Plant Hormones and Their Function(s)

This article is meant to serve as a quick reference for diagnosing or treating possible ailments or deficiencies in plants and trees regarding hormonal inconsistencies. By having a better understanding of what traits the common Phytohormones exhibit or suppress, we can better understand what our plants need and how to aid them.

1. Auxins

• Primary Role: Promote cell elongation and growth.

• Key Functions: Stimulate elongation of cells in the stem and roots. Regulate phototropism (growth towards light) and gravitropism (growth in response to gravity). Control apical dominance (suppression of lateral buds by the main shoot). Promote root formation and adventitious rooting. Involved in fruit development and vascular differentiation.

• Production: Synthesized in the shoot apical meristem, young leaves, and developing seeds. Produced from tryptophan or via a tryptophan-independent pathway.

• Stimuli for Use/Release: Light: Phototropism triggers differential auxin distribution. Gravity: Gravitropism stimulates auxin redistribution. Wounding: Increases auxin production for regeneration. Developmental Signals: Promote root and fruit development.

• Human Applications: Positive Effects: Synthetic auxins (e.g., 2,4-D, IBA, NAA) stimulate rooting, prevent premature fruit drop, and promote vegetative propagation. Detrimental Effects: Auxin transport inhibitors (e.g., TIBA) reduce growth. Overapplication causes toxicity and abnormal development.

2. Cytokinins

• Primary Role: Stimulate cell division and differentiation.

• Key Functions: Promote cell division in roots and shoots. Delay leaf senescence by maintaining chlorophyll content. Work antagonistically with auxins to regulate apical dominance. Promote nutrient mobilization and seed germination.

• Production: Synthesized in root tips and transported via the xylem. Derived from adenine.

• Stimuli for Use/Release: Nutrient Availability: High nitrogen stimulates production. Cell Division Signals: During active cell division. Stress Conditions: Delay senescence during abiotic stresses.

• Human Applications: Positive Effects: Synthetic cytokinins (e.g., kinetin, benzyladenine) promote shoot regeneration and delay senescence. Natural enhancers like coconut water are used in tissue culture. Detrimental Effects: Cytokinin inhibitors (e.g., lovastatin) promote aging and reduce cell division.

3. Gibberellins (GAs)

• Primary Role: Promote stem elongation and seed germination.

• Key Functions: Stimulate stem elongation by promoting cell division and elongation. Break seed dormancy and promote germination. Trigger bolting (rapid stem elongation) in certain plants. Regulate flowering and fruit development. Enhance enzyme production during germination.

• Production: Synthesized in young tissues of the shoot and root, developing seeds, and embryos. Derived from terpenoids via the mevalonate pathway.

• Stimuli for Use/Release: Light/Dark Cycles: Seedlings in darkness produce GAs for elongation. Seed Germination: Triggered by water imbibition and temperature changes. Developmental Triggers: Promote flowering and fruit set.

• Human Applications: Positive Effects: Gibberellic acid (GA3) is used to break dormancy, enhance growth, and improve fruit size. GA inhibitors (e.g., paclobutrazol) control excessive elongation in crops. Detrimental Effects: Overuse can lead to spindly growth and reduced yield quality.

4. Abscisic Acid (ABA)

• Primary Role: Regulate stress responses and seed dormancy.

• Key Functions: Induce stomatal closure to minimize water loss. Maintain seed dormancy by inhibiting germination. Promote leaf senescence under stress conditions. Mediate responses to cold and salinity.

• Production: Synthesized in mature leaves, roots, and seeds. Derived from carotenoids in plastids.

• Stimuli for Use/Release: Water Stress: Drought triggers ABA synthesis. Seed Dormancy: Prevents premature germination. Cold/Salinity Stress: Enhances tolerance.

• Human Applications: Positive Effects: ABA analogs improve drought tolerance by closing stomata. Detrimental Effects: Inhibitors like fluridone prevent seed dormancy but reduce stress tolerance.

5. Ethylene

• Primary Role: Regulate fruit ripening and stress responses.

• Key Functions: Accelerate fruit ripening and senescence. Promote leaf and flower abscission. Induce triple response in seedlings (shortened stem, thickened stem, horizontal growth). Play a role in stress responses like wounding and flooding.

• Production: Synthesized in almost all plant tissues, especially under stress. Derived from methionine via the Yang cycle.

• Stimuli for Use/Release: Ripening: Ethylene production increases in climacteric fruits. Stress: Wounding, flooding, or mechanical stress triggers ethylene. Senescence and Abscission: Promotes leaf/flower shedding.

• Human Applications: Positive Effects: Ethephon releases ethylene to promote ripening and flower induction. Detrimental Effects: Ethylene inhibitors (e.g., 1-MCP) delay ripening.

6. Brassinosteroids

• Primary Role: Promote growth and stress tolerance.

• Key Functions: Enhance cell elongation and division. Promote vascular differentiation. Protect against temperature extremes and salinity. Involved in pollen tube growth and seed development.

• Production: Synthesized in all growing tissues, especially in young shoots, leaves, and seeds. Derived from sterols.

• Stimuli for Use/Release: Growth Signals: Active during cell elongation. Stress: Enhance tolerance under adverse conditions.

• Human Applications: Positive Effects: Synthetic brassinosteroids (e.g., 24-epibrassinolide) improve stress tolerance. Detrimental Effects: Brassinosteroid inhibitors (e.g., brassinazole) reduce growth.

7. Jasmonates (JAs)

• Primary Role: Regulate defense responses and reproduction.

• Key Functions: Mediate plant defense against herbivores and pathogens. Induce production of secondary metabolites for defense. Regulate fruit ripening and flower development.

• Production: Synthesized in chloroplasts and peroxisomes from linolenic acid.

• Stimuli for Use/Release: Herbivore Attack: Triggers jasmonate production. Pathogen Infection: Activates defense genes.

• Human Applications: Positive Effects: Methyl jasmonate (MeJA) enhances plant defense and metabolite production. Detrimental Effects: Overapplication can stunt growth.

8. Salicylic Acid (SA)

• Primary Role: Regulate systemic acquired resistance (SAR) and stress responses.

• Key Functions: Enhance defense against pathogens. Influence flowering and stomatal behavior under stress.

• Production: Synthesized from phenylalanine or the isochorismate pathway.

• Stimuli for Use/Release: Pathogen Attack: Activates SAR. Environmental Stress: Increases under stress.

• Human Applications: Positive Effects: Salicylic acid sprays enhance immunity. Detrimental Effects: Excess application inhibits growth.

9. Strigolactones

• Primary Role: Regulate plant architecture and symbiotic interactions.

• Key Functions: Inhibit lateral shoot branching. Promote symbiosis with mycorrhizal fungi. Stimulate parasitic plant germination.

• Production: Synthesized in roots from carotenoid precursors.

• Stimuli for Use/Release: Low Nutrients: Phosphorus deficiency enhances production.

• Human Applications: Positive Effects: Synthetic strigolactones regulate shoot architecture. Detrimental Effects: Stimulates parasitic plant germination unintentionally.