SPINDLY Is a Nuclear-Localized Repressor of Gibberellin Signal Transduction Expressed throughout the Plant

Abstract

SPY (SPINDLY) encodes a putative O-linked N-acetyl-glucosamine transferase that is genetically defined as a negatively acting component of the gibberellin (GA) signal transduction pathway. Analysis of Arabidopsis plants containing a SPY::GUS reporter gene reveals that SPY is expressed throughout the life of the plant and in most plant organs examined. In addition to being expressed in all organs where phenotypes due to spy mutations have been reported, SPY::GUS is expressed in the root. Examination of the roots of wild-type, spy, and gai plants revealed phenotypes indicating that SPY and GAI play a role in root development. A second SPY::GUS reporter gene lacking part of the SPY promoter was inactive, suggesting that sequences in the first exon and/or intron are required for detectable expression. Using both subcellular fractionation and visualization of a SPY-green fluorescent protein fusion protein that is able to rescue the spy mutant phenotype, the majority of SPY protein was shown to be present in the nucleus. This result is consistent with the nuclear localization of other components of the GA response pathway and suggests that SPY's role as a negative regulator of GA signaling involves interaction with other nuclear proteins and/or O-N-acetyl-glucosamine modification of these proteins.GAs are endogenous plant growth regulators that have been studied for over 70 years. Until relatively recently, most of this research has concentrated on determining the physiological role of various GAs, defining the GA biosynthetic pathway in plants and fungi, and developing practical uses for GAs and chemical inhibitors of GA biosynthesis in agriculture. Over the last decade, considerable progress has also been made in understanding how plants are able to perceive the level of endogenous GAs and the mechanism by which the GA signal is transduced (Thornton et al., 1999a;Lovegrove and Hooley, 2000;Sun, 2000; Richards et al., 2001). This research has been made possible by advances in molecular genetic techniques in model systems such as Arabidopsis, rice (Oryza sativa), and the aleurone layer of cereal grains. In Arabidopsis, several negatively acting components of the GA response pathway have been characterized in some detail, including SPY (SPINDLY; Jacobsen and Olszewski, 1993;Jacobsen et al., 1996), and two members of the GRAS family (Pysh et al., 1999), RGA (REPRESSOR OF ga1-3) and GAI (GA INSENSITIVE; Peng et al., 1997; Silverstone et al., 1998). The cloning of GAI has led to the identification of orthologous genes from other species such as the wheat (Triticum aestivum) rht homeo-alleles that are the genetic basis of the "green revolution" (Peng et al., 1999a). Other potential GA-signaling proteins include SHI (SHORT INTERNODES), SLY (SLEEPY), and PKL (PICKLE) in Arabidopsis (Steber et al., 1998; Fridborg et al., 1999;Ogas et al., 1999), and GAMyb in barley (Hordeum vulgare;Gubler et al., 1999). A role for heterotrimeric G proteins has also been suggested based on work with inhibitors in wild oat (Avena sativa) a...

SPINDLY and GIGANTEA Interact and Act in Arabidopsis thaliana Pathways Involved in Light Responses, Flowering, and Rhythms in Cotyledon Movements

“…This lack of success may be because of the very low abundance of both proteins. Although the existence of a SPY/GI complex has not been demonstrated in planta, these proteins are expected to have opportunities to interact because both are nuclear localized and SPY is expressed throughout the plant (Huq et al, 2000;Swain et al, 2002).…”

Glucose delays seed germination in Arabidopsis thaliana

“…SPY encodes a putative O-linked N-acetyl-glucosamine transferase and is believed to act as a repressor of GA signalling . SPY is expressed during germination and spy mutants were identified in screens for mutants that were PAC insensitive during germination (Jacobsen and Olszewski 1993;Swain et al 2002). The spy mutant does not show insensitivity to glucose during germination but in several independent experiments was somewhat more glucose sensitive compared to WT (Fig.…”

Global Analysis of DELLA Direct Targets in Early Gibberellin Signaling inArabidopsis

“…The animal OGTs modify target proteins by glycosylation of Ser/ Thr residues, which either interfere or compete with kinases for phosphorylation sites (Wells et al, 2001). The recombinant SPY protein exhibits OGT activity in vitro (Thornton et al, 1999), and like animal OGTs, SPY is localized to the cytoplasm and nucleus (Swain et al, 2002). Target proteins of SPY have not been identified, but potential candidates include the DELLA proteins Shimada et al, 2006;Silverstone et al, 2007).…”