Altered expression of SPINDLY affects gibberellin response and plant development

SECRET AGENT and SPINDLY have overlapping roles in the development of Arabidopsis thaliana L. Heyn

O-GlcNAc transferase (OGT) catalyses transfer of GlcNAc (N-acetylglucosamine) to serine or threonine of proteins. The Arabidopsis OGTs, SECRET AGENT (SEC) and SPINDLY (SPY) have overlapping functions during gametogenesis and embryogenesis. SPY functions in a number of processes including circadian, light, and gibberellin (GA) responses. The role of SEC in plant development and GA signalling was investigated by determining the phenotypes of sec-1 and sec-2 plants and the expression pattern of SEC. Similar to SPY, SEC transcripts were ubiquitous. Although there is no evidence of transcript-level regulation by other factors, SEC mRNA levels are elevated in spy plants and SPY mRNA levels are elevated in sec plants. sec-1 and sec-2 plants exhibited few of the defects observed in spy plants and had wild-type GA responses. Compared with wild type, sec plants produced leaves at a reduced rate. Haplo-insufficiency at SEC in a spy ga1 double mutant background suppressed spy during germination and enhanced the production of ovaries with four carpels by spy. By contrast, SPY haplo-insufficiency in a sec ga1 double mutant background caused a novel phenotype, production of a proliferation of pin-like structures instead of a floral shoot. These results are consistent with SEC function overlapping with SPY for leaf production and reproductive development.

The hexosamine signaling pathway: deciphering the "O-GlcNAc code"

A dynamic cycle of addition and removal of O-linked N-acetylglucosamine (O-GlcNAc) at serine and threonine residues is emerging as a key regulator of nuclear and cytoplasmic protein activity. Like phosphorylation, protein O-GlcNAcylation dramatically alters the posttranslational fate and function of target proteins. Indeed, O-GlcNAcylation may compete with phosphorylation for certain Ser/Thr target sites. Like kinases and phosphatases, the enzymes of O-GlcNAc metabolism are highly compartmentalized and regulated. Yet, O-GlcNAc addition is subject to an additional and unique level of metabolic control. O-GlcNAc transfer is the terminal step in a "hexosamine signaling pathway" (HSP). In the HSP, levels of uridine 5'-diphosphate (UDP)-GlcNAc respond to nutrient excess to activate O-GlcNAcylation. Removal of O-GlcNAc may also be under similar metabolic regulation. Differentially targeted isoforms of the enzymes of O-GlcNAc metabolism allow the participation of O-GlcNAc in diverse intracellular functions. O-GlcNAc addition and removal are key to histone remodeling, transcription, proliferation, apoptosis, and proteasomal degradation. This nutrient-responsive signaling pathway also modulates important cellular pathways, including the insulin signaling cascade in animals and the gibberellin signaling pathway in plants. Alterations in O-GlcNAc metabolism are associated with various human diseases including diabetes mellitus and neurodegeneration. This review will focus on current approaches to deciphering the "O-GlcNAc code" in order to elucidate how O-GlcNAc participates in its diverse functions. This ongoing effort requires analysis of the enzymes of O-GlcNAc metabolism, their many targets, and how the O-GlcNAc modification may be regulated.

A Caenorhabditis elegans model of insulin resistance: altered macronutrient storage and dauer formation in an OGT-1 knockout

O-linked N-acetylglucosamine (O-GlcNAc) is an evolutionarily conserved modification of nuclear pore proteins, signaling kinases, and transcription factors. The O-GlcNAc transferase (OGT) catalyzing O-GlcNAc addition is essential in mammals and mediates the last step in a nutrient-sensing "hexosamine-signaling pathway." This pathway may be deregulated in diabetes and neurodegenerative disease. To examine the function of O-GlcNAc in a genetically amenable organism, we describe a putative null allele of OGT in Caenorhabditis elegans that is viable and fertile. We demonstrate that, whereas nuclear pore proteins of the homozygous deletion strain are devoid of O-GlcNAc, nuclear transport of transcription factors appears normal. However, the OGT mutant exhibits striking metabolic changes manifested in a approximately 3-fold elevation in trehalose levels and glycogen stores with a concomitant approximately 3-fold decrease in triglycerides levels. In nematodes, a highly conserved insulin-like signaling cascade regulates macronutrient storage, longevity, and dauer formation. The OGT knockout suppresses dauer larvae formation induced by a temperature-sensitive allele of the insulin-like receptor gene daf-2. Our findings demonstrate that OGT modulates macronutrient storage and dauer formation in C. elegans, providing a unique genetic model for examining the role of O-GlcNAc in cellular signaling and insulin resistance.

Plants with increased expression of ent-kaurene oxidase are resistant to chemical inhibitors of this gibberellin biosynthesis enzyme

The gibberellin (GA) biosynthetic pathway includes the three-step oxidation of ent-kaurene to ent-kaurenoic acid, catalyzed by the enzyme ent-kaurene oxidase (KO). Arabidopsis plants overexpressing the KO cDNA under the control of the cauliflower mosaic virus 35S promoter, with or without a translational fusion to a modified green fluorescent protein (GFP), are very similar to wild-type (WT) plants under normal growth conditions. In contrast, when WT and 35S:KO (or 35S:KO-GFP) seeds, seedlings or pollen tubes are grown in the presence of chemical inhibitors of KO, such as paclobutrazol and uniconazole, plants with increased KO expression are partially resistant to the effects of these inhibitors. In combination with the observation that decreased KO levels increase the sensitivity to KO inhibitors, the 35S:KO phenotypes demonstrate that the modification of KO enzyme levels could be used to create transgenic crop plants with altered KO inhibitor response. These results also suggest that the KO gene could be used as a selectable marker for plant regeneration based on resistance to KO inhibitors. Finally, the observation that pollen tubes expressing 35S:KO or 35S:KO-GFP have decreased sensitivity to KO inhibitors provides further evidence for a physiological role for GAs in pollen tube elongation.

Not just another hole in the wall: understanding intercellular protein trafficking

Development and differentiation of multicellular organisms requires cell-to-cell communication. In plants direct signaling and exchange of macromolecules between cells is possible through plasmodesmata. Recently direct exchange of membrane-bound vesicles and organelles has been demonstrated between animal cells through formation of cytoplasmic bridges (tunneling nanotubes) in vitro. Here we review recent developments in cell-to-cell trafficking of macromolecules in plants and animals.

Gibberellin metabolism and signaling

Gibberellins (GAs) are a family of plant hormones controlling many aspects of plant growth and development including stem elongation, germination, and the transition from vegetative growth to flowering. Cloning of the genes encoding GA biosynthetic and inactivating enzymes has led to numerous insights into the developmental regulation of GA hormone accumulation that is subject to both positive and negative feedback regulation. Genetic and biochemical analysis of GA-signaling genes has revealed that posttranslational regulation of DELLA protein accumulation is a key control point in GA response. The highly conserved DELLA proteins are a family of negative regulators of GA signaling that appear subject to GA-stimulated degradation through the ubiquitin-26S proteasome pathway. This review discusses the regulation of GA hormone accumulation and signaling in the context of its role in plant growth and development.

Cross talk between gibberellin and cytokinin: the Arabidopsis GA response inhibitor SPINDLY plays a positive role in cytokinin signaling

SPINDLY (SPY) is a negative regulator of gibberellin (GA) responses; however, spy mutants exhibit various phenotypic alterations not found in GA-treated plants. Assaying for additional roles for SPY revealed that spy mutants are resistant to exogenously applied cytokinin. GA also repressed the effects of cytokinin, suggesting that there is cross talk between the two hormone-response pathways, which may involve SPY function. Two spy alleles showing severe (spy-4) and mild (spy-3) GA-associated phenotypes exhibited similar resistance to cytokinin, suggesting that SPY enhances cytokinin responses and inhibits GA signaling through distinct mechanisms. GA and spy repressed numerous cytokinin responses, from seedling development to senescence, indicating that cross talk occurs early in the cytokinin-signaling pathway. Because GA3 and spy-4 inhibited induction of the cytokinin primary-response gene, type-A Arabidopsis response regulator 5, SPY may interact with and modify elements from the phosphorelay cascade of the cytokinin signal transduction pathway. Cytokinin, on the other hand, had no effect on GA biosynthesis or responses. Our results demonstrate that SPY acts as both a repressor of GA responses and a positive regulator of cytokinin signaling. Hence, SPY may play a central role in the regulation of GA/cytokinin cross talk during plant development.

SPINDLY and GIGANTEA interact and act in Arabidopsis thaliana pathways involved in light responses, flowering, and rhythms in cotyledon movements

SPINDLY (SPY) is a negative regulator of gibberellin signaling in Arabidopsis thaliana that also functions in previously undefined pathways. The N terminus of SPY contains a protein-protein interaction domain consisting of 10 tetratricopeptide repeats (TPRs). GIGANTEA (GI) was recovered from a yeast two-hybrid screen for proteins that interact with the TPR domain. GI and SPY also interacted in Escherichia coli and in vitro pull-down assays. The phenotypes of spy and spy-4 gi-2 plants support the hypothesis that SPY functions with GI in pathways controlling flowering, circadian cotyledon movements, and hypocotyl elongation. GI acts in the long-day flowering pathway upstream of CONSTANS (CO) and FLOWERING LOCUS T (FT). Loss of GI function causes late flowering and reduces CO and FT RNA levels. Consistent with SPY functioning in the long-day flowering pathway upstream of CO, spy-4 partially suppressed the reduced abundance of CO and FT RNA and the late flowering of gi-2 plants. Like gi, spy affects the free-running period of cotyledon movements. The free-running period was lengthened in spy-4 mutants and shortened in plants that overexpress SPY under the control of the 35S promoter of Cauliflower mosaic virus. When grown under red light, gi-2 plants have a long hypocotyl. This hypocotyl phenotype was suppressed in spy-4 gi-2 double mutants. Additionally, dark-grown and far-red-light-grown spy-4 seedlings were found to have short and long hypocotyls, respectively. The different hypocotyl length phenotypes of spy-4 seedlings grown under different light conditions are consistent with SPY acting in the GA pathway to inhibit hypocotyl elongation and also acting as a light-regulated promoter of elongation.

Numerous posttranslational modifications provide opportunities for the intricate regulation of metabolic enzymes at multiple levels

The metabolic plasticity displayed by plants during normal development, and in response to environmental fluctuations and stressors, is essential for their growth and survival. The capacity to regulate metabolic enzymes intricately arises in part from posttranslational modifications that can affect enzymatic activity, intracellular localization, protein-protein interactions, and stability. Protein phosphorylation and thiol/disulfide redox modulation are important modifications in plants, and it is likely that O-glycosylation and S-nitrosylation will also emerge as important mechanisms. Recent advances in the field of proteomics, in particular the development of novel and specific chemistries for the detection of a diverse number of modifications, are rapidly expanding our awareness of possible modifications and our understanding of the enzymes whose functions are likely to be regulated posttranslationally.

The gar2 and rga alleles increase the growth of gibberellin-deficient pollen tubes in Arabidopsis

Ectopic expression in Arabidopsis of a pea (Pisum sativum) cDNA (2ox2) encoding a gibberellin (GA) 2-oxidase (PsGA2ox2), involved in the deactivation of biologically active GAs, has been used to establish a role for GAs in promoting pollen tube growth. One line, 35S:2ox2/28c, when homozygous for the transgene, exhibits a novel small fruit phenotype. The 28c transgene reduces pollen tube growth, and this results in a reduced number of fertilized seeds that are only present at the end of the silique nearest the stigma. To confirm that the 28c pollen tube phenotype is due to sense expression of the 2ox2 mRNA, a "hairpin" RNA interface silencing construct, designed to silence 2ox2 expression, has been used to restore pollen tube growth and fruit development. The interaction between 28c and other mutants with increased GA response has also been examined to provide further evidence that GAs play an important role in pollen tube growth. Based on the ability of mutant alleles to suppress the 35S:2ox2/28c phenotype, we define new roles for the gar2-1 and rga alleles in GA signaling during pollen tube elongation in addition to their previously established roles in vegetative tissues. In contrast to the constitutive GA response observed in internodes and leaves lacking RGA and GAI, the rga-2 gai-d5 mutant combination is only a partial suppressor of the 28c phenotype. Because the dominant dwarfing gai-1 allele reduces GA response in vegetative tissues, its effect on plant fertility has been examined. Although gai-1 reduces seed set, this appears to reflect defects in reproductive development other than pollen tube function. Finally, we show that the genetic background (Landsberg erecta or Columbia) modifies the 28c phenotype and that this effect is not due to the ER/er difference between these two ecotypes.

Molecular mechanism of gibberellin signaling in plants

The hormone gibberellin (GA) plays an important role in modulating diverse processes throughout plant development. In recent years, significant progress has been made in the identification of upstream GA signaling components and trans- and cis-acting factors that regulate downstream GA-responsive genes in higher plants. GA appears to derepress its signaling pathway by inducing proteolysis of GA signaling repressors (the DELLA proteins). Recent evidence indicates that the DELLA proteins are targeted for degradation by an E3 ubiquitin ligase SCF complex through the ubiquitin-26S proteasome pathway.

Thermoinduction of genes encoding the enzymes of gibberellin biosynthesis and a putative negative regulator of gibberellin signal transduction in Eustoma grandiflorum

Eustoma grandiflorum Shinn requires vernalization for the induction of stem elongation and flowering. To investigate the role of gibberellins (GAs) in vernalization, the expression levels of genes encoding enzymes of GA biosynthesis, copalyl diphosphate synthetase, GA 20-oxidase and GA 3 beta-hydroxylase, were examined using two culitvars that show different responses to vernalization. The three genes were induced in a vernalization- and a cultivar-dependent manner. EgSPY, a putative negative regulator of GA signal transduction, was also induced during the vernalization period. The results suggest that the expression of the genes encoding GAs biosynthesis is regulated by vernalization. We postulate that EgSPY functions as a negative regulator of GA signal transduction during vernalization, inhibiting adventitious shoot elongation during vernalization.

Gibberellins are required for seed development and pollen tube growth in Arabidopsis

Gibberellins (GAs) are tetracyclic diterpenoids that are essential endogenous regulators of plant growth and development. GA levels within the plant are regulated by a homeostatic mechanism that includes changes in the expression of a family of GA-inactivating enzymes known as GA 2-oxidases. Ectopic expression of a pea GA 2-oxidase2 cDNA caused seed abortion in Arabidopsis, extending and confirming previous observations obtained with GA-deficient mutants of pea, suggesting that GAs have an essential role in seed development. A new physiological role for GAs in pollen tube growth in vivo also has been identified. The growth of pollen tubes carrying the 35S:2ox2 transgene was reduced relative to that of nontransgenic pollen, and this phenotype could be reversed partially by GA application in vitro or by combining with spy-5, a mutation that increases GA response. Treatment of wild-type pollen tubes with an inhibitor of GA biosynthesis in vitro also suggested that GAs are required for normal pollen tube growth. These results extend the known physiological roles of GAs in Arabidopsis development and suggest that GAs are required for normal pollen tube growth, a physiological role for GAs that has not been established previously.

GIGANTEA and SPINDLY genes linked to the clock pathway that controls circadian characteristics of transpiration in Arabidopsis

Several "clock" genes that regulate the circadian system in Arabidopsis thaliana have been identified. The GIGANTEA (GI) gene has been shown to participate in the circadian system that is linked to overt rhythms in gene expression, leaf movements, hypocotyl elongation, and photoperiodic control of flowering in Arabidopsis. During continuous light (LL), circadian expression patterns in gi-2 mutants show reduced amplitudes and altered period lengths when compared with controls. Rhythms in stomatal function, such as transpiration, have been shown to be endogenous and persist in constant lighting conditions. In order to test for a physiologic variable that might be affected by the circadian clock via the GI gene, we compared circadian characteristics of transpiration between three Arabidopsis mutants (gi-2, spy-4, spy-4/gi-2) and wild-type (WT) controls in synchronized (LD for 2.5d) and free-running (LL for 3d) conditions. Each genotype showed a significant circadian rhythm in LD at p < 0.001, with acrophases located near the middle of the daily 14h L-span, with average amplitudes for WT: 18.9%, gi-2: 16.1%, spy-4: 7.7%, and spy-4/gi-2: 5.3%. On the first day in LL, the circadian amplitude was dramatically reduced to 3.1% for gi-2 compared with WT (11.9%), while amplitudes for spy-4 (6.9%) and spy-4/gi-2 (5.7%) were not significantly changed from LD. The amplitude for gi-2 remained low during days 2 (4.2%) and 3 (2.1%) in LL, while it slowly dampened for the WT (8.6 and 6.6%). The amplitudes for spy-4 (6.6%) and spy-4/gi-2 (5.6%) on day 2 in LL were indistinguishable from the LD span, but finally dampened on day 3 in LL (1.9 and 2.3%, respectively). These data suggest that transpiration is a physiologic variable controlled by a circadian system that involves both the GI and SPY proteins.

Two O-linked N-acetylglucosamine transferase genes of Arabidopsis thaliana L. Heynh. have overlapping functions necessary for gamete and seed development

The Arabidopsis SECRET AGENT (SEC) and SPINDLY (SPY) proteins are similar to animal O-linked N-acetylglucosamine transferases (OGTs). OGTs catalyze the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to Ser/Thr residues of proteins. In animals, O-GlcNAcylation has been shown to affect protein activity, stability, and/or localization. SEC protein expressed in Escherichia coli had autocatalytic OGT activity. To determine the function of SEC in plants, two tDNA insertional mutants were identified and analyzed. Although sec mutant plants did not exhibit obvious phenotypes, sec and spy mutations had a synthetic lethal interaction. This lethality was incompletely penetrant in gametes and completely penetrant postfertilization. The rate of both female and male sec spy gamete transmission was higher in plants heterozygous for both mutations than in plants heterozygous for sec and homozygous for spy. Double-mutant embryos aborted at various stages of development and no double-mutant seedlings were obtained. These results indicate that OGT activity is required during gametogenesis and embryogenesis with lethality occurring when parentally derived SEC, SPY, and/or O-GlcNAcylated proteins become limiting.

Gibberellin regulates post-microsporogenesis processes in petunia anthers

Previous studies have suggested that gibberellins (GAs) are produced in petunia anthers and transported to the corolla to induce growth and pigmentation. In this work, we studied the role of GA in the regulation of anther development. When petunia plants were treated with the GA-biosynthesis inhibitor paclobutrazol, anther development was arrested. Microscopic analysis of these anthers revealed that paclobutrazol inhibits post-meiotic developmental processes. The treated anthers contained pollen grains but the connective tissue and tapetum cells were degenerated. A similar phenotype was obtained when the Arabidopsis GA-signal repressor, SPY, was over-expressed in transgenic petunia plants, i.e. anther development was arrested following microsporogenesis. The expression of the GA-induced gene, GIP, can be used in petunia as a molecular marker to study GA responses. GA3 treatment of young anthers promoted, and paclobutrazol inhibited, GIP expression, suggesting that the hormone controls the natural activation of the gene in the anthers. Analyses of GIP expression during anther development revealed that the gene is induced only after microsporogenesis. This observation further suggests a role for GA in the regulation of post-meiotic processes during petunia anther development.

SPINDLY is a nuclear-localized repressor of gibberellin signal transduction expressed throughout the plant

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.

The role of SPY and its TPR domain in the regulation of gibberellin action throughout the life cycle of Petunia hybrida plants

SPY acts as a negative regulator of gibberellin (GA) action in Arabidopsis, but its mode of action and regulation are still unknown. SPY over-expression in transgenic petunia plants affected various GA-regulated processes, including seed germination, shoot elongation, flower initiation, flower development and the expression of a GA-induced gene, GIP. A similar phenotype was obtained when wild-type petunia plants were treated with the GA-biosynthesis inhibitor, paclobutrazol. The N-terminus of SPY contains tetratricopeptide repeats (TPR). TPR motifs participate in protein-protein interactions, suggesting that SPY is part of a multiprotein complex. To test this hypothesis, we over-expressed the SPY's TPR region without the catalytic domain in transgenic petunia and generated a dominant-negative SPY mutant. The transgenic seeds were able to germinate on paclobutrazol, suggesting an enhanced GA signal. We cloned the petunia SPY homologue, PhSPY, and showed that its mRNA level is not affected by GA or ABA. The results of this study support the role of SPY as a negative regulator of GA action, suggest that the TPR domain is required for the interaction with other proteins to form an active complex and indicate that different plants use similar mechanisms to transduce the GA signal.

Ectopic expression of the tetratricopeptide repeat domain of SPINDLY causes defects in gibberellin response

The SPINDLY (SPY) protein of Arabidopsis is a negative regulator of gibberellin (GA) response. The SPY protein has 10 copies of the tetratricopeptide repeat (TPR) at the N terminus. TPR motifs function as protein-protein interaction domains. Several spy alleles are affected only in the TPR region suggesting that protein-protein interactions mediated by this domain are important for proper GA signaling. We have used a reverse genetics approach to further investigate the role of the TPR domain. The TPR domain of SPY was overexpressed in wild-type, gai, and spy plants. Expression of the TPR domain alone is not sufficient to rescue spy mutants. Expression of the TPR domain in a wild-type background produces phenotypes similar to those caused by loss-of-function spy mutants including resistance to GA biosynthesis inhibitors, short hypocotyl length, and early flowering. The dwarfing of the floral shoot internodes caused by the gai mutation was suppressed by expression of the TRP domain. Expression of the TPR domain had no effect on the abundance of endogenous SPY mRNA. The TPR domain was found to interact with SPY both in vitro and in yeast two-hybrid assays. These data indicate that the TPR domain of SPY can participate in protein-protein interactions and that these interactions are important for the proper functioning of SPY.