Phytohormones (Plant Hormones): Types, Functions, Tropism, Crosstalk & Agricultural Applications

Introduction to Phytohormones (Plant Hormones)
Types of phytohormones

Auxins
Gibberellins
Cytokinins
Ethylene
Abscisic Acid (ABA)
Brassinosteroids
Jasmonates
Salicylic Acid
Strigolactones
Peptide and Small-Molecule Hormones

Hormone Crosstalk
Agricultural Applications
Recent Advances and Future Perspectives
Conclusion
References

Introduction to Phytohormones (Plant Hormones)

Phytohormones, also known as plant hormones, are organic substances produced naturally by plants in very minute quantities.
They play a crucial role in regulating plant growth and development, as well as coordinating responses to various environmental stimuli.
Unlike animal hormones, phytohormones are not secreted by specialized endocrine glands; instead, they are synthesized in multiple plant tissues.
They act as signaling molecules within the plant, triggering and regulating important physiological processes.
These processes include cell division, cell elongation, and cell differentiation, which are essential for proper plant growth and structural development.
Phytohormones also regulate major developmental events such as flowering and fruiting.
In addition, they help plants respond and adapt to environmental stresses, ensuring survival under changing conditions.

Types of phytohormones

There are various types of phytohormones in plants, including auxins, gibberellins, cytokinins, ethylene, abscisic acid, brassinosteroids, jasmonates, salicylic acid, and strigolactones, each playing distinct roles in plant growth and development.

1. Auxins

Auxins are a group of phytohormones belonging to the indole category and naturally occur as Indole-3-acetic acid (IAA).
Auxins are primarily synthesized in the tips of roots and shoots, young leaves, and germinating seeds.
Within the plant, auxins are transported through polar transport, which helps establish concentration gradients that regulate various growth processes.
Synthetic auxins such as 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene acetic acid (NAA) are widely used in agriculture and horticulture.

Functions of auxin

Cell elongation: Auxins stimulate plasma membrane proton pumps, which activate expansin proteins that loosen the cell wall, allowing it to expand. This process is essential for stem elongation.
Apical dominance: Auxins promote apical dominance, a phenomenon in which the apical bud of the main shoot suppresses the growth of lateral (axillary) buds due to higher auxin concentration at the shoot tip.
Root initiation: Auxins induce the formation of adventitious roots and are widely used in plant cuttings. The application of synthetic auxins enhances root initiation.
Tropism: Auxins regulate tropisms, which are directional growth responses to environmental stimuli such as light (phototropism) and gravity (gravitropism).
Vascular tissue differentiation: Auxins promote the differentiation of vascular tissues, resulting in the formation of continuous vascular strands that efficiently transport water, minerals, and nutrients throughout the plant.
Parthenocarpy: Auxins stimulate the development of seedless fruits without fertilization.

2. Gibberellins

Gibberellins, also known as gibberellic acids, are a class of phytohormones belonging to the diterpenoid group.
More than 100 different gibberellins have been identified, but only a few are known to be physiologically active and perform significant functions.
They are mainly synthesized in young leaves, roots, seeds, and young fruits.
Gibberellins are transported throughout the plant via vascular bundles.

Functions of gibberellins

Stem elongation: Gibberellins are responsible for stem elongation by inducing both cell division and cell elongation in stem internodes. This effect is particularly evident in dwarf plants with genetically shorter internodes, where the exogenous application of gibberellins can restore normal plant height.
Breaking dormancy: Gibberellins play a key role in breaking seed dormancy and promoting germination. They stimulate the production of α-amylase, an enzyme that hydrolyzes starch reserves in the endosperm, thereby providing energy for the developing embryo.
Induction of flowering: Gibberellins induce flowering in certain long-day plant species. In rosette plants, they promote bolting, which is the rapid elongation of the flowering stem.
Fruit growth and parthenocarpy: Gibberellins regulate fruit growth and may also induce parthenocarpy, leading to the development of seedless fruits in some plant species.

3. Cytokinins

Cytokinins are plant phytohormones that stimulate cell division in both shoots and roots.
Zeatin is the most well-known natural cytokinin.
Synthetic cytokinins include kinetin and benzylaminopurine (BAP).
Cytokinins are primarily produced in the root tips.
They are transported to other parts of the plant through the xylem.

Roles of Cytokinins

Cell division: Cytokinins promote cell growth and cell division, especially in the shoots and roots.
Delay of senescence: Cytokinins slow down leaf senescence (aging) by enhancing nutrient mobilization and maintaining chlorophyll content.
Reversal of apical dominance: Cytokinins act antagonistically to auxins and help reverse apical dominance, thereby promoting the outgrowth of lateral buds.
Increase in photosynthesis: Cytokinins support chloroplast maturation, which enhances the rate of photosynthesis.

4. Ethylene

Ethylene is a gaseous phytohormone produced in almost all plant tissues.
Its synthesis increases particularly during senescence, fruit ripening, and various stress conditions.
Ethylene is synthesized from the amino acid methionine through the intermediate compound 1-aminocyclopropane-1-carboxylic acid (ACC).

Functions of Ethylene

Fruit ripening: Ethylene promotes fruit ripening by accelerating cell wall softening, inducing color changes, and enhancing flavor maturation.
Triggering senescence: Ethylene stimulates flower and leaf abscission as well as the process of senescence.
Stress responses: Ethylene production increases under stress conditions such as pathogen infection, flooding, and drought, where it functions as a signaling molecule to activate adaptive responses.
Triple response in etiolated seedlings: In etiolated (dark-grown) seedlings, ethylene induces the triple response, characterized by horizontal growth, reduced elongation growth, and radial swelling of the stem.

5. Abscisic Acid (ABA)

Abscisic acid (ABA) is a sesquiterpenoid phytohormone.
It is present in most plant tissues, particularly in seeds and mature leaves.
Abscisic acid plays a significant role in regulating seed dormancy and mediating stress responses.

Functions of Abscisic Acid

Increase in seed dormancy: Abscisic acid maintains seed dormancy under unfavorable environmental conditions by inhibiting germination.
Stomatal closure: Abscisic acid regulates the closure of stomata during water stress conditions, thereby reducing water loss through transpiration.
Stress tolerance: Abscisic acid accumulates during drought and salinity stress, helping to reduce plant damage and prevent plant death.
Growth inhibition: Abscisic acid generally acts as a growth inhibitor by decreasing the rate of cell elongation and cell division.

6. Brassinosteroids

Brassinosteroids are polyhydroxysteroid compounds that are structurally similar to steroid hormones found in animals.
They are widely distributed throughout the plant body and are especially abundant in pollen, seeds, and young vegetative tissues.

Functions of Brassinosteroids

Cell elongation and division: Brassinosteroids promote cell elongation and cell division, contributing to stem and root growth.
Vascular tissue differentiation: They support vascular tissue differentiation, thereby improving the transport of water and nutrients throughout the plant.
Stress resistance: Brassinosteroids enhance plant tolerance to abiotic stresses such as temperature stress and salinity stress, as well as to pathogen infection.
Regulation of flowering and seed formation: Brassinosteroids play a role in controlling flowering and seed development.

7. Jasmonates

Jasmonates are lipid-derived signaling molecules that include jasmonic acid and its various derivatives.
They are synthesized in response to the perception of abiotic and biotic stress conditions.

Functions of Jasmonates

Defense responses: Jasmonates activate defense mechanisms against pathogens and herbivores by stimulating the production of protective chemicals.
Regulation of reproductive development: Jasmonates play a role in regulating flower formation and pollen maturation.
Growth inhibition and senescence: Under stressful conditions, jasmonates inhibit root growth and promote senescence.

8. Salicylic Acid

Salicylic acid is a phenolic phytohormone involved in plant defense mechanisms.
It plays a crucial role in systemic acquired resistance (SAR), a plant-wide defense response against pathogens.

Functions of salicylic acid

Induction of defense pathways: Salicylic acid activates defense-related genes and signaling pathways, thereby enhancing resistance against pathogenic infections.
Heat production in flowers: In certain plant species, salicylic acid stimulates heat production in flowers to help attract pollinators.
Regulation of growth and development: Salicylic acid also participates in controlling various aspects of plant growth and developmental processes.

9. Strigolactones

Strigolactones are carotenoid-derived phytohormones.
They are primarily produced in plant roots.
Strigolactones are transported to other parts of the plant through the xylem vessels.

Function of Strigolactones

Inhibition of axillary bud growth: Strigolactones suppress the growth of axillary buds, thereby contributing to the maintenance of apical dominance.
Symbiotic interaction with fungi: Strigolactones stimulate hyphal branching in arbuscular mycorrhizal fungi, enhancing nutrient uptake through symbiotic association between plants and fungi.
Stimulation of parasitic seed germination: Strigolactones promote germination in the seeds of certain parasitic plants.

10. Peptide and Small-Molecule Hormones

In addition to the classical plant hormones, recent research has identified peptide hormones and small-molecule hormones that function as important signaling molecules in plant growth, development, and defense.
These peptide hormones include Systemin, Phytosulfokine (PSK), CLAVATA3 (CLV3), and Rapid Alkalinization Factors (RALFs).
Peptide hormones generally act through receptor-like kinases (RLKs) located on the plasma membrane.
They are involved in cell-to-cell communication and play critical roles in processes such as meristem maintenance, root and shoot patterning, and defense responses.
Small-molecule hormones, including strigolactones and nitric oxide, also participate in specialized signaling functions related to environmental adaptation and plant growth regulation.
These newly identified hormones increase the complexity and specificity of plant signaling networks, allowing more precise physiological responses to internal and external stimuli.

Hormone Crosstalk

Plant hormones do not function independently; rather, their actions are coordinated through a complex network of interactions known as hormone crosstalk.
Hormonal crosstalk enables plants to achieve coordinated regulation of growth, development, and defense in response to a constantly changing environment.
Auxins and cytokinins often act antagonistically during shoot and root development, balancing cell division and differentiation in different plant organs.
Gibberellins and abscisic acid (ABA) also exhibit antagonistic interactions during seed germination, where gibberellins promote germination while ABA inhibits it and maintains dormancy.
Ethylene interacts with jasmonates and salicylic acid in modulating plant defense responses against pathogens.
Hormone crosstalk allows integration of internal developmental signals with external environmental stress signals, ensuring optimal allocation of resources between growth and survival.
Understanding hormone crosstalk is essential for unraveling the complexity of adaptive plant responses and for manipulating hormone networks to improve crop development and productivity.

Agricultural Applications

Plant hormones and their synthetic analogs are widely used in agriculture and horticulture to enhance plant growth, improve yield, and increase stress tolerance.
Auxins promote root formation in plant cuttings and help control premature fruit drop.
Synthetic auxins such as 2,4-D are used as herbicides for the selective control of broadleaf weeds.
Gibberellins are applied to promote fruit enlargement, stimulate seed germination, and break seed dormancy in various crops.
Cytokinins delay senescence and are used to maintain the post-harvest freshness of vegetables and cut flowers.
Ethylene-releasing compounds such as ethephon are utilized to induce uniform fruit ripening.
Analogs of abscisic acid are being investigated for their potential role in enhancing drought tolerance in crops.
The precise and regulated application of these plant growth regulators enables farmers to manipulate plant physiology according to agricultural requirements, leading to improved productivity and better crop quality.

Recent Advances and Future Perspectives

Recent developments in molecular biology, genomics, and synthetic biology have greatly improved our understanding of phytohormone biosynthesis, signaling pathways, and hormonal interactions.
Advanced tools such as CRISPR/Cas9 genome editing, transcriptome analysis, and hormone-responsive reporter systems have led to the discovery of new hormone receptors, transporters, and signaling intermediates.
Researchers are uncovering the functions of previously underexplored hormones and identifying new mechanisms of crosstalk between hormonal pathways and environmental signals.
There is increasing focus on engineering hormone pathways in crop plants, aiming to enhance stress tolerance, improve yield, and optimize growth through precise and regulated hormone management.
Future directions in phytohormone research include clarifying the roles of peptide hormones, investigating interactions between plant microbiomes and hormone signaling, and developing targeted agrochemicals that modulate specific components of hormone signaling networks to support sustainable agriculture.

Conclusion

Plant hormones are fundamental regulators of every stage of a plant’s life cycle, from seed germination to senescence, and from organogenesis to stress responses.
Major hormone classes such as auxins, gibberellins, cytokinins, ethylene, and abscisic acid are extensively studied and well characterized.
Newly recognized hormones, including brassinosteroids, jasmonates, and strigolactones, have further expanded our understanding of plant physiology and regulatory complexity.
The identification of peptide hormones and small-molecule hormones has added additional layers to the intricate network of plant signaling systems.
Hormonal crosstalk maintains a fine balance between growth and defense, which is essential for plant survival in dynamic and changing environments.
The practical application of plant growth regulators in agriculture has significantly improved crop productivity and quality.
Continued research in phytohormone biology is uncovering innovative strategies that will shape the future of plant biotechnology and promote sustainable agricultural practices.

References

Biology LibreTexts. (2022, July 27). Plant hormones (Section 4.2). Retrieved from https://bio.libretexts.org/Bookshelves/Botany/The_Science_of_Plants_-Understanding_Plants_and_How_They_Grow(Michaels_et_al.)/04%3A_How_Plants_Grow_Part_2/4.02%3A_Plant_Hormones
Vaishnav, D., & Chowdhury, P. (2023). Types and functions of phytohormones and their roles in stress responses. In IntechOpen eBooks. https://doi.org/10.5772/intechopen.109325
Liao, K., Peng, Y., Yuan, L., Dai, Y., Chen, Q., Yu, L., Bai, M., Zhang, W., Xie, L., & Xiao, S. (2019). Brassinosteroids antagonize jasmonate-activated plant defense responses through BRI1-EMS-SUPPRESSOR1 (BES1). Plant Physiology, 182(2), 1066–1082. https://doi.org/10.1104/pp.19.01220
ScienceDirect. (n.d.). Article PDF retrieved from https://www.sciencedirect.com/science/article/pii/S0981942825003675/pdf
Ali, J., Mukarram, M., Ojo, J., Dawam, N., Riyazuddin, R., Ghramh, H. A., Khan, K. A., Chen, R., Kurjak, D., & Bayram, A. (2024). Harnessing phytohormones: Advancing plant growth and defense strategies for sustainable agriculture. Physiologia Plantarum, 176(3). https://doi.org/10.1111/ppl.14307
Oh, M., Honey, S. H., & Tax, F. E. (2020). Regulation of cell expansion, cell division, and vascular development by brassinosteroids: A historical perspective. International Journal of Molecular Sciences, 21(5), 1743. https://doi.org/10.3390/ijms21051743
International Journal of Molecular Sciences. (n.d.). Special issue on plant phytohormones. Retrieved from https://www.mdpi.com/journal/ijms/special_issues/Plant_Phytohormone
Beale, M. H., & Sponsel, V. M. (1993). Future directions in plant hormone research. Journal of Plant Growth Regulation, 12(4), 227–235. https://doi.org/10.1007/bf00213039
Maxapress. (n.d.). Plant hormones. Retrieved from https://www.maxapress.com/ph
Davies, P. J. (2007). The plant hormones: Their nature, occurrence, and functions. In Springer eBooks (pp. 1–15). https://doi.org/10.1007/978-1-4020-2686-7_1