Indole-3-acetic acid

Indole-3-acetic acid (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. Chemically, IAA is a carboxylic acid in which the carboxyl group is attached through a methylene group to the C-3 position of an indole ring. In appearance, IAA is a colorless solid.

Biosynthesis and biological activity
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.

IAA has many different effects, as all auxins do, 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.

It is formed from tryptophan, along with tryptophol in rats infected by Trypanosoma brucei gambiense.

Synthetic analogs
There are less expensive and metabolically stable synthetic auxin analogs on the market for use in horticulture, such as indole-3-butyric acid (IBA) and 1-naphthaleneacetic acid (NAA).

Studies of IAA in the 1940s led to the development of the phenoxy herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Like IBA and NAA, 2,4-D and 2,4,5-T are metabolically and environmentally more stable analogs of IAA. However, when sprayed on broad-leaf dicot plants, they induce rapid, uncontrolled growth, eventually killing them. First introduced in 1946, these herbicides were in widespread use in agriculture by the middle of the 1950s.

Synthesis
Chemically, it can be synthesized by the reaction of indole with glycolic acid in the presence of base at 250 °C:


 * Synthesis of indole-3-acetic acid.png

Many methods for its synthesis have been developed since its original synthesis from indole-3-acetonitrile.

Toxicity on human organs
Little research has been conducted on the effects of IAA on humans and toxicity data is limited. No data on carcinogenic, teratogenic, or developmental effects have been created. IAA is listed in its MSDS as potentially mutagenic to mammalian somatic cells. It is listed as a potential skin, eye, and respiratory irritant, and users are warned not to ingest it. Protocols for ingestion, inhalation, and skin/eye exposure are standard for moderately poisonous compounds and include thorough rinsing in the case of skin and eyes, fresh air in the case of inhalation, and immediately contacting a physician in all cases to determine the best course of action. Do not induce vomiting in the case of ingestion nor attempt to give anything by mouth to an unconscious person. The NFPA 704 health hazard rating for IAA is 2, which denotes a risk of temporary incapacitation with intense or prolonged, but not chronic exposure, and a possibility of residual injury.

Effects on embryonic rat brain development
Studies have been performed to determine the effects IAA and similar chemicals on the development of the cerebral cortex during early embryogenesis. One study performed in rats determined that such compounds decreased the locomotor activities of the embryos/fetuses. Further, treatment with IAA and analog 1(methyl)-IAA resulted in significantly decreased brain sizes (relative to body weigh)in embryonic day 21 animals.

Cancer
Indole-3-acetic acid (IAA) and horseradish peroxidase (HRP) have been proposed on the basis of polymer transfer and gene-direction, they may be useful in cancer therapy. Radical-IAA IAA in the human body will attach to cells with that characterized by HRP. By this means the HRP reaction cells can be selectively killed. But because of the competitive bonding of the IAA-HRP more studies must be done before clinical studies can begin.