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Originally published as Genetics Published Articles Ahead of Print on October 8, 2006.

Genetics, Vol. 174, 1841-1857, December 2006, Copyright © 2006
doi:10.1534/genetics.106.061044

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An Arabidopsis Basic Helix-Loop-Helix Leucine Zipper Protein Modulates Metal Homeostasis and Auxin Conjugate Responsiveness

Rebekah A. Rampey*,{dagger},1, Andrew W. Woodward{dagger},1,2, Brianne N. Hobbs*,{dagger}, Megan P. Tierney{dagger},3, Brett Lahner{ddagger}, David E. Salt{ddagger} and Bonnie Bartel{dagger},4

* Department of Biology, Harding University, Searcy, Arkansas 72149, {dagger} Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005 and {ddagger} Center for Plant Environmental Stress Physiology, Purdue University, West Lafayette, Indiana 47907

4 Corresponding author: Department of Biochemistry and Cell Biology, Rice University, 6100 S. Main St., MS-140, Houston, TX 77005.
E-mail: bartel{at}rice.edu

The plant hormone auxin can be regulated by formation and hydrolysis of amide-linked indole-3-acetic acid (IAA) conjugates. Here, we report the characterization of the dominant Arabidopsis iaa–leucine resistant3 (ilr3-1) mutant, which has reduced sensitivity to IAA–Leu and IAA–Phe, while retaining wild-type responses to free IAA. The gene defective in ilr3-1 encodes a basic helix-loop-helix leucine zipper protein, bHLH105, and the ilr3-1 lesion results in a truncated product. Overexpressing ilr3-1 in wild-type plants recapitulates certain ilr3-1 mutant phenotypes. In contrast, the loss-of-function ilr3-2 allele has increased IAA–Leu sensitivity compared to wild type, indicating that the ilr3-1 allele confers a gain of function. Microarray and quantitative real-time PCR analyses revealed five downregulated genes in ilr3-1, including three encoding putative membrane proteins similar to the yeast iron and manganese transporter Ccc1p. Transcript changes are accompanied by reciprocally misregulated metal accumulation in ilr3-1 and ilr3-2 mutants. Further, ilr3-1 seedlings are less sensitive than wild type to manganese, and auxin conjugate response phenotypes are dependent on exogenous metal concentration in ilr3 mutants. These data suggest a model in which the ILR3/bHLH105 transcription factor regulates expression of metal transporter genes, perhaps indirectly modulating IAA-conjugate hydrolysis by controlling the availability of metals previously shown to influence IAA–amino acid hydrolase protein activity.






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