Hormones

Hormones are signalling chemicals that are carried to distant organs by sophisticated biological processes to govern physiology and behaviour in multicellular organisms. Animals, plants, and fungus all require hormones for proper development. Because a hormone is defined as a signalling molecule that affects far from its source of production, many different types of molecules can be classified as hormones. Eicosanoids (e.g. prostaglandins and thromboxanes), steroids (e.g. oestrogen and brassinosteroid), amino acid derivatives (e.g. epinephrine and auxin), protein/peptides (e.g. insulin and CLE peptides), and gases are all examples of hormones (e.g ethylene and nitric oxide).

Hormones are chemicals that allow organs and tissues to communicate with one another. Hormones regulate digestion, metabolism, respiration, sensory perception, sleep, excretion, lactation, stress induction, growth and development, locomotion, reproduction, and mood manipulation in vertebrates. Hormones play a role in practically every stage of plant development, from germination through senescence.

Hormones modify the function of distant cells by attaching to certain receptor proteins in the target cell. When a hormone connects to a receptor, a signal transduction cascade is activated, which normally promotes gene transcription and increases the production of target proteins. Hormones can potentially have synergistic effects with genomic effects through non-genomic, fast-acting mechanisms. Second messengers are used by water-soluble hormones (such as peptides and amines) to act on the surface of target cells. Steroids and other lipid-soluble hormones penetrate through the plasma membranes of target cells (both cytoplasmic and nuclear) to function within their nucleus. Brassinosteroids (a class of polyhydroxysteroids that have been recognised as a sixth class of plant hormones and may have utility as an anticancer drug for endocrine-responsive cancers to induce apoptosis and inhibit growth in plants that, despite being lipid soluble, still bind to their receptor at the cell surface) are a notable exception to this rule.

Endocrine glands are specialised structures found in animals that produce hormones into the endocrine signalling system. Hormone secretion is triggered by specific biological cues and is frequently regulated by negative feedback. High blood sugar (serum glucose concentration), for example, stimulates insulin production. Insulin then works to maintain homeostasis by lowering glucose levels, resulting in lower insulin levels. Water soluble hormones are easily carried through the circulatory system after they are secreted. To form ligand-protein complexes, lipid-soluble hormones must attach to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)). Some hormones travel as prohormones that must be activated in specific cells through a series of highly regulated activation stages (as seen in insulin and growth hormones), while others travel as fully active hormones (as seen in insulin and growth hormones).

The endocrine system secretes hormones directly into the bloodstream, usually through fenestrated capillaries, whereas the exocrine system uses ducts to secrete hormones indirectly. Hormones having a paracrine action diffuse to surrounding target tissue through the interstitial spaces.

Plants lack specialised organs for hormone secretion, while hormone production is spatially distributed. The hormone auxin, for example, is mostly produced at the tips of new leaves and in the shoot apical meristem. Due to the lack of specialised glands, the major site of hormone production can shift over a plant’s life, and the location of production is determined by the plant’s age and environment.