Metabolite Profiling to Understand Auxin Homeostasis

Our lab uses the small flowering plant Arabidopsis thaliana as a model for the study of hormonal control of plant growth and development. Our primary goals are to resolve how biosynthesis of the plant hormone auxin is developmentally regulated. The phytohormone auxin is fundamentally important in plant growth and development as a regulator of numerous biological processes. These include apical dominance (the suppression of lateral shoot formation by the primary shoot), fruit ripening, tropisms (response to gravity and light), as well as cell elongation, division, and differentiation. It is still not clear how this hormone is synthesized in plants or how it affects these biological responses at the molecular level. We do know that levels of the primary auxin, indole-3-acetic-acid (IAA) are maintained throughout the plant by a complex network of pathways, many of which are redundant. For example, we know that IAA can be derived from tryptophan or from precursors to tryptophan (see below) and these pathways are utilized differentially throughout the life cycle of the plant.

We are using a combination of biochemical and molecular genetic analysis to identify the intermediates and enzymes in this pathway, with the ultimate goal being to examine the developmental and tissue specific regulation of the genes encoding these enzymes. Complex, redundant pathways such as those involved in IAA synthesis are less amenable to simple visual screens because defects in one pathway are often compensated for by another functional pathway.

As part of a NSF-sponsored Plant Genome Initiative, our lab is working to identify mutants that are involved in all aspects of auxin biology. We are developing a high throughput assay for mutants with small changes in IAA levels. Using automated sample handling and mass spectral quantification we can identify mutants with defects in redundant pathways that would not be detected in a simple visual screen. Sequence information is currently available for the entire Arabidopsis genome, so identifying the genes that correspond to these mutants will be straightforward. Transgenic technology is also simple in Arabidopsis, so any candidate genes can be re-introduced into Arabidopsis in a variety of formats (antisense, overexpression, inducible expression, tissue-specific expression) in order to test their function.

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