Plant Physiology Preview
Published on December 1, 2006; 10.1104/pp.106.091025
OPEN ACCESS ARTICLE
Received October 6, 2006
Accepted November 23, 2006
Functional Analysis of SPINDLY in Gibberellin Signaling in Arabidopsis
Aron L. Silverstone , Tong-Seung Tseng , Stephen M. Swain , Alyssa Dill , Sun Yong Jeong , Neil E. Olszewski , and Tai-ping Sun *
Department of Biology, Duke University, Durham, North Carolina 27708
Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108
* Corresponding author; email: tps{at}duke.edu.
The Arabidopsis 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 3 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 8-9 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 3 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.
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