Auxins are an important class of phytohormones that play a critical role in various aspects of plant growth and development such as cell elongation, cell division, apical dominance, tropisms, and fruit ripening. The most abundant naturally occurring auxin is indole-3-acetic acid (IAA). IAA is found in plants both as the free acid and in conjugated forms. It is believed that the balance between IAA biosynthesis, metabolism, and transport determines the actual levels of IAA in a cell at any given time, but the molecular mechanisms controlling this regulation are largely unknown. In my thesis project, an IAA-alanine resistant mutant from Arabidopsis thaliana, iar4, has been phenotypically characterized and mapped to chromosome 1. Three mutants potentially defective in the last step of IAA biosynthesis have also been characterized. One of these mutants has been tested for complementation with a gene encoding a putative aldehyde oxidase, AtAO4.