PlantHormones on emsenn.net

Abscisic Acid

Thu, 02 Apr 2026 00:00:00 +0000

Abscisic acid: a C₁₅ terpenoid hormone primarily regulating water-stress responses. Despite its name — historically derived from its role in abscission (leaf/fruit drop) — ABA is not the primary driver of abscission (that role belongs to ethylene). Instead, ABA is the plant’s central drought and stress-response hormone, coordinating stomatal closure, seed dormancy, and tolerance mechanisms across tissues. ABA accumulates rapidly in response to water deficit, activating a cascade that prioritizes survival over growth.

Auxin

Thu, 02 Apr 2026 00:00:00 +0000

Auxin: the primary plant growth hormone, regulating cell elongation, apical dominance, phototropism, gravitropism, root initiation, fruit development, and vascular differentiation. The main natural auxin is indole-3-acetic acid (IAA), first isolated by the botanist Went in 1926 following Darwin’s classical observations of phototropic bending in grass seedlings.

Biosynthesis and transport

Auxin is synthesized primarily in the shoot apex through the TAA/YUC (tryptophan aminotransferase / YUCCA) pathway: tryptophan → indole-3-pyruvic acid (IPA) → IAA. The enzyme TAA1 catalyzes the first step; the YUCCA family of flavin monooxygenases catalyzes the second. Once produced, IAA moves through tissues via polar transport — a directional basipetal (downward) movement mediated by PIN-FORMED (PIN) efflux carriers and AUX1/LAX influx carriers on the plasma membrane. This polar transport is unique among plant hormones: auxin cannot move acropetally (upward) against the PIN gradient. The directionality of PIN localization determines the direction of auxin flow and thus the spatial pattern of growth.

Cytokinin

Thu, 02 Apr 2026 00:00:00 +0000

Cytokinin: a class of plant hormones that promote cell division (cytokinesis) and delay senescence (leaf aging). Synthesized primarily in root tips, transported upward in the xylem, and perceived through a two-component phosphorelay system. The name derives from their role in cytokinesis, though their regulatory functions extend far beyond cell division.

Biosynthesis and activation

The rate-limiting step in cytokinin synthesis is catalyzed by isopentenyltransferase (IPT) enzymes, which transfer an isopentenyl group from dimethylallyl diphosphate (DMAPP) to adenine nucleotides, forming isopentenyl adenine (iP)-type cytokinins. CYP735A cytochrome P450 enzymes then convert these to the trans-zeatin (tZ) types, which are typically more bioactive. Inactive cytokinin nucleotides are converted to bioactive free bases by LONELY GUY (LOG) phosphoribohydrolases. The active forms — isopentenyl adenosine, zeatin, and zeatin riboside — are perceived by the plant’s sensory machinery.

Ethylene

Thu, 02 Apr 2026 00:00:00 +0000

Ethylene: the only gaseous plant hormone (C₂H₄), diffusing freely through air and soil to regulate fruit ripening, senescence, leaf and flower abscission, defense responses, and the “triple response” in dark-grown seedlings. Operating at nanomolar concentrations, ethylene is produced in response to stress, wounding, and fruit maturation, coordinating plant responses across multiple tissues simultaneously.

Biosynthesis

Ethylene is synthesized through the Yang cycle: methionine → S-adenosylmethionine (SAM, via SAM synthetase) → 1-aminocyclopropane-1-carboxylic acid (ACC, via ACC synthase / ACS) → ethylene (via ACC oxidase / ACO). ACS is the rate-limiting enzyme; it is encoded by a multigene family and regulated at both transcriptional and post-translational levels. Phosphorylation by MAP kinases (MPK3/MPK6) stabilizes certain ACS isoforms. ACC can also be conjugated to malonyl-ACC or gamma-glutamyl-ACC as a storage form, sequestering the hormone and rendering it inactive until deconjugation is needed.

Gibberellin

Thu, 02 Apr 2026 00:00:00 +0000

Gibberellin: a class of plant growth-promoting hormones that stimulate stem elongation, break seed dormancy, promote flowering, and regulate fruit development. Over 130 distinct gibberellin structures have been identified (GA₁, GA₃, GA₄ being among the most bioactive), but all share a common four-ring gibban skeleton. The hormone was first discovered through study of the “bakanae” (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi, which secretes gibberellins causing extreme stem elongation.