Figure 1.

Principle auxin metabolic pathways in seed plants. Auxin (indole-3-acetic acid, IAA) is synthesized from tryptophan (Trp) via indole-3-pyruvic acid (IPyA), and undergoes irreversible catabolism via oxindole-3-acetic acid (oxIAA). Auxin can be reversibly conjugated to a wide variety of compounds, generally considered to be storage forms of IAA. Conjugation with aspartic acid to make indole-3-acetyl aspartic acid (IAAsp) is irreversible, so this constitutes a second catabolic pathway. The figure also shows reversible conversion to indole-3-butyric acid (IBA), an auxin-like compound with a role in adventitious root formation. Blue boxes indicate the name of the gene family responsible for each pathway. Pink ellipses are added for clarity, to indicate the portion of each chemical structure that differs from IAA.