Auxin

Auxin is a class of plant hormones that regulate growth and development. The most common natural auxin is indole-3-acetic acid (IAA). Auxins influence cell elongation, division, and differentiation, and they play key roles in tropisms, apical dominance, vascular tissue formation, and organ development. They also participate in fruit set and growth and in responses to light and gravity.

Biosynthesis and transport: Auxins are mainly produced in shoot tips and young leaves, with IAA formed via tryptophan-dependent pathways. They are transported directionally from the shoot apex toward developing tissues by a polar transport system. PIN-FORMED (PIN) efflux carriers and AUX1/LAX influx carriers create and maintain auxin gradients that regulate pattern formation.

Mechanism of action: Auxin signaling involves receptors such as TIR1/AFB, which bind auxin and promote degradation of AUX/IAA repressors. Release of ARF transcription factors then alters expression of target genes, coordinating growth responses. Auxin signaling integrates with light, gravity, and other hormone networks to shape development.

Physiological roles: In shoots, auxin promotes cell elongation and helps establish apical dominance by inhibiting lateral bud growth. In roots, it stimulates lateral root initiation and root hair development. Auxin gradients guide tropic responses and vascular differentiation, influence organogenesis in tissue culture, and support fruit set and growth.

Applications and diversity: Synthetic auxins, such as 2,4-D, NAA, and IBA, are used as herbicides, rooting agents, and in plant propagation and tissue culture. Natural and synthetic auxins are employed to induce rooting in cuttings, regulate fruit development, and study developmental processes.

History: The concept of auxin emerged from early 20th-century work by Fritz Went in 1928, who showed that substances produced at shoot tips could trigger growth responses in decapitated coleoptiles. The name auxin derives from Latin augere, to increase; later, indole-3-acetic acid was identified as the principal natural auxin.