Plant hormones are naturally occurring chemical compounds produced by plants in minute quantities. These compounds act as messengers, orchestrating and regulating various physiological processes within the plant to ensure proper growth, development, and responses to environmental stimuli.
What are Plant Hormones?
Plant hormones, also known as “phytohormones” or “growth regulators,” are naturally occurring compounds that guide plant growth and development. Even in low concentrations, these compounds are highly active and influence vital physiological processes. Just like how our body’s chemicals direct different functions, plant hormones guide growth, flowering, and defense mechanisms. There are several types of plant hormones, each playing specific roles.
Types of Plant Growth Regulators
Based on their function, plant hormones fall into two categories:
- Plant Growth Promoters: Auxins, Cytokinins, Gibberellins
- Plant Growth Inhibitors: Abscisic Acid
- Both Promoters and Inhibitors: Ethylene
Auxin Hormones
Auxins act like directors, guiding plant growth and development. Produced in the tips of shoots and roots, auxins influence a wide range of processes.
- Natural Auxins: IAA (Indole-3-Acetic Acid)
- Synthetic Auxins: 2,4-D (2,4-Dichlorophenoxyacetic Acid), NAA (Naphthalene Acetic Acid)
Functions of Auxin
Auxins play a crucial role in plant tissue culture (PTC). Both natural and synthetic auxins are widely used in PTC. In nature, auxins are produced in the apices, shoot meristems, and active growing regions. Auxins promote cell division, callus formation, tissue swelling, and adventitious root formation. At low concentrations, they promote root formation, while at higher concentrations, they promote callus formation. Examples include 2,4-D, NAA, IAA, IBA, and 2,4,5-T.
Cytokinin Hormones
Cytokinins promote cell division and are vital in various growth and developmental processes.
Functions of Cytokinins
Widely used in PTC, cytokinins exist in both natural and synthetic forms:
- Natural Cytokinins: Zeatin, 2iP
- Synthetic Cytokinins: BAP, Kinetin
Cytokinins, derived from adenine, play a significant role in shoot induction and overall growth stimulation. When combined with auxins, they enhance cell division. Higher concentrations promote adventitious shoot formation (1-10 mg/L) while inhibiting root formation. They also reduce apical dominance and encourage shoot growth.
Gibberellin Hormones
Gibberellins regulate stem elongation, seed germination, and flowering.
Functions of Gibberellins
They promote internode elongation and overall growth. In vitro, they are often used for meristem or bud regeneration. Gibberellins generally inhibit both adventitious root and shoot formation.
Abscisic Acid (ABA)
ABA functions as a stress hormone, playing roles in seed dormancy, drought response, and stomatal closure. It contributes to abiotic stress tolerance and is often used in embryogenesis. Depending on the species, ABA may either induce or inhibit callus formation.
Ethylene
Ethylene, a gaseous hormone, regulates fruit ripening, senescence, and stress responses. It has significant applications in agriculture and horticulture.