Indole-3-acetic acid

Indole-3-acetic acid (IAA, 3-IAA) is the most common, naturally occurring, plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, containing a carboxymethyl substituent. It is a colorless solid that is soluble in polar organic solvents. Indole-3-acetic acid (IAA, 3-IAA) is the most common, naturally occurring, plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, containing a carboxymethyl substituent. It is a colorless solid that is soluble in polar organic solvents. IAA is predominantly produced in cells of the apex (bud) and very young leaves of a plant. Plants can synthesize IAA by several independent biosynthetic pathways. Four of them start from tryptophan, but there is also a biosynthetic pathway independent of tryptophan. Plants mainly produce IAA from tryptophan through indole-3-pyruvic acid. IAA is also produced from tryptophan through indole-3-acetaldoxime in Arabidopsis thaliana. In rats, IAA is a product of both endogenous and colonic microbial metabolism from dietary tryptophan along with tryptophol. This was first observed in rats infected by Trypanosoma brucei gambiense. and correlation with protein intake has been confirmed in 2015. Humans did not excrete IAA. As all auxins, IAA has many different effects, such as inducing cell elongation and cell division with all subsequent results for plant growth and development. On a larger scale, IAA serves as signaling molecule necessary for development of plant organs and coordination of growth. IAA enters the plant cell nucleus and binds to a protein complex composed of a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and a ubiquitin ligase (E3), resulting in ubiquitination of Aux/IAA proteins with increased speed. Aux/IAA proteins bind to auxin response factor (ARF) proteins, forming a heterodimer, suppressing ARF activity. In 1997 it was described how ARFs bind to auxin-response gene elements in promoters of auxin regulated genes, generally activating transcription of that gene when an Aux/IAA protein is not bound. IAA inhibits the photorespiratory-dependent cell death in photorespiratory catalase mutants. This suggests a role for auxin signalling in stress tolerance. IAA production is widespread among environmental bacteria that inhabit soils, waters, but also plant and animal hosts. Distribution and substrate specificity of the involved enzymes suggests these pathways play a role beyond plant-microbe interactions. Enterobacter cloacae can produce IAA, from aromatic and branched-chain amino acids. Fungi can form a fungal mantle around roots of perennial plants called ectomycorrhiza. A fungus specific to spruce called Tricholoma vaccinum was shown to produce IAA from tryptophan and excrete it from its hyphae. This induced branching in cultures, and enhanced Hartig net formation. The fungus uses a multidrug and toxic extrusion (MATE) transporter Mte1. Research into IAA-producing fungi to promote plant growth and protection in sustainable agriculture is underway. Chemically, it can be synthesized by the reaction of indole with glycolic acid in the presence of base at 250 °C: