Plant hormones, also known as phytohormones or plant growth regulators, are naturally occurring chemical compounds that regulate various physiological processes and growth patterns in plants. These hormones play a crucial role in controlling plant growth, development, and responses to environmental cues. Plant hormones are produced in specific tissues and are transported throughout the plant to target sites where they exert their effects. There are several major types of plant hormones, each with its own distinct functions:
Auxins: Auxins are responsible for controlling cell elongation, stem growth, and apical dominance (the inhibition of lateral bud growth by the terminal bud). They also influence tropisms, such as phototropism (growth toward light) and gravitropism (response to gravity). Indole-3-acetic acid (IAA) is a well-known natural auxin.
Gibberellins (GAs): Gibberellins promote stem elongation, seed germination, and fruit development. They also play a role in breaking dormancy in seeds and buds. Different forms of gibberellins (e.g., GA1, GA3) have specific effects on plant growth.
Cytokinins: Cytokinins are involved in cell division and differentiation. They promote lateral bud growth, delay senescence (aging) in leaves, and stimulate axillary bud formation. Common cytokinins include zeatin and kinetin.
Abscisic Acid (ABA): ABA is often referred to as the “stress hormone” because it regulates responses to environmental stresses such as drought, salinity, and cold. It promotes stomatal closure to reduce water loss and induces seed dormancy.
Ethylene: Ethylene is a gaseous hormone that plays a role in fruit ripening, leaf and flower senescence, and abscission (shedding) of leaves and fruits. It also has an important role in response to various stress conditions.
Jasmonic Acid (JA) and Salicylic Acid (SA): These hormones are involved in plant defense responses against herbivores and pathogens. Jasmonic acid, in particular, plays a role in inducing the production of defensive compounds.
Brassinosteroids: Brassinosteroids promote cell expansion, cell division, and stem elongation. They also enhance resistance to stress and pathogens.
Strigolactones: Strigolactones play a role in controlling shoot branching by inhibiting the outgrowth of lateral buds. They also have a role in root development and interaction with mycorrhizal fungi.
Peptide Hormones: Plant peptide hormones include systemins, phytosulfokines (PSKs), and CLAVATA3/ESR-related (CLE) peptides, among others. They regulate various processes, such as defense responses, cell differentiation, and stem cell maintenance.
These plant hormones often work in complex networks and crosstalk with each other to fine-tune plant growth and responses to environmental changes. The balance and interactions among these hormones are critical for proper plant development, adaptation to environmental conditions, and the regulation of various physiological processes throughout the plant’s life cycle.
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