Indole3acetic

Indole-3-acetic acid, commonly abbreviated as IAA, is the principal natural auxin, a plant hormone that plays a central role in regulating growth and development. It influences processes such as cell elongation, division, differentiation, and patterning, and is indispensable for responses to light, gravity, and wounding.

Biosynthesis and metabolism: IAA is produced primarily via tryptophan-dependent pathways, with the indole-3-pyruvic acid (IPyA) route being the most prominent in many plants. In this pathway, tryptophan is converted to IPyA by TAA1/TAR enzymes, and IPyA is then converted to IAA by YUCCA family flavin-containing monooxygenases. There are additional tryptophan-dependent routes and evidence for tryptophan-independent routes in some species. IAA activity is regulated by conjugation to sugars or amino acids and by oxidation, which help control the active hormone pool.

Transport and distribution: IAA movement in plants occurs mainly through polar auxin transport, mediated by PIN efflux carriers and AUX1/LAX influx carriers. This directional transport establishes auxin gradients that coordinate growth responses, organ initiation, and vascular development. In shoots, transport tends to move from the apex toward the base, while in other contexts it supports various tropic responses and developmental processes.

Mechanism of action: IAA perception involves the TIR1/AFB receptor family, leading to degradation of AUX/IAA repressors and activation of ARF transcription factors. This signaling network modulates gene expression to produce changes in cell expansion, differentiation, and organ patterning.

Applications and relevance: IAA is used in research to study hormone signaling and development, and in practical horticulture to promote rooting of cuttings. Its levels and effects are highly context-dependent, with precise balance necessary for normal plant growth.