What are phytohormones ?

Plants, like animals, use specific chemical substances to regulate their growth and development. These substances, known as phytohormones or plant hormones, play a crucial role in various aspects of plant life. Perhaps you’ve already wondered how a seed grows into a towering tree, or how certain flowers open in daylight? The answer lies largely in the action of these hormones.

Definition of phytohormones

Phytohormones are organic substances produced by plants that influence various physiological processes, including plant growth and development. Unlike animal hormones, they are not produced by specific glands, but rather by various cells in different parts of the plant. Their presence, even at very low concentrations, can induce significant changes in plant morphology and physiology.

It’s important to note that several types of phytohormones exist, and each has unique functions. Among the best known areauxin, gibberellin, cytokinin andethylene.

Types of phytohormones and their specific functions

Auxine

Auxin is probably the most studied of the plant hormones. It plays a key role in regulating primary plant growth, notably by elongating cells in young shoots. Produced mainly in the tips of stems and roots, it is transported to other parts of the plant where it regulates various processes, such as :

• Phototropism: growth towards light.
• Gravitropism: growth in response to gravity.
• Adventitious root formation.
• Apical dominance: inhibition of lateral bud growth.

Auxin is also essential for organogenesis and wound healing in plants.

Gibberellin

Gibberellins are another major class of phytohormones involved in stimulating plant growth and promoting cell elongation, particularly in stems. They also have effects on :

• Seed germination.
• Flowering.
• L’internody.
• Fruit enlargement.

For example, cultivated rice benefits greatly from this hormone to ensure good yields.

Cytokinin

Cytokinins are primarily responsible for cell division (cytokinesis) and strongly influence the regulation of growth and development. Their production amplifies cell proliferation, supporting processes such as :

• Tissue and organ differentiation.
• Delays leaf senescence.
• Stimulation of lateral buds when apical dominance is suppressed.

Farmers and researchers often exploit cytokinins to improve the harvest profile and extend the shelf life of agricultural produce.

Ethylene

Ethylene is unique among phytohormones in that it is a gaseous hormone. It plays a major role in fruit ripening and environmental stress management. Its influence can be seen through :

• Stimulation of fruit ripening and dropping.
• Response to stress caused by pathogens or abiotic stresses such as mechanical injury.
• Advancing leaf senescence.

This gas is emitted by all parts of the plant and generally acts where it is produced. You may have heard of ethylene being used to help synchronize fruit ripening in the food industry.

Interaction between phytohormones in growth regulation

Phytohormones do not act independently. They interact to harmoniously regulate plant growth and development. These interactions can be synergetic (cooperative) or antagonistic (opposing), and the combined effects often depend on the relative concentration of each hormone and the plant’s stage of development.

For example, auxins and cytokinins work closely together in the stem apical meristem, regulating cell division and organ formation. Auxins encourage organ formation, while cytokinins promote cell division, with a delicate balance between the two being essential for proper cell growth and differentiation.

Ethylene plays a role by interacting with auxins and cytokinins, influencing various processes such as fruit ripening, leaf abscission and responses to environmental stress. It can reinforce the actions of auxins, for example, by stimulating root growth and affecting cell wall plasticity.

Abscisic acid (ABA) is essential for regulating responses to abiotic stress, such as drought, salinity and cold, and works with other phytohormones to orchestrate plant defenses. By interacting with specific transcription factors, ABA can activate genes that increase drought tolerance, helping plants to adapt to difficult conditions.

Gibberellins and brassinosteroids also participate in this network of interactions, regulating seed germination, stem growth, flower formation and root hair growth respectively. They interact with auxins, cytokinins, ethylene and ABA to adjust growth and stress responses.

Practical applications of knowledge about phytohormones

The practical application of knowledge about phytohormones has transformed modern agriculture. Growers often manipulate these hormones to improve yields, optimize crop quality and manage various environmental stresses.

Agronomy and horticulture

Phytohormones play an essential role in agronomy and horticulture, significantly improving crop productivity and optimizing plant growth. The use of synthetic growth regulators, such asnaphthaleneacetic acid (NAA), an auxin analog, and paclobutrazol, a gibberellin inhibitor, enables precise control of plant development.

These regulators promote root formation, limit stem growth or accelerate fruit ripening. Natural biostimulants, including plant and algae extracts rich in cytokinins, auxins and gibberellins, stimulate growth and resilience to stress. They improve germination, shoot development and fruit ripening, while increasing plant resistance to adverse environmental conditions.

Plant biotechnology

Plant biotechnology benefits greatly from advances in phytohormones, paving the way for innovative strategies to increase crop productivity and sustainability. Thanks to genetic engineering and metabolic engineering, it is possible to develop transgenic plants with improved characteristics, such as enhanced resistance to abiotic and biotic stresses.

Phytohormones are also indispensable in the processes of organogenesis and somatic embryogenesis, key to in vitro vegetative propagation and the creation of genetically modified plants. Specific combinations of auxins and cytokinins are essential for inducing somatic bud and embryo formation in tissue culture.

In addition, the study of interactions between phytohormones and beneficial microorganisms, such as arbuscular mycorrhizal fungi (AMF), reveals a potential for improving plant growth and resistance to stress. These microorganisms can secrete phytohormones that promote plant development and strengthen defense against pathogens.

Conclusion

Phytohormones are essential for regulating plant growth, development and stress response. These substances, which include auxins, gibberellins, cytokinins, ethylene and abscisic acid, among others, orchestrate vital plant processes in complex ways. They play a vital role in managing abiotic stress, stimulating growth and adjusting defense mechanisms.

Sources :

https://www.miye.care/categorie-produit/

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1399-3054.1978.tb01531.x