Elucidation of the role of auxin-input pathways in auxin homeostasis.

Auxin is a phytohormone involved in cell elongation and division. Levels of indole-3-acetic acid (IAA), the most abundant auxin, are tightly regulated through biosynthesis, degradation, sequestration, and transport. Sequestration of IAA occurs in reversible processes by adding amino acids, polyol or simple alcohols, or sugars, forming IAA conjugates, or via a two-carbon elongation forming indole-3-butyric acid (IBA). These forms of auxin have decreased activity and are located in multiple organelles. Using a combination of molecular and genetic tools, I examined how these storage forms are working together to contribute to overall IAA levels and IAA response in both the model plant Arabidopsis thaliana and Zea mays. In Arabidopsis, I examined mutants that combine disruptions in the pathways converting IAA conjugates and IBA to free IAA. These mutants show phenotypes indicative of low auxin levels, including abnormal vein patterning and decreased apical dominance. Root phenotypes include changes in root length, root branching, and root hair growth. IAA levels are significantly reduced in the cotyledon tissue but not in meristems and hypocotyls. In the combination mutants, auxin biosynthetic gene expression is increased, particularly in the YUCCA/ TAA pathway, providing a feedback mechanism that allows the plant to compensate for changes in IAA-input pathways and maintain cellular homeostasis. The combination mutants also revealed a novel role for IAA in germination. Gibberellic Acid (GA) is able to rescue this germination defect suggesting an interaction between IAA and GA in seed germination. In maize, I examined how IBA is contributing to IAA levels in the plant. I identified homologs to Arabidopsis genes necessary for IBA conversion to IAA and then used reverse genetics to examine the effects in the plant when IBA metabolism was disrupted. I found several developmental phenotypes similar to other low auxin mutants, including decreased tassel branching and decreased yield. These phenotypes suggest that IBA is playing a significant role in overall auxin homeostasis in maize