comparative Analysis of Plant Hormones
| Hormone Class | Key Functions | Agricultural Uses | Limitations |
|---|---|---|---|
| Auxins (e.g., IAA, NAA, 2,4-D) | Promote root initiation, prevent fruit drop | Rooting agents, fruit thinning | High concentrations inhibit growth |
| Gibberellins (GA₃, GA₄+₇) | Stimulate stem elongation, break dormancy | Bolting control, fruit enlargement | Overuse thins fruit skin, dilutes sugars |
| Cytokinins (e.g., 6-BA, CPPU) | Enhance cell division, delay senescence | Tissue culture, fruit setting | May cause malformed fruits |
| Ethylene (e.g., ethephon) | Ripening, abscission | Fruit maturation, flower induction | Shortens shelf life; phytotoxic |
| Abscisic Acid (ABA) | Stress response, dormancy | Drought resistance, seed priming | Growth suppression at high doses |
| Brassinosteroids (28-HBL) | Balances all above hormones | Growth promotion + stress mitigation | None at recommended doses |
3. Key Functions of Brassinosteroids
3.1 Growth Promotion & Yield Enhancement
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Cell expansion: Upregulates expansin genes, increasing fruit size (e.g., grapes, tomatoes).
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Photosynthesis boost: Elevates chlorophyll content by 15–30%, enhancing sugar accumulation.
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Flower/fruit retention: Reduces abscission by balancing auxin-ethylene ratios.
3.2 Stress Resistance
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Drought/salinity: Activates ROS-scavenging enzymes (SOD, CAT).
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Cold/frost: Increases proline and soluble sugars for cryoprotection.
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Pesticide detox: Accelerates recovery from herbicide injury (e.g., glyphosate).
3.3 Quality Improvement
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Fruit coloration: Stimulates anthocyanin synthesis in apples/berries.
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Shelf-life extension: Lowers ethylene sensitivity in postharvest crops.
4. Practical Applications
4.1 Optimal Use Guidelines
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Dosage: 5–10 g/ha (1500× dilution; 15 kg water).
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Timing: 2–3 sprays/season (e.g., pre-flowering, fruit set, ripening).
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Compatibility: Mixes with non-alkaline agrochemicals (synergistic with chlorpyrifos, fungicides).
4.2 Superiority of 28-Homobrassinolide (28-HBL)
| Parameter | 28-HBL | 24-Epibrassinolide |
|---|---|---|
| Bioactivity | ⭐⭐⭐⭐⭐ | ⭐⭐⭐ |
| Stress resistance | 30% higher | Moderate |
| Cost efficiency | 1 g = 1 ha | 2 g = 1 ha |
4.3 Proven Crop-Specific Benefits
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Cereals (Rice/Wheat): 20% higher tillering; reduced lodging.
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Fruits (Citrus/Apple): 15–25% yield increase; Brix +2°.
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Vegetables (Tomato/Cucumber): 30% less blossom-end rot.
5. Innovative Formulations
5.1 BRs + Cytokinins (e.g., CPPU)
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Synergy: CPPU enlarges cells; BRs enhance nutrient partitioning → larger, sweeter fruits.
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Use: Watermelon, kiwifruit (spray at 5–10 days post-anthesis).
5.2 BRs + Gibberellins (GA₃)
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Application: Mango/lychee flowering – GA₃ induces blooms; BRs reduce drop.
5.3 BRs + Ethephon (Ethylene Generator)
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Solution for Corn: Ethephon shortens stalks; BRs strengthens stems → lodging resistance.
6. Conclusion
Brassinosteroids represent a paradigm shift in sustainable agriculture by:
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Replacing multiple PGRs with a single, low-dose input.
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Mitigating climate stresses (heat, drought, frost).
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Residue-free compliance with EU/China’s Green Development Policy.
Future directions: Nano-encapsulated BRs for slow release; CRISPR-edited crops with enhanced BR biosynthesis.
