Poster: Hormones
Abs #
584: The roles of Arabidopsis IAA-amino acid conjugate hydrolases in auxin homeostasis
Plants regulate auxin levels through complex interactions among de novo synthesis, degradation, influx, efflux, and conjugate synthesis and hydrolysis. A thorough knowledge of identity, regulation, and interaction of these pathways is key to understanding auxin influences on plant growth and development. We are using genetic approaches to understand the function and metabolism of auxin conjugates, particularly amide-linked conjugates of indole-3-acetic acid (IAA). Some IAA conjugates have auxin-like activity in bioassays. We have isolated mutants with altered responses to IAA-conjugates. ilr1( IAA-Leu Resistant) and iar3 ( IAA-Ala Resistant) are defective in amidohydrolases that cleave specific IAA conjugates to release free, active IAA. Based on similarity to ILR1and IAR3, five more members of the Arabidopsis amidohydrolase gene family have been identified (ILL1,ILL2, ILL3, ILL5, ILL6). We have compared the in vitroenzymatic activity of GST fusions of these hydrolases and found that four can hydrolyze IAA-amino acids. The expression patterns of promoter-GUS fusions suggest that the hydrolases have overlapping, yet distinct roles in plant development. We found that the triple hydrolase mutant, ilr1 iar3 ill2,displays several phenotypes suggestive of low endogenous auxin levels. Comparisons of the triple, double, and single hydrolase mutants have revealed the relative importance of each hydrolase in IAA-conjugate hydrolysis in seedlings. Interestingly, IAA-Ala may have auxin activity independent of hydrolysis because the triple hydrolase mutant remains sensitive to high concentrations of IAA-Ala, but is resistant to other conjugates. This work will aid in elucidating the roles of the amidohydrolase gene family and the IAA conjugates in auxin homeostasis.
