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DEVELOPMENT AND HORMONE ACTION

Functional Analysis of SPINDLY in Gibberellin Signaling in Arabidopsis^{1,[C],[W],[OA]}

Department of Biology, Duke University, Durham, North Carolina 27708 (A.L.S., A.D., S.Y.J., T.-p.S.); and Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108 (T.-S.T., S.M.S, N.E.O.)

The Arabidopsis (Arabidopsis thaliana) SPINDLY (SPY) protein negatively regulates the gibberellin (GA) signaling pathway. SPY is an O-linked N-acetylglucosamine (GlcNAc) transferase (OGT) with a protein-protein interaction domain consisting of 10 tetratricopeptide repeats (TPR). OGTs add a GlcNAc monosaccharide to serine/threonine residues of nuclear and cytosolic proteins. Determination of the molecular defects in 14 new spy alleles reveals that these mutations cluster in three TPRs and the C-terminal catalytic region. Phenotypic characterization of 12 spy alleles indicates that TPRs 6, 8, and 9 and the catalytic domain are crucial for GA-regulated stem elongation, floral induction, and fertility. TPRs 8 and 9 and the catalytic region are also important for modulating trichome morphology and inflorescence phyllotaxy. Consistent with a role for SPY in embryo development, several alleles affect seedling cotyledon number. These results suggest that three of the TPRs and the OGT activity in SPY are required for its function in GA signal transduction. We also examined the effect of spy mutations on another negative regulator of GA signaling, REPRESSOR OF ga1-3 (RGA). The DELLA motif in RGA is essential for GA-induced proteolysis of RGA, and deletion of this motif (as in rga-17) causes a GA-insensitive dwarf phenotype. Here, we demonstrate that spy partially suppresses the rga-17 phenotype but does not reduce rga-17 or RGA protein levels or alter RGA nuclear localization. We propose that SPY may function as a negative regulator of GA response by increasing the activity of RGA, and presumably other DELLA proteins, by GlcNAc modification.

² These authors contributed equally to the paper.

³ Present address: Syngenta Biotechnology, Inc., 3054 Cornwallis Rd., Research Triangle Park, NC 27709.

⁴ Present address: Carnegie Institution of Washington, Department of Plant Biology, 260 Panama St., Stanford, CA 94305.

⁵ Present address: CSIRO Plant Industry, Private Mail Bag, Merbein, VIC 3505, Australia.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Tai-ping Sun (tps{at}duke.edu).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.091025

^* Corresponding author; e-mail tps{at}duke.edu; fax 919–613–8177.

Received October 6, 2006; accepted November 23, 2006; published December 1, 2006.

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