Plant hormones are naturally occurring organic compounds that regulate physiological processes at low concentrations. The major classes include auxins, cytokinins, gibberellins, abscisic acid, ethylene, brassinosteroids, jasmonates, salicylic acid, and strigolactones.

Auxin (primarily indole-3-acetic acid) regulates cell elongation, apical dominance, tropic responses, and root initiation. It is synthesized in shoot meristems and young leaves and transported polarly through the plant. The auxin signaling pathway involves the TIR1/AFB receptor, Aux/IAA repressor degradation, and ARF transcription factor activation. Synthetic auxins are used as rooting hormones and herbicides.

Cytokinins promote cell division, shoot formation, and delay senescence. They are adenine derivatives synthesized primarily in root tips. The cytokinin signal is perceived by histidine kinase receptors that activate a phosphorelay cascade leading to type-B ARR transcription factors. Cytokinins are used in tissue culture to stimulate shoot proliferation.

Gibberellins promote stem elongation, seed germination, and fruit development. Over 130 gibberellins are known, with GA1 and GA4 being the most active. The GID1 receptor and DELLA repressor proteins mediate gibberellin signaling. Gibberellin inhibitors are used to control plant height in agriculture.

Abscisic acid regulates stomatal closure, seed dormancy, and stress responses. It is synthesized in response to drought and cold stress. PYR/PYL/RCAR receptors initiate a signaling cascade that inhibits PP2C phosphatases, activating SnRK2 kinases.

Ethylene is a gaseous hormone that regulates fruit ripening, senescence, and abscission. It is perceived by ETR1 family receptors that regulate the EIN2/EIN3 signaling pathway. Ethylene inhibitors are used to delay fruit ripening during storage.