Biogenesis and Degradation of Indoleacetic Acid in Plants

Tryptophan has been clearly established as the precursor of indoleacetic acids in both plants (e.g., 303, 921, 922) and fungi (e.g., 864), and in plant tumor tissue (e.g., 378, 948). Two routes are possible for indoleacetic acid formation from tryptophan as follows  

There is evidence that both these routes can occur. The enzymes converting tryptophan to indoleacetic acid can be obtained in maize embryo juice the tryptophan is thought to arise from the endosperm (964). Indolepyruvic acid is also present in maize endosperm (837, 838), suggesting it to be an intermediate. On the other hand, tryptamine is converted to indoleacetic acid in plants (304, 815) and the amine oxidase responsible has been studied by Kenten and Mann (464). Consideration of the biogenesis of alkaloids, discussed later, suggests that both tryptamine and indoleacetaldehyde are likely to occur in plants. 

Indoleacetic acid is degraded in plants by a specific indoleacetic acid oxidase. This is a light-activatable flavoprotein enzyme coupled through hydrogen peroxide to a peroxidase (285 but cf. 463a, 805b). It apparently uses phenols as cofactors (296) but can be inhibited by polyphenols (305). The product of the reaction is still unidentified (836). 

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