Phytohormones revisited: what makes a compound a hormone in plants

Since the discovery of auxins as cell division factors a century ago, impressive scientific advances related to phytohormones have revolutionized plant sciences and human progress. This review examines the key features of the ten hormonal groups that operate in plants, here referred to as the 'classical ten': auxins, gibberellins (GAs), cytokinins (CKs), abscisic acid (ABA), ethylene (ETH), salicylates (SAs), jasmonates (JAs), brassinosteroids (BRs), peptide hormones (PEPs), and strigolactones (SLs). By leveraging historical data and sharpening the essentials of each hormonal group, their major functions are presented, with a discussion on what makes a compound a phytohormone and how it can be differentiated from hormone-like compounds and other signaling molecules. Hormonal receptors, long-distance transport, and differences between hormonal crosstalk, interactions, and complementation are discussed to illustrate the basics of hormonal action in plants.