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

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

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

^* Department of Biology, Harding University, Searcy, Arkansas 72149, Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005 and 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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