Isolation of cDNA of an auxin-regulated gene encoding a G protein beta subunit-like protein from tobacco BY-2 cells

Proteomic analysis and interactions network in leaves of mycorrhizal and nonmycorrhizal sorghum plants under water deficit

For understanding the water deficit stress mechanism in sorghum, we conducted a physiological and proteomic analysis in the leaves of Sorghum bicolor L. Moench (a drought tolerant crop model) of non-colonized and colonized plants with a consortium of arbuscular mycorrhizal fungi. Physiological results indicate that mycorrhizal fungi association enhances growth and photosynthesis in plants, under normal and water deficit conditions. 2D-electrophoresis profiles revealed 51 differentially accumulated proteins in response to water deficit, of which HPLC/MS successfully identified 49. Bioinformatics analysis of protein–protein interactions revealed the participation of different metabolic pathways in nonmycorrhizal compared to mycorrhizal sorghum plants under water deficit. In noninoculated plants, the altered proteins are related to protein synthesis and folding (50S ribosomal protein L1, 30S ribosomal protein S10, Nascent polypeptide-associated complex subunit alpha), coupled with multiple signal transduction pathways, guanine nucleotide-binding beta subunit (Rack1) and peptidyl-prolyl-cis-trans isomerase (ROC4). In contrast, in mycorrhizal plants, proteins related to energy metabolism (ATP synthase-24kDa, ATP synthase β), carbon metabolism (malate dehydrogenase, triosephosphate isomerase, sucrose-phosphatase), oxidative phosphorylation (mitochondrial-processing peptidase) and sulfur metabolism (thiosulfate/3-mercaptopyruvate sulfurtransferase) were found. Our results provide a set of proteins of different metabolic pathways involved in water deficit produced by sorghum plants alone or associated with a consortium of arbuscular mycorrhizal fungi isolated from the tropical rain forest Los Tuxtlas Veracruz, México.

Profiling of the Receptor for Activated C Kinase 1a (RACK1a) interaction network in Arabidopsis thaliana

As a scaffold protein, Receptor for Activated C Kinase 1a (RACK1) interacts with many proteins and is involved in multiple biological processes in Arabidopsis. However, the global RACK1 protein interaction network in higher plants remains poorly understood. Here, we generated a yeast two-hybrid library using mixed samples from different developmental stages of Arabidopsis thaliana. Using RACK1a as bait, we performed a comprehensive screening of the resulting library to identify RACK1a interactors at the whole-transcriptome level. We selected 1065 independent positive clones that led to the identification of 215 RACK1a interactors. We classified these interactors into six groups according to their potential functions. Several interactors were selected for bimolecular fluorescence complementation (BiFC) analysis and their interaction with RACK1a was confirmed in vivo. Our results provide further insight into the molecular mechanisms through which RACK1a regulates various growth and development processes in higher plants.

Functional analysis of drought and salt tolerance mechanisms of mulberry RACK1 gene

The receptor for activated C kinase 1 (RACK1) protein acts as a central hub for the integration of many physiological processes in eukaryotic organisms. Plant RACK1 is implicated in abiotic stress responses, but the underlying molecular mechanisms of stress adaptation remain largely unknown. Here, the overexpression of the mulberry (Morus alba L.) RACK1 gene in Arabidopsis decreased tolerance to drought and salt stresses and MaRACK1 overexpression changed expression levels of genes in response to stress and stimuli. We developed a simple and efficient transient transformation system in mulberry, and the mulberry seedlings transiently expressing MaRACK1 were hypersensitive to drought and salt stresses. The expression levels of guanine nucleotide-binding protein (G-protein) encoding genes in mulberry and Arabidopsis were not affected by MaRACK1 overexpression. The interactions between RACK1 and G-proteins were confirmed, and the RACK1 proteins from mulberry and Arabidopsis could not interact with their respective G-proteins, which indicated that RACK1 may regulate stress responses independently of G-proteins. Additionally, MaRACK1 may regulate drought and salt stress tolerances by interacting with a fructose 1, 6-bisphosphate aldolase. Our findings provide new insights into the mechanisms underlying RACK1 functions in abiotic stress responses and important information for their further characterization.

Integration of RACK1 and ethylene signaling regulates plant growth and development in Arabidopsis

Arabidopsis RACK1 (Receptors for Activated C Kinase 1) are versatile scaffold proteins that have been shown to be involved in the regulation of plant response to plant hormones including auxin, ABA, gibberellin and brassinosteroid, but not ethylene. By characterizing the double and triple mutants of RACK1 genes, we found that rack1 mutants showed reduced sensitivity to ethylene. By characterizing double and high order mutants generated between ein2, a loss-of-function mutant of the key ethylene signaling regulator gene EIN2 (Ethylene INsensitive 2), and rack1 mutants, we found that loss-of-function of EIN2 partially recovered some phenotypes observed in the rack1 mutants, such as low-fertility and reduced root length and rosette size. On the other hand, the ein2 rack1 mutants produced more rosette leaves, and flowered late when compared with ein2 and the corresponding rack1 mutants. We also found that the curled leaves and twisted petioles phenotypes observed in the ein2 mutants were enhanced in the ein2 rack1 mutants. However, assays in yeast indicated that EIN2 may not physically interact with RACK1. On the other hand, RT-PCR results showed that the expression level of EIN2 was reduced in the rack1 mutants. Taken together, our results suggest that RACKl may integrate ethylene signaling to regulate plant growth and development in Arabidopsis.

Kale BoRACK1 is involved in the plant response to salt stress and Peronospora brassicae Gaumann

The receptor for activated C kinase 1 (RACK1) belongs to a protein subfamily containing a tryptophan-aspartic acid-domain (WD) repeat structure. Compelling evidence indicates that RACK1 can interact with many signal molecules and affect different signal transduction pathways. In this study, a kale (Brassica oleracea var. acephala f.tricolor) RACK1 gene (BoRACK1) was cloned by RT-PCR. The amino acid sequence of BoRACK1 had seven WD repeats in which there were typical GH (glycine-histidine) and WD dipeptides. Comparison with AtRACK1 from Arabidopsis revealed 87.1% identity at the amino acid level. Expression pattern analysis by RT-PCR showed that BoRACK1 was expressed in all analyzed tissues of kale and that its transcription in leaves was down-regulated by salt, abscisic acid, and H₂O₂ at a high concentration. Overexpression of BoRACK1 in kale led to a reduction in symptoms caused by Peronospora brassicae Gaumann on kale leaves. The expression levels of the pathogenesis-related protein genes, PR-1 and PRB-1, increased 2.5-4-fold in transgenic kale, and reactive oxygen species production was more active than in the wild-type. They also exhibited increased tolerance to salt stress in seed germination. H₂O₂ may also be involved in the regulation of BoRACK1 during seed germination under salt stress. Quantitative real-time PCR analyses showed that the transcript levels of BoRbohs genes were significantly higher in overexpression of BoRACK1 transgenic lines. Yeast two-hybrid assays showed that BoRACK1 could interact with WNK8, eIF6, RAR1, and SGT1. This study and previous work lead us to believe that BoRACK1 may form a complex with regulators of plant salt and disease resistance to coordinate kale reactions to pathogens.

Inhibition of Cycloartenol Synthase (CAS) Function in Tobacco BY-2 Cell Suspensions: A Proteomic Analysis

The effect of an inhibitor of cycloartenol synthase (CAS, EC 5.4.99.8) on the proteome of tobacco BY-2 cells has been examined. CAS catalyzes the first committed step in phytosterol synthesis in plants. BY-2 cells were treated with RO 48-8071, a potent inhibitor of oxidosqualene cyclization. Proteins were separated by two-dimensional electrophoresis and spots, that clearly looked differentially accumulated after visual inspection, were cut, in-gel trypsin digested, and peptides were analyzed by nano-HPLC-MS/MS. Distinct peptides were compared to sequences in the data banks and attributed to corresponding proteins and genes. Inhibition of CAS induced proteins that appear to mitigate the negative effects of the chemical exposure. However, as all enzymes that are directly involved in phytosterol biosynthesis are low-abundant proteins, significant changes in their levels could not be observed. Differences could be seen with enzymes involved in primary metabolism (glycolysis, pentose phosphate pathway etc.), in proteins of the chaperonin family, and those, like actin, that participate in formation and strengthening of the cytoskeleton and have some impact on cell growth and division.

Arabidopsis receptor of activated C kinase1 phosphorylation by WITH NO LYSINE8 KINASE

Receptor of activated C kinase1 (RACK1) is a versatile scaffold protein that binds to numerous proteins to regulate diverse cellular pathways in mammals. In Arabidopsis (Arabidopsis thaliana), RACK1 has been shown to regulate plant hormone signaling, stress responses, and multiple processes of growth and development. However, little is known about the molecular mechanism underlying these regulations. Here, we show that an atypical serine (Ser)/threonine (Thr) protein kinase, WITH NO LYSINE8 (WNK8), phosphorylates RACK1. WNK8 physically interacted with and phosphorylated RACK1 proteins at two residues: Ser-122 and Thr-162. Genetic epistasis analysis of rack1 wnk8 double mutants indicated that RACK1 acts downstream of WNK8 in the glucose responsiveness and flowering pathways. The phosphorylation-dead form, RACK1A(S122A/T162A), but not the phosphomimetic form, RACK1A(S122D/T162E), rescued the rack1a null mutant, implying that phosphorylation at Ser-122 and Thr-162 negatively regulates RACK1A function. The transcript of RACK1A(S122D/T162E) accumulated at similar levels as those of RACK1(S122A/T162A). However, although the steady-state level of the RACK1A(S122A/T162A) protein was similar to wild-type RACK1A protein, the RACK1A(S122D/T162E) protein was nearly undetectable, suggesting that phosphorylation affects the stability of RACK1A proteins. Taken together, these results suggest that RACK1 is phosphorylated by WNK8 and that phosphorylation negatively regulates RACK1 function by influencing its protein stability.

The Receptor for Activated C Kinase in Plant Signaling: Tale of a Promiscuous Little Molecule

Two decades after the first report of the plant homolog of the Receptor for Activated C Kinase 1 (RACK1) in cultured tobacco BY2 cells, a significant advancement has been made in the elucidation of its cellular and molecular role. The protein is now implicated in many biological functions including protein translation, multiple hormonal responses, developmental processes, pathogen infection resistance, environmental stress responses, and miRNA production. Such multiple functional roles are consistent with the scaffolding nature of the plant RACK1 protein. A significant advance was achieved when the β-propeller structure of the Arabidopsis RACK1A isoform was elucidated, thus revealing that its conserved seven WD repeats also assembled into this typical topology. From its crystal structure, it became apparent that it shares the structural platform for the interaction with ligands identified in other systems such as mammals. Although RACK1 proteins maintain conserved Protein Kinase C binding sites, the lack of a bona fide PKC adds complexity and enigma to the nature of the ligand partners with which RACK1 interacts in plants. Nevertheless, ligands recently identified using the split-ubiquitin based and conventional yeast two-hybrid assays, have revealed that plant RACK1 is involved in several processes that include defense response, drought and salt stress, ribosomal function, cell wall biogenesis, and photosynthesis. The information acquired indicates that, in spite of the high degree of conservation of its structure, the functions of the plant RACK1 homolog appear to be distinct and diverse from those in yeast, mammals, insects, etc. In this review, we take a critical look at the novel information regarding the many functions in which plant RACK1 has been reported to participate, with a special emphasis on the information on its currently identified and missing ligand partners.

In tobacco BY-2 cells xyloglucan oligosaccharides alter the expression of genes involved in cell wall metabolism, signalling, stress responses, cell division and transcriptional control

Xyloglucan oligosaccharides (XGOs) are breakdown products of XGs, the most abundant hemicelluloses of the primary cell walls of non-Poalean species. Treatment of cell cultures or whole plants with XGOs results in accelerated cell elongation and cell division, changes in primary root growth, and a stimulation of defence responses. They may therefore act as signalling molecules regulating plant growth and development. Previous work suggests an interaction with auxins and effects on cell wall loosening, however their mode of action is not fully understood. The effect of an XGO extract from tamarind (Tamarindus indica) on global gene expression was therefore investigated in tobacco BY-2 cells using microarrays. Over 500 genes were differentially regulated with similar numbers and functional classes of genes up- and down-regulated, indicating a complex interaction with the cellular machinery. Up-regulation of a putative XG endotransglycosylase/hydrolase-related (XTH) gene supports the mechanism of XGO action through cell wall loosening. Differential expression of defence-related genes supports a role for XGOs as elicitors. Changes in the expression of genes related to mitotic control and differentiation also support previous work showing that XGOs are mitotic inducers. XGOs also affected expression of several receptor-like kinase genes and transcription factors. Hence, XGOs have significant effects on expression of genes related to cell wall metabolism, signalling, stress responses, cell division and transcriptional control.

OsRACK1 is involved in abscisic acid- and H2O2-mediated signaling to regulate seed germination in rice (Oryza sativa, L.)

The receptor for activated C kinase 1 (RACK1) is one member of the most important WD repeat–containing family of proteins found in all eukaryotes and is involved in multiple signaling pathways. However, compared with the progress in the area of mammalian RACK1, our understanding of the functions and molecular mechanisms of RACK1 in the regulation of plant growth and development is still in its infancy. In the present study, we investigated the roles of rice RACK1A gene (OsRACK1A) in controlling seed germination and its molecular mechanisms by generating a series of transgenic rice lines, of which OsRACK1A was either over-expressed or under-expressed. Our results showed that OsRACK1A positively regulated seed germination and negatively regulated the responses of seed germination to both exogenous ABA and H₂O₂. Inhibition of ABA biosynthesis had no enhancing effect on germination, whereas inhibition of ABA catabolism significantly suppressed germination. ABA inhibition on seed germination was almost fully recovered by exogenous H₂O₂ treatment. Quantitative analyses showed that endogenous ABA levels were significantly higher and H₂O₂ levels significantly lower in OsRACK1A-down regulated transgenic lines as compared with those in wildtype or OsRACK1A-up regulated lines. Quantitative real-time PCR analyses showed that the transcript levels of OsRbohs and amylase genes, RAmy1A and RAmy3D, were significantly lower in OsRACK1A-down regulated transgenic lines. It is concluded that OsRACK1A positively regulates seed germination by controlling endogenous levels of ABA and H₂O₂ and their interaction.

Maize ZmRACK1 is involved in the plant response to fungal phytopathogens

The receptor for activated C kinase 1 (RACK1) belongs to a protein subfamily containing a tryptophan-aspartic acid-domain (WD) repeat structure. Compelling evidence indicates that RACK1 can interact with many signal molecules and affect different signal transduction pathways. In this study, we cloned a maize RACK1 gene (ZmRACK1) by RT-PCR. The amino acid sequence of ZmRACK1 had seven WD repeats in which there were typical GH (glycine-histidine) and WD dipeptides. Comparison with OsRACK1 from rice revealed 89% identity at the amino acid level. Expression pattern analysis by RT-PCR showed that ZmRACK1 was expressed in all analyzed tissues of maize and that its transcription in leaves was induced by abscisic acid and jasmonate at a high concentration. Overexpression of ZmRACK1 in maize led to a reduction in symptoms caused by Exserohilum turcicum (Pass.) on maize leaves. The expression levels of the pathogenesis-related protein genes, PR-1 and PR-5, increased 2.5-3 times in transgenic maize, and reactive oxygen species production was more active than in the wild-type. Yeast two-hybrid assays showed that ZmRACK1 could interact with RAC1, RAR1 and SGT1. This study and previous work leads us to believe that ZmRACK1 may form a complex with regulators of plant disease resistance to coordinate maize reactions to pathogens.

Arabidopsis scaffold protein RACK1A modulates rare sugar D-allose regulated gibberellin signaling

As energy sources and structural components, sugars are the central regulators of plant growth and development. In addition to the abundant natural sugars in plants, more than 50 different kinds of rare sugars exist in nature, several of which show distinct roles in plant growth and development. Recently, one of the rare sugars, D-allose, an epimer of D-glucose at C3, is found to suppress plant hormone gibberellin (GA) signaling in rice. Scaffold protein RACK1A in the model plant Arabidopsis is implicated in the GA pathway as rack1a knockout mutants show insensitivity to GA in GA-induced seed germination. Using genetic knockout lines and a reporter gene, the functional role of RACK1A in the D-allose pathway was investigated. It was found that the rack1a knockout seeds showed hypersensitivity to D-allose-induced inhibition of seed germination, implicating a role for RACK1A in the D-allose mediated suppression of seed germination. On the other hand, a functional RACK1A in the background of the double knockout mutations in the other two RACK1 isoforms, rack1b/rack1c, showed significant resistance to the D-allose induced inhibition of seed germination. The collective results implicate the RACK1A in the D-allose mediated seed germination inhibition pathway. Elucidation of the rare sugar signaling mechanism will help to advance understanding of this less studied but important cellular signaling pathway.

A crosstalk of auxin and GA during tuber development

Several hormones have been studied for their effect on tuber initiation and development. Until recently, the hormone with the most prominent role in tuber initiation was attributed to GA. Genes involved in GA degradation do exhibit an upregulated profile during early stages of tuber development, leading to a rapid decrease of active GA content, thereby facilitating stolon-tip swelling. While GA is known to be involved in shoot and stolon elongation, the development of the new tuberorgan requires changes in meristem identity and the reorientation ofthe plane of cell division. In other developmental processes, such as embryo patterning, flower development and lateral root initiation auxin plays a key role. Recent evidence on the involvement of auxin in tuber formation was providedby the measurement of auxin content in swelling stolons. Auxin content in the stolon tips increased several fold prior to tuber swelling. In vitro tuberisation experiments with auxin applications support the role of auxin during tuber initiation. Taken together, it is becoming clear that the initiation and induction of tubers in potato is a developmental process that appears to be regulated by a crosstalk between GA and auxin.

Changes in RACK1 expression induce defects in nodulation and development in Phaseolus vulgaris

RACK1 is a scaffold protein with the ability to interact in a regulated manner with a diverse number of ligands from distinct signal-transduction pathways. This assessment allowed us to infer that it may be involved in different processes such as nodulation. In a recent study we showed by silencing, that PvRACK1 has a pivotal role in cell expansion and in symbiosome and bacteroid integrity during nodule development in Phaseolus vulgaris. On the other hand, we have also observed that its over-expression provokes a dramatic phenotype in: (a) seedlings that have been exposed to heat, in which systemic necrosis is induced; and (b) in Agrobacterium rhizogenes-transformed roots, where nodulation is strongly inhibited and nodules show early senescent symptoms. These findings indicate that PvRACK1 may be an integrator of diverse signal-transduction pathways in processes as varied as nodulation, cell expansion, heat stress responses, and systemic activation of necrosis. 

Comparative transcriptional analysis reveals differential gene expression between asymmetric and symmetric zygotic divisions in tobacco

Asymmetric cell divisions occur widely during many developmental processes in plants. In most angiosperms, the first zygotic cell division is asymmetric resulting in two daughter cells of unequal size and with distinct fates. However, the critical molecular mechanisms regulating this division remain unknown. Previously we showed that treatment of tobacco zygotes with beta-glucosyl Yariv (βGlcY) could dramatically alter the first zygotic asymmetric division to produce symmetric two-celled proembryos. In the present study, we isolated zygotes and two-celled asymmetric proembryos in vivo by micromanipulation, and obtained symmetric, two-celled proembryos by in vitro cell cultures. Using suppression-subtractive hybridization (SSH) and macroarray analysis differential gene expression between the zygote and the asymmetric and symmetric two-celled proembryos was investigated. After sequencing of the differentially expressed clones, a total of 1610 EST clones representing 685 non-redundant transcripts were obtained. Gene ontology (GO) term analysis revealed that these transcripts include those involved in physiological processes such as response to stimulus, regulation of gene expression, and localization and formation of anatomical structures. A homology search against known genes from Arabidopsis indicated that some of the above transcripts are involved in asymmetric cell division and embryogenesis. Quantitative real-time PCR confirmed the up- or down-regulation of the selected candidate transcripts during zygotic division. A few of these transcripts were expressed exclusively in the zygote, or in either type of the two-celled proembryos. Expression analyses of select genes in different tissues and organs also revealed potential roles of these transcripts in fertilization, seed maturation and organ development. The putative roles of few of the identified transcripts in the regulation of zygotic division are discussed. Further functional work on these candidate transcripts will provide important information for understanding asymmetric zygotic division, generation of apical-basal polarity and cell fate decisions during early embryogenesis.

PvRACK1 loss-of-function impairs cell expansion and morphogenesis in Phaseolus vulgaris L. root nodules

Receptor for activated C kinase (RACK1) is a highly conserved, eukaryotic protein of the WD-40 repeat family. Its peculiar β-propeller structure allows its interaction with multiple proteins in various plant signal-transduction pathways, including those arising from hormone responses, development, and environmental stress. During Phaseolus vulgaris root development, RACK1 (PvRACK1) mRNA expression was induced by auxins, abscissic acid, cytokinin, and gibberellic acid. In addition, during P. vulgaris nodule development, PvRACK1 mRNA was highly accumulated at 12 to 15 days postinoculation, suggesting an important role after nodule meristem initiation and Rhizobium nodule infection. PvRACK1 transcript accumulation was downregulated by a specific RNA interference construct which was expressed in transgenic roots of composite plants of P. vulgaris inoculated with Rhizobium tropici. PvRACK1 downregulated transcript levels were monitored by quantitative reverse-transcription polymerase chain reaction analysis in individual transgenic roots and nodules. We observed a clear phenotype in PvRACK1-knockdown nodules, in which nodule number and nodule cell expansion were impaired, resulting in altered nodule size. Microscopic analysis indicated that, in PvRACK1-knockdown nodules, infected and uninfected cells were considerably smaller (80 and 60%, respectively) than in control nodules. In addition, noninfected cells and symbiosomes in silenced nodules showed significant defects in membrane structure under electron microscopy analysis. These findings indicate that PvRACK1 has a pivotal role in cell expansion and in symbiosome and bacteroid integrity during nodule development.

Involvement of Arabidopsis RACK1 in protein translation and its regulation by abscisic acid

Earlier studies have shown that RACK1 functions as a negative regulator of abscisic acid (ABA) responses in Arabidopsis (Arabidopsis thaliana), but the molecular mechanism of the action of RACK1 in these processes remains elusive. Global gene expression profiling revealed that approximately 40% of the genes affected by ABA treatment were affected in a similar manner by the rack1 mutation, supporting the view that RACK1 is an important regulator of ABA responses. On the other hand, coexpression analysis revealed that more than 80% of the genes coexpressed with RACK1 encode ribosome proteins, implying a close relationship between RACK1's function and the ribosome complex. These results implied that the regulatory role for RACK1 in ABA responses may be partially due to its putative function in protein translation, which is one of the major cellular processes that mammalian and Saccharomyces cerevisiae RACK1 is involved in. Consistently, all three Arabidopsis RACK1 homologous genes, namely RACK1A, RACK1B, and RACK1C, complemented the growth defects of the S. cerevisiae cross pathway control2/rack1 mutant. In addition, RACK1 physically interacts with Arabidopsis Eukaryotic Initiation Factor6 (eIF6), whose mammalian homolog is a key regulator of 80S ribosome assembly. Moreover, rack1 mutants displayed hypersensitivity to anisomycin, an inhibitor of protein translation, and displayed characteristics of impaired 80S functional ribosome assembly and 60S ribosomal subunit biogenesis in a ribosome profiling assay. Gene expression analysis revealed that ABA inhibits the expression of both RACK1 and eIF6. Taken together, these results suggest that RACK1 may be required for normal production of 60S and 80S ribosomes and that its action in these processes may be regulated by ABA.

The transcription factor RSG regulates negative feedback of NtGA20ox1 encoding GA 20-oxidase

Gibberellins (GAs) are phytohormones that regulate growth and development throughout the life cycle of plants. RSG (REPRESSION OF SHOOT GROWTH) is a tobacco (Nicotiana tabacum) transcriptional activator with a basic leucine zipper domain that regulates the endogenous amount of GAs by control of GA biosynthetic enzymes. Negative feedback contributes to homeostasis of the GA levels. Previous studies suggested that RSG is directly or indirectly involved in the GA negative feedback of NtGA20ox1 encoding GA 20-oxidase. Using transgenic tobacco plants, we have identified a cis-acting region that is responsible for the feedback regulation of NtGA20ox1. This region contains an RSG-binding sequence. A mutation in the RSG-binding sequence abolished negative feedback of NtGA20ox1 in transgenic plants. Chromatin immunoprecipitation (ChIP) assays showed that RSG binds to the NtGA20ox1 promoter in vivo in response to a decrease in GA levels, and that this binding is abolished within 3 h after GA treatment. Furthermore, decreases in GA levels promote modifications of active histone marks in the promoter of NtGA20ox1. Our results suggest that RSG plays a role in the homeostasis of GAs through direct binding to the NtGA20ox1 promoter.

Alteration of substrate specificity: the variable N-terminal domain of tobacco Ca(2+)-dependent protein kinase is important for substrate recognition

Protein kinases are major signaling molecules that are involved in a variety of cellular processes. However, the molecular mechanisms whereby protein kinases discriminate specific substrates are still largely unknown. Ca(2+)-dependent protein kinases (CDPKs) play central roles in Ca(2+) signaling in plants. Previously, we found that a tobacco (Nicotiana tabacum) CDPK1 negatively regulated the transcription factor REPRESSION OF SHOOT GROWTH (RSG), which is involved in gibberellin feedback regulation. Here, we found that the variable N-terminal domain of CDPK1 is necessary for the recognition of RSG. A mutation (R10A) in the variable N-terminal domain of CDPK1 reduced both RSG binding and RSG phosphorylation while leaving kinase activity intact. Furthermore, the R10A mutation suppressed the in vivo function of CDPK1. The substitution of the variable N-terminal domain of an Arabidopsis thaliana CDPK, At CPK9, with that of Nt CDPK1 conferred RSG kinase activities. This chimeric CDPK behaved according to the identity of the variable N-terminal domain in transgenic plants. Our results open the possibility of engineering the substrate specificity of CDPK by manipulation of the variable N-terminal domain, enabling a rational rewiring of cellular signaling pathways.

Germination behavior, biochemical features and sequence analysis of the RACK1/arcA homolog from Phaseolus vulgaris

Partial peptide sequence of a 36 kDa protein from common bean embryo axes showed 100% identity with a reported beta-subunit of a heterotrimeric G protein from soybean. Analysis of the full sequence showed 96.6% identity with the reported soybean G(beta)-subunit, 86% with RACK1B and C from Arabidopsis and 66% with human and mouse RACK1, at the amino acid level. In addition, it showed 85.5, 85 and 83% identities with arcA from Solanum lycopersicum, Arabidopsis (RACK1A) and Nicotiana tabacum, respectively. The amino acid sequence displayed seven WD40 domains and two sites for activated protein kinase C binding. The protein showed a constant expression level but the mRNA had a maximum at 32 h post-imbibition. Western immunoblotting showed the protein in vegetative plant tissues, and in both microsomal and soluble fractions from embryo axes. Synthetic auxin treatment during germination delayed the peak of RACK1 mRNA expression to 48 h but did not affect the protein expression level while the polar auxin transport inhibitor, naphtylphtalamic acid had no effect on either mRNA or protein expression levels. Southern blot and genomic DNA amplification revealed a small gene family with at least one member without introns in the genome. Thus, the RACK1/arcA homolog from common bean has the following features: (1) it is highly conserved; (2) it is both soluble and insoluble within the embryo axis; (3) it is encoded by a small gene family; (4) its mRNA has a peak of expression at the time point of germination stop and (5) its expression is only slightly affected by auxin but unaffected by an auxin transport blocker.

RACK1 is a negative regulator of ABA responses in Arabidopsis

Receptor for Activated C Kinase 1 (RACK1) is viewed as a versatile scaffold protein in mammals. The protein sequence of RACK1 is highly conserved in eukaryotes. However, the function of RACK1 in plants remains poorly understood. Accumulating evidence suggested that RACK1 may be involved in hormone responses, but the precise role of RACK1 in any hormone signalling pathway remains elusive. Molecular and genetic evidence that Arabidopsis RACK1 is a negative regulator of ABA responses is provided here. It is shown that three RACK1 genes act redundantly to regulate ABA responses in seed germination, cotyledon greening and root growth, because rack1a single and double mutants are hypersensitive to ABA in each of these processes. On the other hand, plants overexpressing RACK1A displayed ABA insensitivity. Consistent with their proposed roles in seed germination and early seedling development, all three RACK1 genes were expressed in imbibed, germinating and germinated seeds. It was found that the ABA-responsive marker genes, RD29B and RAB18, were up-regulated in rack1a mutants. Furthermore, the expression of all three RACK1 genes themselves was down-regulated by ABA. Consistent with the view that RACK1 negatively regulates ABA responses, rack1a mutants lose water significantly more slowly from the rosettes and are hypersensitive to high concentrations of NaCl during seed germination. In addition, the expression of some putative RACK1-interacting, ABA-, or abiotic stress-regulated genes was mis-regulated in rack1a rack1b double mutants in response to ABA. Taken together, these findings provide compelling evidence that RACK1 is a critical, negative regulator of ABA responses.

A tobacco calcium-dependent protein kinase, CDPK1, regulates the transcription factor REPRESSION OF SHOOT GROWTH in response to gibberellins

The homeostasis of gibberellins (GAs) is maintained by negative feedback in plants. REPRESSION OF SHOOT GROWTH (RSG) is a tobacco (Nicotiana tabacum) transcriptional activator that has been suggested to play a role in GA feedback by the regulation of GA biosynthetic enzymes. The 14-3-3 signaling proteins negatively regulate RSG by sequestering it in the cytoplasm in response to GAs. The phosphorylation on Ser-114 of RSG is essential for 14-3-3 binding of RSG. Here, we identified tobacco Ca(2+)-dependent protein kinase (CDPK1) as an RSG kinase that promotes 14-3-3 binding to RSG by phosphorylation of Ser-114 of RSG. CDPK1 interacts with RSG in a Ca(2+)-dependent manner in vivo and in vitro and specifically phosphorylates Ser-114 of RSG. Inhibition of CDPK repressed the GA-induced phosphorylation of Ser-114 of RSG and the GA-induced nuclear export of RSG. Overexpression of CDPK1 inhibited the feedback regulation of a GA 20-oxidase gene and resulted in sensitization to the GA biosynthetic inhibitor. Our results suggest that CDPK1 decodes the Ca(2+) signal produced by GAs and regulates the intracellular localization of RSG.

RACK1 genes regulate plant development with unequal genetic redundancy in Arabidopsis

BACKGROUND

RACK1 is a versatile scaffold protein in mammals, regulating diverse developmental processes. Unlike in non-plant organisms where RACK1 is encoded by a single gene, Arabidopsis genome contains three RACK1 homologous genes, designated as RACK1A, RACK1B and RACK1C, respectively. Previous studies indicated that the loss-of-function alleles of RACK1A displayed multiple defects in plant development. However, the functions of RACK1B and RACK1C remain elusive. Further, the relationships between three RACK1 homologous genes are unknown.

RESULTS

We isolated mutant alleles with loss-of-function mutations in RACK1B and RACK1C, and examined the impact of these mutations on plant development. We found that unlike in RACK1A, loss-of-function mutations in RACK1B or RACK1C do not confer apparent defects in plant development, including rosette leaf production and root development. Analyses of rack1a, rack1b and rack1c double and triple mutants, however, revealed that rack1b and rack1c can enhance the rack1a mutant's developmental defects, and an extreme developmental defect and lethality were observed in rack1a rack1b rack1c triple mutant. Complementation studies indicated that RACK1B and RACK1C are in principle functionally equivalent to RACK1A. Gene expression studies indicated that three RACK1 genes display similar expression patterns but are expressed at different levels. Further, RACK1 genes positively regulate each other's expression.

CONCLUSION

These results suggested that RACK1 genes are critical regulators of plant development and that RACK1 genes function in an unequally redundant manner. Both the difference in RACK1 gene expression level and the cross-regulation are likely the molecular determinants of their unequal genetic redundancy.

Structure of a signal transduction regulator, RACK1, from Arabidopsis thaliana

The receptor for activated C-kinase 1 (RACK1) is a highly conserved WD40 repeat scaffold protein found in a wide range of eukaryotic species from Chlamydymonas to plants and humans. In tissues of higher mammals, RACK1 is ubiquitously expressed and has been implicated in diverse signaling pathways involving neuropathology, cellular stress, protein translation, and developmental processes. RACK1 has established itself as a scaffold protein through physical interaction with a myriad of signaling proteins ranging from kinases, phosphatases, ion channels, membrane receptors, G proteins, IP3 receptor, and with widely conserved structural proteins associated with the ribosome. In the plant Arabidopsis thaliana, RACK1A is implicated in diverse developmental and environmental stress pathways. Despite the functional conservation of RACK1-mediated protein-protein interaction-regulated signaling modes, the structural basis of such interactions is largely unknown. Here we present the first crystal structure of a RACK1 protein, RACK1 isoform A from Arabidopsis thaliana, at 2.4 A resolution, as a C-terminal fusion of the maltose binding protein. The structure implicates highly conserved surface residues that could play critical roles in protein-protein interactions and reveals the surface location of proposed post-transcriptionally modified residues. The availability of this structure provides a structural basis for dissecting RACK1-mediated cellular signaling mechanisms in both plants and animals.

RACK1 functions in rice innate immunity by interacting with the Rac1 immune complex

A small GTPase, Rac1, plays a key role in rice (Oryza sativa) innate immunity as part of a complex of regulatory proteins. Here, we used affinity column chromatography to identify rice RACK1 (for Receptor for Activated C-Kinase 1) as an interactor with Rac1. RACK1 functions in various mammalian signaling pathways and is involved in hormone signaling and development in plants. Rice contains two RACK1 genes, RACK1A and RACK1B, and the RACK1A protein interacts with the GTP form of Rac1. Rac1 positively regulates RACK1A at both the transcriptional and posttranscriptional levels. RACK1A transcription was also induced by a fungal elicitor and by abscisic acid, jasmonate, and auxin. Analysis of transgenic rice plants and cell cultures indicates that RACK1A plays a role in the production of reactive oxygen species (ROS) and in resistance against rice blast infection. Overexpression of RACK1A enhances ROS production in rice seedlings. RACK1A was shown to interact with the N terminus of NADPH oxidase, RAR1, and SGT1, key regulators of plant disease resistance. These results suggest that RACK1A functions in rice innate immunity by interacting with multiple proteins in the Rac1 immune complex.

A novel cell division factor from tobacco 2B-13 cells that induced cell division in auxin-starved tobacco BY-2 cells

Effects of auxin as plant hormones are widespread; in fact in almost all aspects of plant growth and development auxin plays a pivotal role. Although auxin is required for propagating cell division in plant cells, its effect upon cell division is least understood. If auxin is depleted from the culture medium, cultured cells cease to divide. It has been demonstrated in this context that the addition of auxin to auxin-starved nondividing tobacco BY-2 cells induced semisynchronous cell division. On the other hand, there are some cell lines, named habituated cells, that can grow without auxin. The cause and reason for the habituated cells have not been clarified. A habituated cell line named 2B-13 is derived from the tobacco BY-2 cell line, which has been most intensively studied among plant cell lines. When we tried to find the difference between two cell lines of BY-2 and 2B-13 cells, we found that the addition of culture filtrated from the auxin-habituated 2B-13 cells induced semisynchronous cell division in auxin-starved BY-2 cells. The cell division factor (CDF) that is responsible for inducing cell division in auxin-starved BY-2 cells was purified to near-homogeneity by sequential passage through a hydroxyapatite column, a ConA Sepharose column and a Sephadex gel filtration column. The resulting purified fraction appeared as a single band of high molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels by silver staining and was able to induce cell division in auxin-starved BY-2 cells. Identification of the protein by MALD-TOF-MS/MS revealed that it is structurally related to P-glycoprotein from Gossypioides kirkii, which belongs to ATP-binding cassette (ABC)-transporters. The significance of CDF as a possible ABC-transporter is discussed in relationship to auxin-autotrophic growth and auxin-signaling pathway.

Cell cycle synchronization of tobacco BY-2 cells

Synchronization is a powerful technique for understanding cell cycle events. Here, we describe the procedure for synchronizing tobacco bright yellow 2 (BY-2) cell line, with which an exceptionally high level of synchrony can be achieved. It basically relies on an "arrest-and-release" strategy using aphidicolin, an inhibitor of DNA replication, and propyzamide, a plant-microtubule disruptant. In a single-step process using aphidicolin alone, a cell population with about 70% of the cells at mitosis can be achieved, whereas by a two-step method using the two inhibitors sequentially, the level of synchrony can reach over 90%. The method of choice depends not only on the peak mitotic cell proportion but also on the cell cycle stage that is targeted for analysis. Both procedures take about 1.5 days, and cell cycle progression can be observed from the S phase to the next G1 phase at about 12 h after a 24 h-period treatment with aphidicolin.

G proteins Go green: a plant G protein signaling FAQ sheet

Plants, like animals, use signal transduction pathways based on heterotrimeric guanine nucleotide-binding proteins (G proteins) to regulate many aspects of development and cell signaling. Some components of G protein signaling are highly conserved between plants and animals and some are not. This Viewpoint compares key aspects of G protein signal transduction in plants and animals and describes the current knowledge of this system in plants, the questions that still await exploration, and the value of research on plant G proteins to scientists who do not study plants. Pathways in Science's Signal Transduction Knowledge Environment Connections Maps database provide details about the emerging roles of G proteins in several cellular processes of plants.

Tuber on a chip: differential gene expression during potato tuber development

Potato tuber development has proven to be a valuable model system for studying underground sink organ formation. Research on this topic has led to the identification of many genes involved in this complex process and has aided in the unravelling of the mechanisms underlying starch synthesis. However, less attention has been paid to the biochemical pathways of other important metabolites or to the changing metabolic fluxes occurring during potato tuber development. In this paper, we describe the construction of a potato complementary DNA (cDNA) microarray specifically designed for genes involved in processes related to tuber development and tuber quality traits. We present expression profiles of 1315 cDNAs during tuber development where the predominant profiles were strong up- and down-regulation. Gene expression profiles showing transient increases or decreases were less abundantly represented and followed more moderate changes, mainly during tuber initiation. In addition to the confirmation of gene expression patterns during tuber development, many novel differentially expressed genes were identified and are considered as candidate genes for direct involvement in potato tuber development. A detailed analysis of starch metabolism genes provided a unique overview of expression changes during tuber development. Characteristic expression profiles were often clearly different between gene family members. A link between differential gene expression during tuber development and potato tissue specificity is described. This dataset provides a firm basis for the identification of key regulatory genes in a number of metabolic pathways that may provide researchers with new tools to achieve breeding goals for use in industrial applications.

Analysis of leaf proteome after UV-B irradiation in maize lines differing in sensitivity

UV-B radiation causes diverse morphological and physiological responses in plants, but the underlying mechanisms governing these integrated responses are unknown. In this study, we systematically surveyed responses of maize leaves to UV-B radiation using DIGE 2D gels and identified selected proteins by mass spectrometry and immunodetection analysis. To identify changes in protein accumulation in response to UV-B radiation, a line (b, pl W23) deficient in flavonoid sunscreen compounds and hence similar to commercial corn was used. In addition, its proteome in natural UV-B conditions was compared with that of two maize landraces from high altitudes (Cacahuacintle and Confite Puneño) that have improved UV-B tolerance. Protein patterns in adult maize leaves (Zea mays) were documented after growth for 21 days in sunlight depleted of UV-B radiation or growth in sunlight including an 8-h UV-B supplementation during 1 day in the field. We found that there is a very high correlation between previously documented mRNA accumulation assessed by microarray hybridization and quantitative real time reverse transcription-PCR and protein expression after UV-B irradiation in leaves of W23. Multiple isoforms were confirmed for some proteins; at least one protein, pyruvate, phosphate dikinase, is regulated post-translationally by phosphorylation by UV-B exposure. Proteins differentially regulated by UV-B radiation in W23 with higher levels under similar UV-B conditions in high altitude plants were also identified. These could be genetically fixed traits conferring UV-B tolerance and offer clues to specific adaptations to living in high ambient UV-B conditions.

Involvement of 14-3-3 signaling protein binding in the functional regulation of the transcriptional activator REPRESSION OF SHOOT GROWTH by gibberellins

REPRESSION OF SHOOT GROWTH (RSG) is a tobacco (Nicotiana tabacum) transcriptional activator with a basic Leu zipper domain that regulates endogenous amounts of gibberellins (GAs) by the control of a GA biosynthetic enzyme. The 14-3-3 signaling proteins have been suggested to suppress RSG by sequestering it in the cytoplasm. Here, we show that RSG phosphorylation on Ser-114 is important for 14-3-3 binding. We found that GA levels regulate the intracellular localization of RSG. RSG translocated into the nucleus in response to a reduction in GA levels. GA treatment could reverse this nuclear accumulation. The GA-induced disappearance of RSG-green fluorescent protein from the nucleus did not depend on protein degradation. By contrast, the mutant RSG (S114A) that could not bind to 14-3-3 continued to be localized predominantly in the nucleus after GA application. Analysis of the mRNA levels of GA biosynthetic genes showed that the feedback regulation of the GA 20-oxidase gene was inhibited in transgenic plants expressing a dominant negative form of RSG. Our results suggest that RSG is negatively modulated by GAs by 14-3-3 binding and might be involved in GA homeostasis.

The tobacco bZIP transcription factor BZI-1 binds the GH3 promoter in vivo and modulates auxin-induced transcription

Summary In order to establish the biological function of the tobacco basic leucine zipper (bZIP) transcription factor BZI-1 in hormone signalling, we have analysed transgenic plants which were altered with respect to the protein level or the activation potential of BZI-1. Overexpression of a dominant-negative derivative of BZI-1, lacking the N-terminal activation domain, resulted in plants displaying reduced internode size, enhanced lateral shoot formation and small, curly leaves. The response to auxin monitored with reference to root organogenesis, epinastic leaf curvature and transcription of the auxin-induced GH3 gene was reduced. In vitro, BZI-1 specifically binds to ACGT elements (ACEs) present in the GH3 promoter. In vivo, binding to the GH3 promoter was confirmed by chromatin immunoprecipitation (ChIP). Overexpression of BZI-1 in transgenic plants did not lead to a significant activation of the GH3 target gene. In contrast, plants expressing a VP16 (Herpes simplex virion protein 16)-BZI-1 fusion protein showed enhanced auxin-induced GH3 transcription. However, VP16-BZI-1 is insufficient to trigger GH3 expression independently of the auxin stimulus. Whereas auxin responsiveness has been shown to be mediated by ARF (auxin response factor) transcription factors, we discuss a function of BZI-1 assisting in fine-tuning of auxin-induced transcription.

Construction of a two-dimensional gel electrophoresis protein database for the Nicotiana tabacum cv. Bright Yellow-2 cell suspension culture

Using two-dimensional gel electrophoresis (2-DE) and electrospray-tandem mass spectrometry (ESI-MS/MS), we have started the proteome analysis of the cell line Nicotiana tabacum cv. Bright Yellow-2 (tobacco BY-2). The BY-2 cell suspension culture is widely used as a model system to study the growth and development of plant cells. We present a protocol describing the sample preparation and 2-DE, enabling us to separate and display more than 1000 proteins from this cell culture. A reference gel was generated, using immobilized pH gradient isoelectric focusing in a linear gradient from pH 3 to 10 and 12% Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Although the tobacco genome is not sequenced yet, a range of protein spots from this reference map was identified by means of a semi-automated liquid chromatography-ESI-quadrupole time of flight-tandem MS (LC-ESI-QTOF-MS-MS) setup and cross-species matching. These data were integrated in a database, which can be accessed at http://tby2-www.uia.ac.be/tby2/. On the on-line reference map, the identified protein spots are hyperlinked to individual protein entries. Each protein entry contains all identification information, as well as links to relevant entries in other on-line databases. Comprehensive search functions are implemented. Especially for an unsequenced but widespread model organism like tobacco BY-2, such a reference database is a convenient source for protein information that brings protein identification within reach without the need for extensive MS. This publicly accessible database provides a solid basis for tobacco BY-2 proteomics in the future.

Transfer of T-DNA and Vir proteins to plant cells by Agrobacterium tumefaciens induces expression of host genes involved in mediating transformation and suppresses host defense gene expression

Agrobacterium tumefaciens is a plant pathogen that incites crown gall tumors by transferring to and expressing a portion of a resident plasmid in plant cells. Currently, little is known about the host response to Agrobacterium infection. Using suppressive subtractive hybridization and DNA macroarrays, we identified numerous plant genes that are differentially expressed during early stages of Agrobacterium-mediated transformation. Expression profiling indicates that Agrobacterium infection induces plant genes necessary for the transformation process while simultaneously repressing host defense response genes, thus indicating successful utilization of existing host cellular machinery for genetic transformation purposes. A comparison of plant responses to different strains of Agrobacterium indicates that transfer of both T-DNA and Vir proteins modulates the expression of host genes during the transformation process.

The effect of pH and various cations on the GTP hydrolysis of rice heterotrimeric G-protein alpha subunit expressed in Escherichia coli

Previously, we reported the biochemical properties of RGA1 that is expressed in Escherichia coli (Seo et al., 1997). The activities of RGA1 that hydrolyzes and binds guanine nucleotide were dependent on the MgCl(2) concentration. The steady state rate constant (k(cat) ) for GTP hydrolysis of RGA1 at 2 mM MgCl(2) was 0.0075 +/- 0.0001 min(-1). Here, we examined the effects of pH and cations on the GTPase activity. The optimum pH at 2 mM MgCl(2) was approximately 6.0; whereas, the pH at 2 mM NH(4)Cl was approximately 4.0. The result from the cation dependence on the GTPase (guanosine 5'-triphosphatase) activity of RGA1 under the same condition showed that the GTP hydrolysis rate (k(cat)= 0.0353 min(-1)) under the condition of 2 mM NH(4)Cl at pH 4.0 was the highest. It corresponded to about 3.24-fold of the k(cat) value of 0.0109 min(-1) in the presence of 2 mM MgCl(2) at pH 6.0.

Gamma-tubulin distribution during cortical microtubule reorganization at the M/G1 interface in tobacco BY-2 cells

Cortical microtubules are considered to regulate the direction of cellulose microfibril deposition. Despite their significant role in determining cell morphology, cortical microtubules completely disappear from the cell cortex during M phase and become reorganized at G1 phase. The mechanism by which these microtubules become properly formed again is, however, still unclear. We have proposed that the origin of cortical microtubules is on the daughter nuclear surface, but further cortical microtubule reorganization occurs at the cell cortex. Hence it is probable that the locations of microtubule organizing centers (MTOCs) are actively changing. However, the actual MTOC sites of cortical microtubules were not clearly determined. In this paper, we have examined the distribution of gamma-tubulin, one of the key molecules of MTOCs in various organisms, during cortical microtubule reorganization using both immunofluorescence and a GFP reporter system. Using a monoclonal antibody (clone G9) that recognizes highly conserved residues in y-tubulin, y-tubulin was found to be constitutively expressed and to be clearly localized to microtubule structures, such as the preprophase bands, spindles, and phragmoplasts, specific to each cell cycle stage. This distribution pattern was confirmed by the GFP reporter system. During cortical microtubule reorganization at the M to G1 transition phase, gamma-tubulin first accumulated at the daughter nuclear surfaces, and then seemed to spread onto the cell cortex along with microtubules elongating from the daughter nuclei. Based on the results, it was confirmed that daughter nuclear surfaces acted as origins of cortical microtubules, and that further reorganization occurred on the cell cortex.

Phenotypic subunit composition of the tobacco (Nicotiana tabacum L.) vacuolar-type H(+)-translocating ATPase

The model plant tobacco (Nicotiana tabacum L.) was chosen for a survey of the subunit composition of the V-ATPase at the protein level. V-ATPase was purified from tobacco leaf cell tonoplasts by solubilization with the nonionic detergent Triton X-100 and immunoprecipitation. In the purified fraction 12 proteins were present. By matrix-assisted laser-desorption ionization mass spectrometry (MALDI-MS) and amino acid sequencing 11 of these polypeptides could be identified as subunits A, B, C, D, F, G, c, d and three different isoforms of subunit E. The polypeptide which could not be identified by MALDI analysis might represent subunit H. The data presented here, for the first time, enable an unequivocal identification of V-ATPase subunits after gel electrophoresis and open the possibility to assign changes in polypeptide composition to variations in respective V-ATPase subunits occurring as a response to environmental conditions or during plant development.

The possible involvement of a phosphate-induced transcription factor encoded by phi-2 gene from tobacco in ABA-signaling pathways

A novel phosphate-induced gene, phi-2, has been identified by its induction on addition of phosphate to phosphate-starved tobacco BY-2 cells. The predicted gene product of phi-2 has significant homology to a group of bZIP proteins involved in ABA-signaling pathways, and phi-2 also responded to ABA treatment. A previously isolated phosphate-induced gene, phi-1, (Sano et al. (1999) Plant Cell Physiol. 40: 1) was also responsive to ABA. Although phosphate addition induced semi-synchronous cell division in phosphate-starved tobacco BY-2 cells, ABA adversely affected cell division. Detailed examination revealed that the high levels of phosphate required to induce semi-synchronous cell division seemed to be perceived as indicators of stress by the cells. One of the stress indicators perceived by the cells is a cytoplasmic pH change, to which phi-2 and phi-1 genes respond. The different components of the cell's response to phosphate induction are discussed.

14-3-3 proteins regulate intracellular localization of the bZIP transcriptional activator RSG

Gibberellins (GAs) are essential regulators of many aspects of plant development, including stem elongation, seed germination, and flowering. RSG is a transcriptional activator with a basic leucine zipper domain that regulates endogenous amounts of GAs through the control of a GA biosynthetic enzyme. The ubiquitous expression of RSG in plant organs suggests an involvement of post-transcriptional and/or post-translational modifications of the transcription factor. Here, we identify the 14-3-3 signaling proteins as RSG binding partners. The mutant version of RSG that could not bind to 14-3-3 proteins exhibited a higher transcriptional activity than did wild-type RSG. Consistent with this observation, the mutant RSG that could not bind to 14-3-3 proteins was localized predominantly in the nucleus, whereas wild-type RSG was distributed throughout the cell. Using the nuclear export inhibitor leptomycin B, we also showed that RSG, apparently statically localized in the cytoplasm, is capable of shuttling in and out of the nucleus. These results suggest that 14-3-3 proteins negatively modulate RSG, which is involved in the regulation of endogenous amounts of GAs, by controlling its intracellular localization.

14-3-3 proteins regulate intracellular localization of the bZIP transcriptional activator RSG

Gibberellins (GAs) are essential regulators of many aspects of plant development, including stem elongation, seed germination, and flowering. RSG is a transcriptional activator with a basic leucine zipper domain that regulates endogenous amounts of GAs through the control of a GA biosynthetic enzyme. The ubiquitous expression of RSG in plant organs suggests an involvement of post-transcriptional and/or post-translational modifications of the transcription factor. Here, we identify the 14-3-3 signaling proteins as RSG binding partners. The mutant version of RSG that could not bind to 14-3-3 proteins exhibited a higher transcriptional activity than did wild-type RSG. Consistent with this observation, the mutant RSG that could not bind to 14-3-3 proteins was localized predominantly in the nucleus, whereas wild-type RSG was distributed throughout the cell. Using the nuclear export inhibitor leptomycin B, we also showed that RSG, apparently statically localized in the cytoplasm, is capable of shuttling in and out of the nucleus. These results suggest that 14-3-3 proteins negatively modulate RSG, which is involved in the regulation of endogenous amounts of GAs, by controlling its intracellular localization.

Fate of nascent microtubules organized at the M/G1 interface, as visualized by synchronized tobacco BY-2 cells stably expressing GFP-tubulin: time-sequence observations of the reorganization of cortical microtubules in living plant cells

Transgenic BY-2 cells stably expressing a GFP (green fluorescent protein)-tubulin fusion protein (BY-GT16) were subcultured in a modified Linsmaier and Skoog medium. The BY-GT16 cells could be synchronized by aphidicolin and the dynamics of their microtubules (MTs) were monitored by the confocal laser scanning microscopy (CLSM). We have succeeded in investigating the mode of reorganization of cortical MTs at the M/G1 interface. The cortical MTs were initially organized in the perinuclear regions and then they elongated to reach the cell cortex, forming the bright spots there. Subsequently, the first cortical MTs rapidly elongated from the spots and they were oriented parallel to the long axis towards the distal end of the cells. Around the time when the tips of the parallel MTs reached the distal end, the formation of transverse cortical MTs followed in the cortex near the division site, as we had previously suggested [Hasezawa and Nagata (1991) Bot. Acta 104: 206, Nagata et al. (1994) Planta 193: 567]. It was confirmed in independent observations that the appearance of the parallel MTs was followed by the appearance of the transverse MTs in each cell. We found that the transverse MTs spread through the whole cell cortex within about 20-30 min, while the parallel MTs disappeared. The significance of these observations on the mode of cortical MT organization is discussed.

Heterotrimeric G-proteins in plant cell signaling

Heterotrimeric G-proteins, which couple cell surface receptors with internal effectors, are evident in all eukaryotes. Their operation involves receptor activation, GTP/GDP exchange and modulation of effector activity; deactivation occurs by an intrinsic GTPase activity. Structurally, G-proteins comprise three dissimilar subunits; Gα, Gβ and Gγ. The Gα subunit consists of an α-helical and a GTPase domain, the latter is responsible for interaction with Gβγ, receptor and effector. Gβ and Gγ form a tightly associated heterodimer which can also modulate effector activity when released by the activated Gα. Genome sequence and other data suggest that, in plants, there are several (~8-10?) Gα, one or two Gβ and one Gγ. These proteins are expressed throughout the plant, mainly in the plasma membrane and endoplasmic reticulum. In vivo, there is strong evidence for G-protein control of ion channels, particularly K⁺ , in the response pathways to fungal and bacterial pathogens as well as in some aspects of gibberellin, abscisic acid and auxin signaling pathways. Finally, future prospects for understanding plant G-protein linked signaling will rely on new and emerging technologies; these include antisense suppression, gene knockouts, yeast two-hybrid and phage display molecular approaches, intracellular immunization using recombinant single chain antibodies and expression of peptide encoding minigenes.