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

Investigation of the Nature of CgCDPK and CgbHLH001 Interaction and the Function of bHLH Transcription Factor in Stress Tolerance in Chenopodium glaucum

Calcium-dependent protein kinase (CDPK) and its substrates play important roles in plant response to stress. So far, the documentation on the characterization of the CDPK and downstream interaction components (especially transcription factors, TFs) is limited. In the present study, an interaction between CgCDPK (protein kinase) (accession no. MW26306) and CgbHLH001 (TF) (accession no. MT797813) from a halophyte Chenopodium glaucum was further dissected. Firstly, we revealed that the probable nature between the CgCDPK and CgbHLH001 interaction was the phosphorylation, and the N-terminus of CgbHLH001, especially the 96th serine (the potential phosphorylation site) within it, was essential for the interaction, whereas the mutation of ⁹⁶Ser to alanine did not change its nuclear localization, which was determined by the N-terminus and bHLH domain together. Furthermore, we verified the function of CgbHLH001 gene in response to stress by ectopic overexpression in tobacco; the transgenic lines presented enhanced stress tolerance probably by improving physiological performance and stress-related gene expression. In conclusion, we characterized the biological significance of the interaction between CDPK and bHLH in C. glaucum and verified the positive function of CgbHLH001 in stress tolerance, which may supply more evidence in better understanding of the CDPK signaling pathway in response to adversity.

Phytohormone-Mediated Stomatal Response, Escape and Quiescence Strategies in Plants under Flooding Stress

Generally, flooding causes waterlogging or submergence stress which is considered as one of the most important abiotic factors that severely hinders plant growth and development. Plants might not complete their life cycle even in short duration of flooding. As biologically intelligent organisms, plants always try to resist or survive under such adverse circumstances by adapting a wide array of mechanisms including hormonal homeostasis. Under this mechanism, plants try to adapt through diverse morphological, physiological and molecular changes, including the closing of stomata, elongating of petioles, hollow stems or internodes, or maintaining minimum physiological activity to store energy to combat post-flooding stress and to continue normal growth and development. Mainly, ethylene, gibberellins (GA) and abscisic acid (ABA) are directly and/or indirectly involved in hormonal homeostasis mechanisms. Responses of specific genes or transcription factors or reactive oxygen species (ROS) maintain the equilibrium between stomatal opening and closing, which is one of the fastest responses in plants when encountering flooding stress conditions. In this review paper, the sequential steps of some of the hormone-dependent survival mechanisms of plants under flooding stress conditions have been critically discussed.

Shade-induced nuclear localization of PIF7 is regulated by phosphorylation and 14-3-3 proteins in Arabidopsis

Shade avoidance syndrome enables shaded plants to grow and compete effectively against their neighbors. In Arabidopsis, the shade-induced de-phosphorylation of the transcription factor PIF7 (PHYTOCHROME-INTERACTING FACTOR 7) is the key event linking light perception to stem elongation. However, the mechanism through which phosphorylation regulates the activity of PIF7 is unclear. Here, we show that shade light induces the de-phosphorylation and nuclear accumulation of PIF7. Phosphorylation-resistant site mutations in PIF7 result in increased nuclear localization and shade-induced gene expression, and consequently augment hypocotyl elongation. PIF7 interacts with 14-3-3 proteins. Blocking the interaction between PIF7 and 14-3-3 proteins or reducing the expression of 14-3-3 proteins accelerates shade-induced nuclear localization and de-phosphorylation of PIF7, and enhances the shade phenotype. By contrast, the 14-3-3 overexpressing line displays an attenuated shade phenotype. These studies demonstrate a phosphorylation-dependent translocation of PIF7 when plants are in shade and a novel mechanism involving 14-3-3 proteins, mediated by the retention of PIF7 in the cytoplasm that suppresses the shade response.

Shade-induced nuclear localization of PIF7 is regulated by phosphorylation and 14-3-3 proteins in Arabidopsis

Shade avoidance syndrome enables shaded plants to grow and compete effectively against their neighbors. In Arabidopsis, the shade-induced de-phosphorylation of the transcription factor PIF7 (PHYTOCHROME-INTERACTING FACTOR 7) is the key event linking light perception to stem elongation. However, the mechanism through which phosphorylation regulates the activity of PIF7 is unclear. Here, we show that shade light induces the de-phosphorylation and nuclear accumulation of PIF7. Phosphorylation-resistant site mutations in PIF7 result in increased nuclear localization and shade-induced gene expression, and consequently augment hypocotyl elongation. PIF7 interacts with 14-3-3 proteins. Blocking the interaction between PIF7 and 14-3-3 proteins or reducing the expression of 14-3-3 proteins accelerates shade-induced nuclear localization and de-phosphorylation of PIF7, and enhances the shade phenotype. By contrast, the 14-3-3 overexpressing line displays an attenuated shade phenotype. These studies demonstrate a phosphorylation-dependent translocation of PIF7 when plants are in shade and a novel mechanism involving 14-3-3 proteins, mediated by the retention of PIF7 in the cytoplasm that suppresses the shade response.

Autophosphorylation of Ser-6 via an intermolecular mechanism is important for the rapid reduction of NtCDPK1 kinase activity for substrate RSG

Tobacco (Nicotiana tabacum) Ca2+-dependent protein kinase 1 (NtCDPK1) is involved in feedback regulation of the plant hormone gibberellin through the phosphorylation of the transcription factor, REPRESSION OF SHOOT GROWTH (RSG). Previously, Ser-6 and Thr-21 were identified as autophosphorylation sites in NtCDPK1. Autophosphorylation of Ser-6 and Thr-21 not only decreases the binding affinity of NtCDPK1 for RSG, but also inhibits the homodimerization of NtCDPK1. Furthermore, autophosphorylation decreases the phosphorylation efficiency of RSG. We demonstrated that Ser-6 and Thr-21 of NtCDPK1 are phosphorylated in response to GAs in plants. The substitution of these autophosphorylation sites with Ala enhances the NtCDPK1 overexpression-induced sensitization of seeds to a GA biosynthetic inhibitor during germination. These findings suggested that autophosphorylation of Ser-6 and Thr-21 prevents excessive phosphorylation of RSG. In this study, we attempted to determine which autophosphorylation site is responsible for the functional regulation of NtCDPK1. Ser-6 was autophosphorylated within 1 min, whereas Thr-21 required over 5 min to be completely autophosphorylated. Furthermore, we found that Ser-6 and Thr-21 were autophosphorylated by inter- and intramolecular mechanisms, respectively, which may be reflected in the faster autophosphorylation of Ser-6. Although both autophosphorylation sites were involved in the reduction of the binding affinity of NtCDPK1 for RSG and the inhibition of NtCDPK1 homodimerization, autophosphorylation of Ser-6 alone was sufficient to decrease the kinase activity of NtCDPK1 for RSG. These results suggest that autophosphorylation of Ser-6 is important for the rapid reduction of NtCDPK1 kinase activity for RSG, whereas that of Thr-21 may play an auxiliary role.

The Arabidopsis GORK K+-channel is phosphorylated by calcium-dependent protein kinase 21 (CPK21), which in turn is activated by 14-3-3 proteins

Potassium (K⁺) is a vital ion for many processes in the plant and fine-tuned ion channels control the K⁺-fluxes across the plasma membrane. GORK is an outward-rectifying K⁺-channel with important functions in stomatal closure and in root K⁺-homeostasis. In this study, post-translational modification of the Arabidopsis GORK ion channel and its regulation by 14-3-3 proteins was investigated. To investigate the possible interaction between GORK and 14-3-3s an in vivo pull-down from an Arabidopsis protein extract with recombinant GORK C-terminus (GORK-C) indeed identified endogenous 14-3-3s (LAMBDA, CHI, NU) as binding partners in a phosphorylation dependent manner. However, a direct interaction between 14-3-3's and GORK-C could not be demonstrated. Since the pull-down of 14-3-3s was phosphorylation dependent, we determined GORK-C as substrate for CPK21 phosphorylation and identified three CPK21 phospho-sites in the GORK protein (T³⁴⁴, S⁵¹⁸ and S⁶⁴⁹). Moreover, interaction of 14-3-3 to CPK21 strongly stimulates its kinase activity; an effect that can result in increased GORK phosphorylation and change in activity. Using the non-invasive vibrating probe technique, we measured the predominantly GORK mediated salt induced K⁺-efflux from wild-type, gork, cpk21, aha2 and 14-3-3 mutant roots. The mutants cpk21 and aha2 did not show statistical significant differences compared to WT. However, two (out of six) 14-3-3 isoforms, CHI and PHI, have a clear function in the salt induced K⁺-efflux. In conclusion, our results show that GORK can be phosphorylated by CPK21 and suggest that 14-3-3 proteins control GORK activity through binding with and activation of CPK21.

14-3-3 Proteins in Plant Hormone Signaling: Doing Several Things at Once

In this review we highlight the advances achieved in the investigation of the role of 14-3-3 proteins in hormone signaling, biosynthesis, and transport. 14-3-3 proteins are a family of conserved molecules that target a number of protein clients through their ability to recognize well-defined phosphorylated motifs. As a result, they regulate several cellular processes, ranging from metabolism to transport, growth, development, and stress response. High-throughput proteomic data and two-hybrid screen demonstrate that 14-3-3 proteins physically interact with many protein clients involved in the biosynthesis or signaling pathways of the main plant hormones, while increasing functional evidence indicates that 14-3-3-target interactions play pivotal regulatory roles. These advances provide a framework of our understanding of plant hormone action, suggesting that 14-3-3 proteins act as hubs of a cellular web encompassing different signaling pathways, transducing and integrating diverse hormone signals in the regulation of physiological processes.

Calcium-Dependent Protein Kinases in Phytohormone Signaling Pathways

Calcium-dependent protein kinases (CPKs/CDPKs) are Ca²⁺-sensors that decode Ca²⁺ signals into specific physiological responses. Research has reported that CDPKs constitute a large multigene family in various plant species, and play diverse roles in plant growth, development, and stress responses. Although numerous CDPKs have been exhaustively studied, and many of them have been found to be involved in plant hormone biosynthesis and response mechanisms, a comprehensive overview of the manner in which CDPKs participate in phytohormone signaling pathways, regulating nearly all aspects of plant growth, has not yet been undertaken. In this article, we reviewed the structure of CDPKs and the mechanism of their subcellular localization. Some CDPKs were elucidated to influence the intracellular localization of their substrates. Since little work has been done on the interaction between CDPKs and cytokinin signaling pathways, or on newly defined phytohormones such as brassinosteroids, strigolactones and salicylic acid, this paper mainly focused on discussing the integral associations between CDPKs and five plant hormones: auxins, gibberellins, ethylene, jasmonates, and abscisic acid. A perspective on future work is provided at the end.

The bHLH transcription factor CgbHLH001 is a potential interaction partner of CDPK in halophyte Chenopodium glaucum

Plants have evolved different abilities to adapt to the ever-fluctuating environments for sessility. Calcium-dependent protein kinase (CDPK) is believed to play a pivotal role in abiotic stress signaling. So far, study on the specific substrates that CDPK recognized in response to adversity is limited. In the present study, we revealed a potential interaction between CDPK and a bHLH transcription factor under salt stress in Chenopodium glaucum. First, we identified a CgCDPK, which was up-regulated under salt and drought stress; then by Y2H screening, CgCDPK was detected to be involved in interaction with a bHLH TF (named as CgbHLH001), which also positively respond to salt and drought stress. Further computational prediction and experiments including GST-pulldown and BiFC assays revealed that potential interaction existed between CgCDPK and CgbHLH001, and they might interact on the plasma membrane. In addition, CgCDPK-overexpressed transgenic tobacco line could significantly accumulate transcripts of NtbHLH (a homolog of CgbHLH001 in N. tabacum), which provided another evidence of correlation between CgCDPK and CgbHLH001. Our results suggest that CgbHLH001 can interact with CgCDPK in signal transduction pathway in response to abiotic stress, which should provide new evidence for further understanding of the substrate specificity of plant CDPK signaling pathway.

Subcellular localization of VIP1 is regulated by phosphorylation and 14-3-3 proteins

Arabidopsis basic leucine zipper transcription factor VIRE2-interacting protein 1 (VIP1) changes its localization from the cytosol to the nucleus when cells are subjected to mechanical or hypo-osmotic stress, although the mechanism of this change is not known. In this study, we show that change in VIP1 subcellular localization is synchronized with a change in the VIP1 phosphorylation state that is induced by mechanical/hypo-osmotic stress. VIP1 has three phosphorylatable serine residues in HXRXXS motifs, which are 14-3-3-binding targets. Mutations of these residues results in the lack of 14-3-3 binding and prevents cytosolic localization of VIP1. These results suggest that dephosphorylation of VIP1 resulting from mechanical or hypo-osmotic stress induces nuclear localization via 14-3-3 dissociation.

The Tudor Staphylococcal Nuclease Protein of Entamoeba histolytica Participates in Transcription Regulation and Stress Response

Entamoeba histolytica is the protozoa parasite responsible of human amoebiasis, disease that causes from 40,000 to 100,000 deaths annually worldwide. However, few are known about the expression regulation of molecules involved in its pathogenicity. Transcription of some virulence-related genes is positively controlled by the cis-regulatory element named URE1. Previously we identified the transcription factor that binds to URE1, which displayed a nuclear and cytoplasmic localization. This protein belongs to the Tudor Staphyococcal nuclease (TSN) family, which in other systems participates in virtually all pathways of gene expression, suggesting that this amoebic transcription factor (EhTSN; former EhURE1BP) could also play multiple functions in E. histolytica. The aim of this study was to identify the possible cellular events where EhTSN is involved. Here, we found that EhTSN in nucleus is located in euchromatin and close to, but not into, heterochromatin. We also showed the association of EhTSN with proteins involved in transcription and that the knockdown of EhTSN provokes a diminishing in the mRNA level of the EhRabB gene, which in its promoter region contains the URE1 motif, confirming that EhTSN participates in transcription regulation. In cytoplasm, this protein was found linked to the membrane of small vesicles and to plasma membrane. Through pull-down assays and mass spectrometry we identity thirty two candidate proteins to interact with EhTSN. These proteins participate in transcription, metabolism, signaling, and stress response, among other cellular processes. Interaction of EhTSN with some candidate proteins involved in metabolism, and signaling was validated by co-immunoprecipitation or co-localization. Finally we showed the co-localization of EhTSN and HSP70 in putative stress granules during heat shock and that the knockdown of EhTSN increases the cell death during heat shock treatment, reinforcing the hypothesis that EhTSN has a role during stress response. All data support the proposal that EhTSN is a multifunctional protein of E. histolytica.

StCDPK3 Phosphorylates In Vitro Two Transcription Factors Involved in GA and ABA Signaling in Potato: StRSG1 and StABF1

Calcium-dependent protein kinases, CDPKs, decode calcium (Ca2+) transients and initiate downstream responses in plants. In order to understand how CDPKs affect plant physiology, their specific target proteins must be identified. In tobacco, the bZIP transcription factor Repression of Shoot Growth (NtRSG) that modulates gibberellin (GA) content is a specific target of NtCDPK1. StCDPK3 from potato is homologous (88% identical) to NtCDPK1 even in its N-terminal variable domain. In this work, we observe that NtRSG is also phosphorylated by StCDPK3. The potato RSG family of transcription factors is composed of three members that share similar features. The closest homologue to NtRSG, which was named StRSG1, was amplified and sequenced. qRT-PCR data indicate that StRSG1 is mainly expressed in petioles, stems, lateral buds, and roots. In addition, GA treatment affected StRSG1 expression. StCDPK3 transcripts were detected in leaves, petioles, stolons, roots, and dormant tubers, and transcript levels were modified in response to GA. The recombinant StRSG1-GST protein was produced and tested as a substrate for StCDPK3 and StCDPK1. 6xHisStCDPK3 was able to phosphorylate the potato StRSG1 in a Ca2+-dependent way, while 6xHisStCDPK1 could not. StCDPK3 also interacts and phosphorylates the transcription factor StABF1 (ABRE binding factor 1) involved in ABA signaling, as shown by EMSA and phosphorylation assays. StABF1 transcripts were mainly detected in roots, stems, and stolons. Our data suggest that StCDPK3 could be involved in the cross-talk between ABA and GA signaling at the onset of tuber development.

Class-Specific Evolution and Transcriptional Differentiation of 14-3-3 Family Members in Mesohexaploid Brassica rapa

14-3-3s are highly conserved, multigene family proteins that have been implicated in modulating various biological processes. The presence of inherent polyploidy and genome complexity has limited the identification and characterization of 14-3-3 proteins from globally important Brassica crops. Through data mining of Brassica rapa, the model Brassica genome, we identified 21 members encoding 14-3-3 proteins namely, BraA.GRF14.a to BraA.GRF14.u. Phylogenetic analysis indicated that B. rapa contains both ε (epsilon) and non-ε 14-3-3 isoforms, having distinct intron-exon structural organization patterns. The non-ε isoforms showed lower divergence rate (Ks < 0.45) compared to ε protein isoforms (Ks > 0.48), suggesting class-specific divergence pattern. Synteny analysis revealed that mesohexaploid B. rapa genome has retained 1-5 orthologs of each Arabidopsis 14-3-3 gene, interspersed across its three fragmented sub-genomes. qRT-PCR analysis showed that 14 of the 21 BraA.GRF14 were expressed, wherein a higher abundance of non-ε transcripts was observed compared to the ε genes, indicating class-specific transcriptional bias. The BraA.GRF14 genes showed distinct expression pattern during plant developmental stages and in response to abiotic stress, phytohormone treatments, and nutrient deprivation conditions. Together, the distinct expression pattern and differential regulation of BraA.GRF14 genes indicated the occurrence of functional divergence of B. rapa 14-3-3 proteins during plant development and stress responses.

Regulation of the Regulators: Post-Translational Modifications, Subcellular, and Spatiotemporal Distribution of Plant 14-3-3 Proteins

14-3-3 proteins bind to and modulate the activity of phosphorylated proteins that regulate a variety of metabolic processes in eukaryotes. Multiple 14-3-3 isoforms are expressed in most organisms and display redundancy in both sequence and function. Plants contain the largest number of 14-3-3 isoforms. For example, Arabidopsis thaliana contains thirteen 14-3-3 genes, each of which is expressed. Interest in the plant 14-3-3 field has swelled over the past decade, largely due to the vast number of possibilities for 14-3-3 metabolic regulation. As the field progresses, it is essential to understand these proteins' activities at both the spatiotemporal and subcellular levels. This review summarizes current knowledge of 14-3-3 proteins in plants, including 14-3-3 interactions, regulatory functions, isoform specificity, and post-translational modifications. We begin with a historical overview and structural analysis of 14-3-3 proteins, which describes the basic principles of 14-3-3 function, and then discuss interactions and regulatory effects of plant 14-3-3 proteins in specific tissues and subcellular compartments. We conclude with a summary of 14-3-3 phosphorylation and current knowledge of the functional effects of this modification in plants.

Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in high light stress tolerance, the effects of AOX on chlorophyll synthesis are unclear. Previous studies indicated that during greening, chlorophyll accumulation was largely delayed in plants whose mitochondrial cyanide-resistant respiration was inhibited by knocking out nuclear encoded AOX gene. Here, we showed that this delay of chlorophyll accumulation was more significant under high light condition. Inhibition of cyanide-resistant respiration was also accompanied by the increase of plastid NADPH/NADP(+) ratio, especially under high light treatment which subsequently blocked the import of multiple plastidial proteins, such as some components of the photosynthetic electron transport chain, the Calvin-Benson cycle enzymes and malate/oxaloacetate shuttle components. Overexpression of AOX1a rescued the aox1a mutant phenotype, including the chlorophyll accumulation during greening and plastidial protein import. It thus suggests that light intensity affects chlorophyll synthesis during greening process by a metabolic signal, the AOX-derived plastidial NADPH/NADP(+) ratio change. Further, our results thus revealed a molecular mechanism of chloroplast-mitochondria interactions.

Integrative proteome analysis of Brachypodium distachyon roots and leaves reveals a synergetic responsive network under H2O2 stress

The plant oxidative stress response is vital for defense against various abiotic and biotic stresses. In this study, ultrastructural changes and the proteomic response to H2O2 stress in roots and leaves of the model plant Brachypodium distachyon were studied. Transmission electron microscopy (TEM) showed that the ultrastructural damage in roots was more serious than in leaves. Particularly, the ultrastructures of organelles and the nucleus in root tip cells were damaged, leading to the inhibition of normal biological activities of roots, which then spread throughout the plant. Based on two-dimensional electrophoresis (2-DE) and MALDI-TOF/TOF-MS, 84 and 53 differentially accumulated protein (DAP) spots representing 75 and 45 unique proteins responsive to H2O2 stress in roots and leaves, respectively, were identified. These protein species were mainly involved in signal transduction, energy metabolism, redox homeostasis/stress defense, protein folding/degradation, and cell wall/cell structure. Interestingly, two 14-3-3 proteins (GF14-B and GF14-D) were identified as DAPs in both roots and leaves. Protein-protein interaction (PPI) analysis revealed a synergetic H2O2-responsive network.

Hd3a promotes lateral branching in rice

Accumulating evidence indicates that the FLOWERING LOCUS T (FT) protein is the mobile floral signal known as florigen. A rice FT homolog, Heading date 3a (Hd3a), is transported from the phloem to shoot apical cells, where it interacts with 14-3-3 proteins and transcription factor OsFD1 to form a florigen activation complex (FAC) that activates a rice homolog of the floral identity gene APETALA1. Recent studies showed that florigen has roles in plant development beyond flowering; however, the exact nature of these roles is not well understood. It is not clear whether FT is transported to organs outside the shoot apex, and whether FAC formation is required for processes other than flowering. We show here that the Hd3a protein accumulates in axillary meristems to promote branching, and that FAC formation is required. Analysis of transgenic plants revealed that Hd3a promotes branching through lateral bud outgrowth. Hd3a protein produced in the phloem reached the axillary meristem in the lateral bud, and its transport was required for promotion of branching. Moreover, mutant Hd3a proteins defective in FAC formation but competent with respect to transport did not promote branching. Finally, we show that Hd3a promotes branching independently from strigolactone and FC1, a transcription factor that inhibits branching in rice. Together, these results suggest that Hd3a functions as a mobile signal for branching in rice.

Cotton (Gossypium hirsutum) 14-3-3 proteins participate in regulation of fibre initiation and elongation by modulating brassinosteroid signalling

Cotton (Gossypium hirsutum) fibre is an important natural raw material for textile industry in the world. Understanding the molecular mechanism of fibre development is important for the development of future cotton varieties with superior fibre quality. In this study, overexpression of Gh14-3-3L in cotton promoted fibre elongation, leading to an increase in mature fibre length. In contrast, suppression of expression of Gh14-3-3L, Gh14-3-3e and Gh14-3-3h in cotton slowed down fibre initiation and elongation. As a result, the mature fibres of the Gh14-3-3 RNAi transgenic plants were significantly shorter than those of wild type. This 'short fibre' phenotype of the 14-3-3 RNAi cotton could be partially rescued by application of 2,4-epibrassinolide (BL). Expression levels of the BR-related and fibre-related genes were altered in the Gh14-3-3 transgenic fibres. Furthermore, we identified Gh14-3-3 interacting proteins (including GhBZR1) in cotton. Site mutation assay revealed that Ser163 in GhBZR1 and Lys51/56/53 in Gh14-3-3L/e/h were required for Gh14-3-3-GhBZR1 interaction. Nuclear localization of GhBZR1 protein was induced by BR, and phosphorylation of GhBZR1 by GhBIN2 kinase was helpful for its binding to Gh14-3-3 proteins. Additionally, 14-3-3-regulated GhBZR1 protein may directly bind to GhXTH1 and GhEXP promoters to regulate gene expression for responding rapid fibre elongation. These results suggested that Gh14-3-3 proteins may be involved in regulating fibre initiation and elongation through their interacting with GhBZR1 to modulate BR signalling. Thus, our study provides the candidate intrinsic genes for improving fibre yield and quality by genetic manipulation.

Pear 14-3-3a gene (Pp14-3-3a) is regulated during fruit ripening and senescense, and involved in response to salicylic acid and ethylene signalling

14-3-3 proteins play important roles in regulating plant development and phytohormone (abscisic acid, gibberellin and brassinosteroids) signalling. However, their regulation in fruit ripening and senescense, and response to salicylic acid and ethylene signalling are yet to be illustrated. One cDNA encoding putative 14-3-3 protein was isolated from pear (Pyrus pyrifolia) and designated Pp14-3-3a. Phylogenetic analysis clearly demonstrated that Pp14-3-3a belonged to ε-like group of 14-3-3 superfamilies. Real-time quantitative PCR analysis indicated that the expression of Pp14-3-3a gene was developmentally regulated in the fruit. Further study demonstrated that Pp14-3-3a expression was inhibited by salicylic acid and induced by ethylene precursor 1-aminocyclopropane-1-carboxylic acid in pear fruit. These data suggested that Pp14-3-3a might be involved in response to salicylic acid and ethylene signalling during fruit ripening and senescence of pear.

Overexpression of the Jatropha curcas JcERF1 gene coding an AP2/ERF-type transcription factor increases tolerance to salt in transgenic tobacco

The JcERF1 gene, which is related to the ERF family (ethylene responsive factor coding genes), was isolated and characterized from the oil tree Jatropha curcas. The JcERF1 protein contains conserved an AP2/EREBP DNA-binding domain of 58 amino acid residues. The JcERF1 gene could be induced by abscisic acid, high salinity, hormones, and osmotic stress, suggesting that JcERF1 is regulated by certain components of the stress-signaling pathway. The full-length and C-terminus of JcERF1 driven by the GAL4 promoter functioned effectively as a transactivator in yeast, while its N-terminus was completely inactive. Transient expression analysis using a JcERF1-mGFP fusion gene in onion epidermal cells revealed that the JcERF1 protein is targeted to the nucleus. Transgenic tobacco plants carrying CaMV35S::JcERF1 fragments were shown to be much more salt tolerant compared to wild-type plants. Our results indicate that JcERF1 is a new member of the ERF transcription factors family that may play an important role in tolerance to environmental stress.

Higher order Arabidopsis 14-3-3 mutants show 14-3-3 involvement in primary root growth both under control and abiotic stress conditions

Arabidopsis 14-3-3 proteins are a family of conserved proteins that interact with numerous partner proteins in a phospho-specific manner, and can affect the target proteins in a number of ways; e.g. modification of enzymatic activity. We isolated T-DNA insertion lines in six 14-3-3 genes within the non-epsilon group that phylogenetically group in three closely related gene pairs. In total, 6 single, 3 double, 12 triple, and 3 quadruple mutants were generated. The mutants were phenotyped for primary root growth on control plates: single and double mutants were indistinguishable from WT, whereas six triples and all quadruples showed a shorter primary root. In addition, length of the first epidermal cell with a visible root hair bulge (LEH) was used to determine primary root elongation on medium containing mannitol and 1-aminocyclopropane-1-carboxylic acid (ACC). This analysis showed clear differences depending on the stress and 14-3-3 gene combinations. Next to the phenotypic growth analyses, a 14-3-3 pull-down assay on roots treated with and without mannitol showed that mannitol stress strongly affects the 14-3-3 interactome. In conclusion, we show gene specificity and functional redundancy among 14-3-3 proteins in primary root elongation under control and under abiotic stress conditions and changes in the 14-3-3 interactome during the onset of stress adaptation.

Analysis of functions of VIP1 and its close homologs in osmosensory responses of Arabidopsis thaliana

VIP1 is a bZIP protein in Arabidopsis thaliana. VIP1 accumulates in the nucleus under hypo-osmotic conditions and interacts with the promoters of hypo-osmolarity-responsive genes, CYP707A1 and CYP707A3 (CYP707A1/3), but neither overexpression of VIP1 nor truncation of its DNA-binding region affects the expression of CYP707A3 in vivo, raising the possibility that VIP and other proteins are functionally redundant. Here we show further analyses on VIP1 and its close homologs, namely, Arabidopsis group I bZIP proteins. The patterns of the signals of the GFP-fused group I bZIP proteins were similar in onion and Arabidopsis cells, suggesting that they have similar subcellular localization. In a yeast one-hybrid assay, the group I bZIP proteins caused reporter gene activation in the yeast reporter strain. VIP1 and other group I bZIP proteins showed positive results in a yeast two-hybrid assay and a bimolecular fluorescence complementation assay, suggesting that they physically interact. These results support the idea that they have somewhat similar functions. By gel shift assays, VIP1-binding sequences in the CYP707A1/3 promoters were confirmed to be AGCTGT/G. Their presence in the promoters of the genes that respond to hypo-osmotic conditions was evaluated using previously published microarray data. Interestingly, a significantly higher proportion of the promoters of the genes that were up-regulated by rehydration treatment and/or submergence treatment (treatment by a hypotonic solution) and a significantly lower proportion of the promoters of the genes that were down-regulated by such treatment shared AGCTGT/G. To further assess the physiological role of VIP1, constitutively nuclear-localized variants of VIP1 were generated. When overexpressed in Arabidopsis, some of them as well as VIP1 caused growth retardation under a mannitol-stressed condition, where VIP1 is localized mainly in the cytoplasm. This raises the possibility that the expression of VIP1 itself rather than its nuclear localization is responsible for regulating the mannitol responses.

Identification of putative TAL effector targets of the citrus canker pathogens shows functional convergence underlying disease development and defense response

Background

Transcriptional activator-like (TAL) effectors, formerly known as the AvrBs3/PthA protein family, are DNA-binding effectors broadly found in Xanthomonas spp. that transactivate host genes upon injection via the bacterial type three-secretion system. Biologically relevant targets of TAL effectors, i.e. host genes whose induction is vital to establish a compatible interaction, have been reported for xanthomonads that colonize rice and pepper; however, citrus genes modulated by the TAL effectors PthA“s” and PthC“s” of the citrus canker bacteria Xanthomonas citri (Xc) and Xanthomonas aurantifolii pathotype C (XaC), respectively, are poorly characterized. Of particular interest, XaC causes canker disease in its host lemon (Citrus aurantifolia), but triggers a defense response in sweet orange.

Results

Based on, 1) the TAL effector-DNA binding code, 2) gene expression data of Xc and XaC-infiltrated sweet orange leaves, and 3) citrus hypocotyls transformed with PthA2, PthA4 or PthC1, we have identified a collection of Citrus sinensis genes potentially targeted by Xc and XaC TAL effectors. Our results suggest that similar with other strains of Xanthomonas TAL effectors, PthA2 and PthA4, and PthC1 to some extent, functionally converge. In particular, towards induction of genes involved in the auxin and gibberellin synthesis and response, cell division, and defense response. We also present evidence indicating that the TAL effectors act as transcriptional repressors and that the best scoring predicted DNA targets of PthA“s” and PthC“s” in citrus promoters predominantly overlap with or localize near to TATA boxes of core promoters, supporting the idea that TAL effectors interact with the host basal transcriptional machinery to recruit the RNA pol II and start transcription.

Conclusions

The identification of PthA“s” and PthC“s” targets, such as the LOB (LATERAL ORGAN BOUNDARY) and CCNBS genes that we report here, is key for the understanding of the canker symptoms development during host susceptibility, or the defenses of sweet orange against the canker bacteria. We have narrowed down candidate targets to a few, which pointed out the host metabolic pathways explored by the pathogens.

A Glycine soja 14-3-3 protein GsGF14o participates in stomatal and root hair development and drought tolerance in Arabidopsis thaliana

It is well established that 14-3-3 proteins are key regulators of multiple stress signal transduction cascades. However, the biological functions of soybean 14-3-3 proteins, especially in plant drought response, are not yet known. In this study, we characterized a Glycine soja 14-3-3 gene, GsGF14o, which is involved in plant development and drought response. GsGF14o expression was greatly induced by drought stress, as evidenced by the quantitative real-time PCR and β-glucuronidase (GUS) activity analysis. GsGF14o overexpression in Arabidopsis thaliana resulted in decreased drought tolerance during seed germination and seedling growth. Furthermore, silencing of AtGF14µ, the most homologous 14-3-3 gene of GsGF14o, led to enhanced drought tolerance at both the seed germination and seedling stage. Unexpectedly, GsGF14o transgenic lines showed reduced water loss and transpiration rates compared with wild-type plants, which was demonstrated to be the consequence of the decreased stomatal size. At the same time, the smaller stomata due to GsGF14o overexpression led to a relatively slow net photosynthesis rate, which led to a growth penalty under drought stress. We further demonstrated that GsGF14o overexpression caused deficits in root hair formation and development, and thereby reduced the water intake capacity of the transgenic root system. In addition, GsGF14o overexpression down-regulated the transcript levels of drought-responsive marker genes. Finally, we also investigated the tissue-specific accumulation of GsGF14o by using a GUS activity assay. Collectively, the results presented here confirm that GsGF14o plays a dual role in drought stress responses through its involvement in the regulation of stomatal size and root hair development.

NYC4, the rice ortholog of Arabidopsis THF1, is involved in the degradation of chlorophyll - protein complexes during leaf senescence

Yellowing/chlorophyll breakdown is a prominent phenomenon in leaf senescence, and is associated with the degradation of chlorophyll - protein complexes. From a rice mutant population generated by ionizing radiation, we isolated nyc4-1, a stay-green mutant with a defect in chlorophyll breakdown during leaf senescence. Using gene mapping, nyc4-1 was found to be linked to two chromosomal regions. We extracted Os07g0558500 as a candidate for NYC4 via gene expression microarray analysis, and concluded from further evidence that disruption of the gene by a translocation-related event causes the nyc4 phenotype. Os07g0558500 is thought to be the ortholog of THF1 in Arabidopsis thaliana. The thf1 mutant leaves show variegation in a light intensity-dependent manner. Surprisingly, the Fv /Fm value remained high in nyc4-1 during the dark incubation, suggesting that photosystem II retained its function. Western blot analysis revealed that, in nyc4-1, the PSII core subunits D1 and D2 were significantly retained during leaf senescence in comparison with wild-type and other non-functional stay-green mutants, including sgr-2, a mutant of the key regulator of chlorophyll degradation SGR. The role of NYC4 in degradation of chlorophyll and chlorophyll - protein complexes during leaf senescence is discussed.

Plant 14-3-3 proteins as spiders in a web of phosphorylation

Protein phosphorylation is essential for many aspects of plant growth and development. To fully modulate the activity of specific proteins after phosphorylation, interaction with members of the 14-3-3 family is necessary. 14-3-3 Proteins are important for many processes because they "assist" a wide range of target proteins with divergent functions. In this review, we will describe how plant 14-3-3 proteins are as spiders in a web of phosphorylation: they act as sensors for phospho-motifs, they themselves are phosphorylated with unknown consequences and they have kinases as target, where some of these phosphorylate 14-3-3 binding motifs in other proteins. Two specific classes of 14-3-3 targets, protein kinases and transcription factors of the bZIP and basic helix-loop-helix-like families, with important and diverse functions in the plant as a whole will be discussed. An important question to be addressed in the near future is how the interaction with 14-3-3 proteins has diverged, both structurally and functionally, between different members of the same protein family, like the kinases and transcription factors.

Functional diversification of FD transcription factors in rice, components of florigen activation complexes

Florigen, a protein encoded by the FLOWERING LOCUS T (FT) in Arabidopsis and Heading date 3a (Hd3a) in rice, is the universal flowering hormone in plants. Florigen is transported from leaves to the shoot apical meristem and initiates floral evocation. In shoot apical cells, conserved cytoplasmic 14-3-3 proteins act as florigen receptors. A hexameric florigen activation complex (FAC) composed of Hd3a, 14-3-3 proteins, and OsFD1, a transcription factor, activates OsMADS15, a rice homolog of Arabidopsis APETALA1, leading to flowering. Because FD is a key component of the FAC, we characterized the FD gene family and their functions. Phylogenetic analysis of FD genes indicated that this family is divided into two groups: (i) canonical FD genes that are conserved among eudicots and non-Poaceae monocots; and (ii) Poaceae-specific FD genes that are organized into three subgroups: Poaceae FD1, FD2 and FD3. The Poaceae FD1 group shares a small sequence motif, T(A/V)LSLNS, with FDs of eudicots and non-Poaceae monocots. Overexpression of OsFD2, a member of the Poaceae FD2 group, produced smaller leaves with shorter plastochrons, suggesting that OsFD2 controls leaf development. In vivo subcellular localization of Hd3a, 14-3-3 and OsFD2 suggested that in contrast to OsFD1, OsFD2 is restricted to the cytoplasm through its interaction with the cytoplasmic 14-3-3 proteins, and interaction of Hd3a with 14-3-3 facilitates nuclear translocation of the FAC containing OsFD2. These results suggest that FD function has diverged between OsFD1 and OsFD2, but formation of a FAC is essential for their function.

Isolation and molecular characterization of GmERF7, a soybean ethylene-response factor that increases salt stress tolerance in tobacco

Ethylene-response factors (ERFs) play an important role in regulating gene expression in plant responses to biotic and abiotic stresses. In this study, a new ERF transcription factor, GmERF7, was isolated from soybean. Sequence analysis showed that GmERF7 contained an AP2/ERF domain with 58 amino acids, two putative nuclear localization signal (NLS) domains, an acidic amino acid-rich transcriptional activation domain and a conserved N-terminal motif [MCGGAI(I/L)]. The expression of GmERF7 was induced by drought, salt, methyl jasmonate (MeJA), ethylene (ETH) and abscisic acid (ABA) treatments. However, the expression of GmERF7 decreased under cold treatment. GmERF7 localized to the nucleus when transiently expressed in onion epidermal cells. Furthermore, GmERF7 protein bound to the GCC-box element in vitro and activated the expression of the β-glucuronidase (GUS) reporter gene in tobacco leaves. Activities of GmERF7 promoter (GmERF7P) upregulated in tobacco leaves with 10h drought, salt and ETH treatments. However, activities of GmERF7P decreased with 10h cold and ABA treatments. Overexpression of GmERF7 in tobacco plants led to higher levels of chlorophyll and soluble carbohydrates and a lower level of malondialdehyde compared with wild-type tobacco plants under salt stress conditions, which indicated that GmERF7 enhanced salt tolerance in transgenic plants.

Phytochrome B inhibits binding of phytochrome-interacting factors to their target promoters

Phytochromes are red and far-red light receptors in plants that mediate critical responses to light throughout the lifecycle. They achieve this in part by targeting negatively acting bHLH transcription factors called phytochrome-interacting factors (PIFs) for degradation within the nucleus. However, it is not known whether protein degradation is the primary mechanism by which phytochromes inhibit these repressors of photomorphogenesis. Here, we use chromatin immunoprecipitation to show that phyB inhibits the regulatory activity of PIF1 and PIF3 by releasing them from their DNA targets. The N-terminal fragment of phyB (NG-GUS-NLS; NGB) also inhibits binding of PIF3 to its target promoters. However, unlike full-length phyB, NGB does not promote PIF3 degradation, establishing the activity of NGB reflects its ability to inhibit PIF binding to DNA. We further show that Pfr forms of both full-length phyB and NGB inhibit DNA binding of PIF1 and PIF3 in vitro. Taken together, our results indicate that phyB inhibition of PIF function involves two separate processes: sequestration and protein degradation.

Nicotiana tabacum Tsip1-interacting ferredoxin 1 affects biotic and abiotic stress resistance

Tsip1, a Zn finger protein that was isolated as a direct interactor with tobacco stress-induced 1 (Tsi1), plays an important role in both biotic and abiotic stress signaling. To further understand Tsip1 function, we searched for more Tsip1-interacting proteins by yeast two-hybrid screening using a tobacco cDNA library. Screening identified a new Tsip1-interacting protein, Nicotiana tabacum Tsip1-interacting ferredoxin 1 (NtTfd1), and binding specificity was confirmed both in vitro and in vivo. The four repeats of a cysteine-rich motif (CXXCXGXG) of Tsip1 proved important for binding to NtTfd1. Virus-induced gene silencing of NtTfd1, Tsip1, and NtTfd1/Tsip1 rendered plants more susceptible to salinity stress compared with TRV2 control plants. NtTfd1- and Tsip1-silenced tobacco plants were more susceptible to infection by Cucumber mosaic virus compared with control plants. These results suggest that NtTfd1 might be involved in the regulation of biotic and abiotic stresses in chloroplasts by interaction with Tsip1.

14-3-3 proteins regulate the intracellular localization of the transcriptional activator GmMYB176 and affect isoflavonoid synthesis in soybean

Isoflavonoids are legume-specific natural plant compounds that play important functions in nitrogen fixation as well as biotic and abiotic stress responses. Many clinical studies have suggested a role for isoflavonoids in human health and nutrition. We have recently identified an R1 MYB transcription factor GmMYB176 that regulates CHS8 gene expression and isoflavonoid biosynthesis. Here we demonstrate that binding of 14-3-3 proteins to GmMYB176 modulates this function. GmMYB176 interacts with all 16 14-3-3 proteins (SGF14s) in soybean (Glycine max) with varying activity. The detailed analysis of 14-3-3-binding sites within GmMYB176 identified a critical motif (D2) where Ser29 is potentially phosphorylated. Deletion of the D2 motif from GmMYB176 or substitution of Ser29 with an alanine abolished binding with SGF14 proteins, which altered the subcellular localization of GmMYB176. Overexpression of SGF14l in soybean hairy roots did not affect the transcript level of GmMYB176 but it reduced the expression levels of key isoflavonoid genes and isoflavonoid accumulation in soybean hairy root. Our results suggest that SGF14-GmMYB176 interaction regulates the intracellular localization of GmMYB176, thereby affecting isoflavonoid biosynthesis in soybean.

Expression of OsSPY and 14-3-3 genes involved in plant height variations of ion-beam-induced KDML 105 rice mutants

The culm length of two semidwarf rice mutants (PKOS1, HyKOS1) obtained from low-energy N-ion beam bombardments of dehusked Thai jasmine rice (Oryza sativa L. cv. KDML 105) seeds showed 25.7% and 21.5% height reductions and one spindly rice mutant (TKOS4) showed 21.4% increase in comparison with that of the KDML 105 control. A cDNA-RAPD analysis identified differential gene expression in internode tissues of the rice mutants. Two genes identified from the cDNA-RAPD were OsSPY and 14-3-3, possibly associated with stem height variations of the semidwarf and spindly mutants, respectively. The OsSPY gene encoded the SPY protein which is considered to be a negative regulator of gibberellin (GA). On the other hand, the 14-3-3 encoded a signaling protein which can bind and prevent the RSG (repression of shoot growth) protein function as a transcriptional repressor of the kaurene oxidase (KO) gene in the GA biosynthetic pathway. Expression analysis of OsSPY, 14-3-3, RSG, KO, and SLR1 was confirmed in rice internode tissues during the reproductive stage of the plants by semi-quantitative RT-PCR technique. The expression analysis showed a clear increase of the levels of OsSPY transcripts in PKOS1 and HyKOS1 tissue samples compared to that of the KDML 105 and TKOS4 samples at the age of 50-60 days which were at the ages of internode elongation. The 14-3-3 expression had the highest increase in the TKOS4 samples compared to those in KDML 105, PKOS1 and HyKOS1 samples. The expression analysis of RSG and KO showed an increase in TKOS4 samples compared to that of the KDML 105 and that of the two semidwarf mutants. These results indicate that changes of OsSPY and 14-3-3 expression could affect internode elongation and cause the phenotypic changes of semidwarf and spindly rice mutants, respectively.

Subcellular localization of class II HDAs in Arabidopsis thaliana: nucleocytoplasmic shuttling of HDA15 is driven by light

Class II histone deacetylases in humans and other model organisms undergo nucleocytoplasmic shuttling. This unique functional regulatory mechanism has been well elucidated in eukaryotic organisms except in plant systems. In this study, we have paved the baseline evidence for the cytoplasmic and nuclear localization of Class II HDAs as well as their mRNA expression patterns. RT-PCR analysis on the different vegetative parts and developmental stages reveal that Class II HDAs are ubiquitously expressed in all tissues with minimal developmental specificity. Moreover, stable and transient expression assays using HDA-YFP/GFP fusion constructs indicate cytoplasmic localization of HDA5, HDA8, and HDA14 further suggesting their potential for nuclear transport and deacetylating organellar and cytoplasmic proteins. Organelle markers and stains confirm HDA14 to abound in the mitochondria and chloroplasts while HDA5 localizes in the ER. HDA15, on the other hand, shuttles in and out of the nucleus upon light exposure. In the absence of light, it is exported out of the nucleus where further re-exposition to light treatments signals its nuclear import. Unlike HDA5 which binds with 14-3-3 proteins, HDA15 fails to interact with these chaperones. Instead, HDA15 relies on its own nuclear localization and export signals to navigate its subcellular compartmentalization classifying it as a Class IIb HDA. Our study indicates that nucleocytoplasmic shuttling is indeed a hallmark for all eukaryotic Class II histone deacetylases.

Expression of the maize ZmGF14-6 gene in rice confers tolerance to drought stress while enhancing susceptibility to pathogen infection

14-3-3 proteins are found in all eukaryotes where they act as regulators of diverse signalling pathways associated with a wide range of biological processes. In this study the functional characterization of the ZmGF14-6 gene encoding a maize 14-3-3 protein is reported. Gene expression analyses indicated that ZmGF14-6 is up-regulated by fungal infection and salt treatment in maize plants, whereas its expression is down-regulated by drought stress. It is reported that rice plants constitutively expressing ZmGF14-6 displayed enhanced tolerance to drought stress which was accompanied by a stronger induction of drought-associated rice genes. However, rice plants expressing ZmGF14-6 either in a constitutive or under a pathogen-inducible regime showed a higher susceptibility to infection by the fungal pathogens Fusarium verticillioides and Magnaporthe oryzae. Under infection conditions, a lower intensity in the expression of defence-related genes occurred in ZmGF14-6 rice plants. These findings support that ZmGF14-6 positively regulates drought tolerance in transgenic rice while negatively modulating the plant defence response to pathogen infection. Transient expression assays of fluorescently labelled ZmGF14-6 protein in onion epidermal cells revealed a widespread distribution of ZmGF14-6 in the cytoplasm and nucleus. Additionally, colocalization experiments of fluorescently labelled ZmGF14-6 with organelle markers, in combination with cell labelling with the endocytic tracer FM4-64, revealed a subcellular localization of ZmGF14-6 in the early endosomes. Taken together, these results improve our understanding of the role of ZmGF14-6 in stress signalling pathways, while indicating that ZmGF14-6 inversely regulates the plant response to biotic and abiotic stresses.

Dual role of BKI1 and 14-3-3 s in brassinosteroid signaling to link receptor with transcription factors

The plasma membrane-localized plant steroid hormone receptor, BRASSINOSTEROID INSENSITIVE 1 (BRI1), is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1). Here, we report that the steroid-induced, plasma membrane-dissociated and phosphorylated BKI1 also plays positive roles in BR signaling by interacting with a subset of 14-3-3 proteins. The cytosolic fraction of BKI1 carboxyl terminal region enhances BR signaling. Mutations of two serine residues in this region lead to reduced phosphorylation by the BRI1 kinase and constitutive plasma membrane localization. The 14-3-3 proteins can interact with the phosphorylated BKI1 through a motif that contains the two phosphorylation sites to release inhibition of BRI1 by BKI1. Meanwhile, the cytosolic BKI1 antagonizes the 14-3-3 s and enhances accumulation of BRI1 EMS SUPPRESSOR 1 (BES1)/BRASSINAZOLE RESISTANT 1 (BZR1) in the nucleus to regulate BR-responses.

14-3-3 proteins in plant physiology

Plant 14-3-3 isoforms, like their highly conserved homologues in mammals, function by binding to phosphorylated client proteins to modulate their function. Through the regulation of a diverse range of proteins including kinases, transcription factors, structural proteins, ion channels and pathogen defense-related proteins, they are being implicated in an expanding catalogue of physiological functions in plants. 14-3-3s themselves are affected, both transcriptionally and functionally, by the extracellular and intracellular environment of the plant. They can modulate signaling pathways that transduce inputs from the environment and also the downstream proteins that elicit the physiological response. This review covers some of the key emerging roles for plant 14-3-3s including their role in the response to the plant extracellular environment, particularly environmental stress, pathogens and light conditions. We also address potential key roles in primary metabolism, hormone signaling, growth and cell division.

The mechanism of substrate recognition of Ca2+-dependent protein kinases

Ca2+-dependent protein kinases (CDPKs) are encoded by a multigene family and are thought to play central roles in Ca2+ signaling in plants. Although the primary structures of CDPK isoforms are highly conserved, several studies suggested a distinct physiological function for each CDPK isoform in plants. Hence, there should be mechanisms by which individual CDPK specifically recognizes its substrate. Recently, the variable N-terminal domain of NtCDPK1 was shown to play an essential role in the specific recognition of the substrate. Because the variable N-terminal domain of other CDPKs may also be involved in the substrate recognition, the search for interacting proteins of the variable N-terminal domain would provide important clues to identify the physiological substrates of each CDPK. Additionally, manipulation of the variable N-terminal domain may enable us to engineer the substrate specificity of CDPK, leading a rational rewiring of cellular signaling pathways.

Soybean 14-3-3 gene family: identification and molecular characterization

The 14-3-3s are a group of proteins that are ubiquitously found in eukaryotes. Plant 14-3-3 proteins are encoded by a large multigene family and are involved in signaling pathways to regulate plant development and protection from stress. Recent studies in Arabidopsis and rice have demonstrated the isoform specificity in 14-3-3s and their client protein interactions. However, detailed characterization of 14-3-3 gene family in legumes has not been reported. In this study, soybean 14-3-3 proteins were identified and their molecular characterization performed. Data mining of soybean genome and expressed sequence tag databases identified 18 14-3-3 genes, of them 16 are transcribed. All 16 SGF14s have higher expression in embryo tissues suggesting their potential role in seed development. Subcellular localization of all transcribed SGF14s demonstrated that 14-3-3 proteins in soybean have isoform specificity, however, some overlaps were also observed between closely related isoforms. A comparative analysis of SGF14s with Arabidopsis and rice 14-3-3s indicated that SGF14s also group into epsilon and non-epsilon classes. However, unlike Arabidopsis and rice 14-3-3s, SGF14s contained only one kind of gene structure belonging to each class. Overall, soybean consists of the largest family of 14-3-3 proteins characterized to date. Our results provide a solid framework for further investigations into the role of SGF14s and their involvement in legume-specific functions.

Nucleo-cytoplasmic transport of proteins and RNA in plants

Transport of macromolecules between the nucleus and the cytoplasm is an essential necessity in eukaryotic cells, since the nuclear envelope separates transcription from translation. In the past few years, an increasing number of components of the plant nuclear transport machinery have been characterised. This progress, although far from being completed, confirmed that the general characteristics of nuclear transport are conserved between plants and other organisms. However, plant-specific components were also identified. Interestingly, several mutants in genes encoding components of the plant nuclear transport machinery were investigated, revealing differential sensitivity of plant-specific pathways to impaired nuclear transport. These findings attracted attention towards plant-specific cargoes that are transported over the nuclear envelope, unravelling connections between nuclear transport and components of signalling and developmental pathways. The current state of research in plants is summarised in comparison to yeast and vertebrate systems, and special emphasis is given to plant nuclear transport mutants.

bZIP transcription factor RSG controls the feedback regulation of NtGA20ox1 via intracellular localization and epigenetic mechanism

Gibberellins (GAs) are phytohormones that regulate growth and development throughout the life cycle of plants. Negative feedback contributes to homeostasis of GA levels. DELLA proteins are involved in this process. Since DELLA proteins do not have apparent DNA binding motifs, other DNA binding proteins might act as a mediator downstream of DELLA proteins in the GA feedback regulation. In this review, we highlight the mechanisms of GA feedback regulation, specifically the differential regulation of GA 20-oxidase (GA20ox) and GA 3-oxidase (GA3ox) by transcription factors. RSG (REPRESSION OF SHOOT GROWTH) is a tobacco (Nicotiana tabacum) transcriptional activator with a basic leucine zipper domain that controls the levels of endogenous GAs through the regulation of GA biosynthesis genes. Recently we reported that RSG not only regulates the expression of ent-kaurene oxidase gene but is also involved in the negative feedback of NtGA20ox1 by GAs. RSG plays a role in the homeostasis of GAs through direct binding to the NtGA20ox1 promoter triggered by a decrease in GA levels in the cell. Furthermore, decreases in GA levels promote modifications of active histone marks on the NtGA20ox1 promoter. We have developed a hypothetical model to explain how RSG regulates dual target genes via epigenetic regulation.

Temperature controls nuclear import of Tam3 transposase in Antirrhinum

It has been proposed that environmental stimuli can activate transposable elements (TEs), whereas few substantial mechanisms have been shown so far. The class-II element Tam3 from Antirrhinum majus exhibits a unique property of low-temperature-dependent transposition (LTDT). LTDT has proved invaluable in developing the gene isolation technologies that have underpinned much of modern plant developmental biology. Here, we reveal that LTDT involves differential subcellular localization of the Tam3 transposase (TPase) in cells grown at low (15°C) and high (25°C) temperatures. The mechanism is associated with the nuclear import of Tam3 TPase in Antirrhinum cells. At high temperature, the nuclear import of Tam3 TPase is severely restricted in Antirrhinum cells, whereas at low temperature, the nuclear localization of Tam3 TPase is observed in about 20% of the cells. However, in tobacco BY-2 and Allium cepa (onion) cells, Tam3 TPase is transported into most nuclei. In addition to three nuclear localization signals (NLSs), the Tam3 TPase is equipped with a nuclear localization inhibitory domain (NLID), which functions to abolish nuclear import of the TPase at high temperature in Antirrhinum. NLID in Tam3 TPase is considered to interact with Antirrhinum-specific factor(s). The host-specific regulation of the nuclear localization of transposase represents a new repertoire controlling class-II TEs.

Interactome analysis of the six cotton 14-3-3s that are preferentially expressed in fibres and involved in cell elongation

Proteins of the 14-3-3 family regulate a divergent set of signalling pathways in all eukaryotic organisms. In this study, several cDNAs encoding 14-3-3 proteins were isolated from a cotton fibre cDNA library. The Gh14-3-3 genes share high sequence homology at the nucleotide level in the coding region and at the amino acid level. Real-time quantitative RT-PCR analysis indicated that the expression of these Gh14-3-3 genes is developmentally regulated in fibres, and reached their peak at the stage of rapid cell elongation of fibre development. Furthermore, overexpression of Gh14-3-3a, Gh14-3-3e, and Gh14-3-3L in fission yeast promoted atypical longitudinal growth of the host cells. Yeast two-hybrid analysis revealed that the interaction between cotton 14-3-3 proteins is isoform selective. Through yeast two-hybrid screening, 38 novel interaction partners of the six 14-3-3 proteins (Gh14-3-3a, Gh14-3-3e, Gh14-3-3f, Gh14-3-3g, Gh14-3-3h, and Gh14-3-3L), which are involved in plant development, metabolism, signalling transduction, and other cellular processes, were identified in cotton fibres. Taking these data together, it is proposed that the Gh14-3-3 proteins may participate in regulation of fibre cell elongation. Thus, the results of this study provide novel insights into the 14-3-3 signalling related to fibre development of cotton.

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.

Isolation and characterization of a novel EAR-motif-containing gene GmERF4 from soybean (Glycine max L.)

Transcriptional repressors are emerging as central regulators of development and stress responses in different organisms. The ERF-associated amphiphilic repression (EAR) motif was identified as essential for transcriptional repression. To gain a better understanding of this type of protein, we reported here a novel GmERF4 protein from soybean. Sequence alignment showed that GmERF4 contains one AP2/ERF domain, two putative nuclear localization signal regions and one EAR motif. The GmERF4 protein was preferentially localized to the nucleus of onion epidermis cells and bound specifically to the GCC box and DRE/CRT element in vitro. Furthermore, the expression of GmERF4 was induced by ethylene, JA, SA, cold, salt, drought, and soybean mosaic virus, and repressed by ABA. Constitutive expression of GmERF4 in transgenic tobacco plants increased tolerance to salt and drought stresses compared with wild-type plants, but did not exhibit detectable resistance against bacterial infection.

The Arabidopsis nuclear pore and nuclear envelope

The nuclear envelope is a double membrane structure that separates the eukaryotic cytoplasm from the nucleoplasm. The nuclear pores embedded in the nuclear envelope are the sole gateways for macromolecular trafficking in and out of the nucleus. The nuclear pore complexes assembled at the nuclear pores are large protein conglomerates composed of multiple units of about 30 different nucleoporins. Proteins and RNAs traffic through the nuclear pore complexes, enabled by the interacting activities of nuclear transport receptors, nucleoporins, and elements of the Ran GTPase cycle. In addition to directional and possibly selective protein and RNA nuclear import and export, the nuclear pore gains increasing prominence as a spatial organizer of cellular processes, such as sumoylation and desumoylation. Individual nucleoporins and whole nuclear pore subcomplexes traffic to specific mitotic locations and have mitotic functions, for example at the kinetochores, in spindle assembly, and in conjunction with the checkpoints. Mutants of nucleoporin genes and genes of nuclear transport components lead to a wide array of defects from human diseases to compromised plant defense responses. The nuclear envelope acts as a repository of calcium, and its inner membrane is populated by functionally unique proteins connected to both chromatin and-through the nuclear envelope lumen-the cytoplasmic cytoskeleton. Plant nuclear pore and nuclear envelope research-predominantly focusing on Arabidopsis as a model-is discovering both similarities and surprisingly unique aspects compared to the more mature model systems. This chapter gives an overview of our current knowledge in the field and of exciting areas awaiting further exploration.

Diversity, regulation, and evolution of the gibberellin biosynthetic pathway in fungi compared to plants and bacteria

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. GAs were first isolated as metabolites of a fungal rice pathogen, Gibberella fujikuroi, since renamed Fusarium fujikuroi. Although higher plants and the fungus produce structurally identical GAs, significant differences in their GA pathways, enzymes involved and gene regulation became apparent with the identification of GA biosynthetic genes in Arabidopsis thaliana and F. fujikuroi. Recent identifications of GA biosynthetic gene clusters in two other fungi, Phaeosphaeria spp. and Sphaceloma manihoticola, and the high conservation of GA cluster organization in these distantly related fungal species indicate that fungi evolved GA and other diterpene biosynthetic pathways independently from plants. Furthermore, the occurrence of GAs and recent identification of the first GA biosynthetic genes in the bacterium Bradyrhizobium japonicum make it possible to study evolution of GA pathways in general. In this review, we summarize our current understanding of the GA biosynthesis pathway, specifically the genes and enzymes involved as well as gene regulation and localization in the genomes of different fungi and compare it with that in higher and lower plants and bacteria.

Defect in non-yellow coloring 3, an alpha/beta hydrolase-fold family protein, causes a stay-green phenotype during leaf senescence in rice

Chlorophyll degradation is an important phenomenon in the senescence process. It is necessary for the degradation of certain chlorophyll-protein complexes and thylakoid membranes during leaf senescence. Mutants retaining greenness during leaf senescence are known as 'stay-green' mutants. Non-functional type stay-green mutants, which possess defects in chlorophyll degradation, retain greenness but not leaf functionality during senescence. Here, we report a new stay-green mutant in rice, nyc3. nyc3 retained a higher chlorophyll a and chlorophyll b content than the wild-type but showed a decrease in other senescence parameters during dark incubation, suggesting that it is a non-functional stay-green mutant. In addition, a small amount of pheophytin a, a chlorophyll a-derivative without Mg(2+) ions in its tetrapyrrole ring, accumulated in the senescent leaves of nyc3. nyc3 shows a similar but weaker phenotype to stay green (sgr), another non-functional stay-green mutant in rice. The chlorophyll content of nyc3 sgr double mutants at the late stage of leaf senescence was also similar to that of sgr. Linkage analysis revealed that NYC3 is located near the centromere region of chromosome 6. Map-based cloning of genes near the centromere is very difficult because of the low recombination rate; however, we overcame this problem by using ionizing radiation-induced mutant alleles harboring deletions of hundreds of kilobases. Thus, it was revealed that NYC3 encodes a plastid-localizing alpha/beta hydrolase-fold family protein with an esterase/lipase motif. The possible function of NYC3 in the regulation of chlorophyll degradation is discussed.

CDPK1, a calcium-dependent protein kinase, regulates transcriptional activator RSG in response to gibberellins

The homeostasis of gibberellins (GAs) is maintained by negative-feedback regulation in plant cells. REPRESSION OF SHOOT GROWTH (RSG) is a transcriptional activator with a basic Leu zipper domain suggested to contribute GA feedback regulation by the transcriptional regulation of genes encoding 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; however, the kinase that catalyzes the reaction is unknown. Recently a Ca(2+)-dependent protein kinase (CDPK) was identified as an RSG kinase that promotes 14-3-3 binding of RSG by phosphorylation of the Ser-114 of RSG. Our results suggest that CDPK decodes the Ca(2+) signal produced by GAs and regulates the intracellular localization of RSG in plant cells.

Dual role for 14-3-3 proteins and ABF transcription factors in gibberellic acid and abscisic acid signalling in barley (Hordeum vulgare) aleurone cells

The balance of gibberellins [gibberellic acid (GA)] and abscisic acid (ABA) is a determining factor during transition of embryogenesis and seed germination. Recently, we showed that 14-3-3 proteins are important in ABA signalling in barley aleurone cells. Using 14-3-3 RNAi constructs in the barley aleurone transient expression system, we demonstrate here that silencing of each 14-3-3 isoform suppresses GA induction of the alpha-amylase gene. 14-3-3 Proteins interact with ABA-responsive element (ABRE) binding factors HvABF1, 2 and 3, and here we show that these transcription factors also interact with the ABA-responsive kinase PKABA1, a kinase that mediates cross-talk between the GA and ABA pathway. ABF1 and ABF2 have a function in both signalling pathways as: (1) ectopic expression of wild-type ABF1 and mutant ABF2, lacking the 14-3-3 interaction domain, transactivates the ABA inducible HVA1 gene; and (2) GA induction of the alpha-amylase gene is repressed by ectopic expression of wild-type ABF1 and 2. Mutant ABF1 and 2 were still effective repressors of GA signalling. In summary, our data provide evidence that 14-3-3 proteins and members of the ABF transcription factor family have a regulatory function in the GA pathway and suggest that PKABA1 and ABF transcription factors are cross-talk intermediates in ABA and GA signalling.