AUXIN BIOSYNTHESIS

Indole-3-acetic acid (IAA) is the most abundant naturally occurring au xin. Plants produce active IAA both by de novo synthesis and by releas ing IAA from conjugates. This review emphasizes recent genetic experim ents and complementary biochemical analyses that are beginning to unra vel the complexities of IAA biosynthesis in plants. Multiple pathways exist for de novo IAA synthesis in plants, and a number of plant enzym es can liberate IAA from conjugates. This multiplicity has contributed to the current situation in which no pathway of IAA biosynthesis in p lants has been unequivocally established. Genetic and biochemical expe riments have demonstrated both tryptophan-dependent and tryptophan-ind ependent routes of IAA biosynthesis. The recent application of precise and sensitive methods for quantitation of IAA and its metabolites to plant mutants disrupted in various aspects of IAA regulation is beginn ing to elucidate the multiple pathways that control IAA levels in the plant.