Plant hormone

Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis,^[1] the regulation of organ size, pathogen defense,^[2][3] stress tolerance^[4][5] and reproductive development.^[6] Unlike in animals (in which hormone production is restricted to specialized glands) each plant cell is capable of producing hormones.^[7][8] Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.^[9]

Phytohormones occur across the plant kingdom, and even in algae, where they have similar functions to those seen in vascular plants ("higher plants").^[10] Some phytohormones also occur in microorganisms, such as unicellular fungi and bacteria, however in these cases they do not play a hormonal role and can better be regarded as secondary metabolites.^[11]