Auxin homeostasis: the DAO of catabolism.

Nearly all programmed and plastic plant growth responses are at least partially regulated by auxins, such as indole-3-acetic acid (IAA). Although vectorial, long distance auxin transport is essential to its regulatory function, all auxin responses are ultimately localized in individual target cells. As a consequence, cellular auxin concentrations are tightly regulated via coordinated biosynthesis, transport, conjugation, and oxidation. The primary auxin oxidative product across species is 2-oxindole-3-acetic acid (oxIAA), followed by glucose and amino acid conjugation to oxIAA. Recently, the enzymes catalyzing the oxidative reaction were characterized in Arabidopsis thaliana. DIOXYGENASE OF AUXIN OXIDATION (DAO) comprises a small subfamily of the 2-oxoglutarate and Fe(II) [2-OG Fe(II)] dependent dioxygenase superfamily. Biochemical and genetic studies have revealed critical physiological functions of DAO during plant growth and development. Thus far, DAO has been identified in three species by homology. Here, we review historical and recent studies and discuss future perspectives regarding DAO and IAA oxidation.