Phytohormone

Overview of phytohormones in plant biology

Phytohormones, also known as plant hormones, are naturally occurring organic compounds that play crucial roles in regulating various physiological processes in plants. These hormones are essential for plant growth, development, and responses to environmental stimuli. Unlike animal hormones, phytohormones are not produced in specialized glands but are synthesized in various parts of the plant.

Types of Phytohormones

Phytohormones can be classified into several major groups, each with distinct functions:

Auxins

Auxins are a class of phytohormones that primarily regulate cell elongation, apical dominance, and root initiation. The most common auxin is indole-3-acetic acid (IAA). Auxins are produced in the shoot apical meristem and young leaves and are transported down the plant to influence growth.

Cytokinins

Cytokinins promote cell division and differentiation, delay leaf senescence, and work in conjunction with auxins to control apical dominance. They are synthesized in roots and transported to other parts of the plant.

Gibberellins

Gibberellins are involved in promoting stem elongation, seed germination, and flowering. They are produced in young leaves, roots, and seeds.

Ethylene

Ethylene is a gaseous hormone that regulates fruit ripening, leaf abscission, and response to stress. It is produced in most plant tissues, especially during senescence and ripening.

Abscisic Acid

Abscisic acid (ABA) is primarily involved in stress responses, such as drought tolerance, and regulates stomatal closure. It also plays a role in seed dormancy.

Brassinosteroids

Brassinosteroids are involved in cell expansion, vascular differentiation, and stress responses. They are structurally similar to animal steroid hormones.

Jasmonates

Jasmonates are involved in plant defense responses and reproductive development. They are synthesized from fatty acids and play a role in response to wounding and pathogen attack.

Salicylic Acid

Salicylic acid is important for plant defense against pathogens and systemic acquired resistance. It also plays a role in thermogenesis in some plant species.

Functions of Phytohormones

Phytohormones regulate a wide range of processes in plants, including:

• Growth and Development: Phytohormones control cell division, elongation, and differentiation, influencing overall plant architecture.
• Reproduction: They regulate flowering, fruit development, and seed germination.
• Stress Responses: Phytohormones mediate plant responses to biotic and abiotic stresses, such as drought, salinity, and pathogen attack.
• Senescence and Abscission: Hormones like ethylene and abscisic acid regulate leaf senescence and fruit drop.

Mechanism of Action

Phytohormones exert their effects by binding to specific receptors, which then trigger signal transduction pathways leading to changes in gene expression. These pathways often involve secondary messengers and protein phosphorylation cascades.

Research and Applications

Understanding phytohormones has significant implications for agriculture and horticulture. Manipulating hormone levels can improve crop yield, enhance stress tolerance, and control plant growth patterns.

Also see

• Plant physiology
• Signal transduction
• Plant development
• Agricultural biotechnology