Christopher S. Walsh
Plant hormones can be defined as naturally occurring substances that regulate one or more developmental events in plants. To be classified as a plant hormone, a chemical is
• not one of the sixteen essential elements required for plant growth;
• synthesized from two or more of the following elements: carbon, hydrogen, oxygen, and nitrogen;
• not a sugar, vitamin, or enzyme cofactor;
• present and active in extremely low concentrations;
• endogenous, meaning that it occurs naturally within the plant; and
• synthesized in one or more locations, where it demonstrates activity, or is able to act at locations other than its site of synthesis.
Past research has identified five broad groups of compounds that meet these criteria. Modern hormonal theory recognizes that auxin, gibberellin, cytokinin, abscisic acid, and ethylene are endogenous hormones that work alone, and in concert, to regulate plant development. Auxin, gibberellin, and cytokinin are generally described as "growth-promoting hormones," while abscisic acid is known as an "inhibitor." Ethylene does not fit neatly into either category. As it is inextricably involved in plant senescence, some classify ethylene as an inhibitor. With recent advances in molecular biology, perhaps ethylene is better described as a "promoter of senescence." A synopsis of each hormone is presented below, including a brief history of its discovery, its sites of synthesis in the plant, its movement, and some of the developmental events regulated by the plant. The commercial applications of plant hormones and plant growth regulators are covered in the PLANT GROWTH REGULATION chapter.
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