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

14-3-3 and FHA domains mediate phosphoprotein interactions

Many aspects of plant growth and development require specific protein interactions to carry out biochemical and cellular functions. Several proteins mediate these interactions, two of which specifically recognize phosphoproteins: 14-3-3 proteins and proteins with FHA domains. These are the only phosphobinding domains identified in plants. Both domains are present in animals and plants, and are used by plant proteins to regulate metabolic, developmental, and signaling pathways. 14-3-3s regulate sugar metabolism, proton gradients, and control transcription factor localization. FHA domains are modular domains often found in multidomain proteins that are involved in signal transduction and plant development.

Two short-chain dehydrogenase/reductases, NON-YELLOW COLORING 1 and NYC1-LIKE, are required for chlorophyll b and light-harvesting complex II degradation during senescence in rice

Yellowing, which is related to the degradation of chlorophyll and chlorophyll-protein complexes, is a notable phenomenon during leaf senescence. NON-YELLOW COLORING 1 (NYC1) in rice encodes a membrane-localized short-chain dehydrogenase/reductase (SDR) that is thought to represent a chlorophyll b reductase necessary for catalyzing the first step of chlorophyll b degradation. Analysis of the nyc1 mutant, which shows the stay-green phenotype, revealed that chlorophyll b degradation is required for the degradation of light-harvesting complex II and thylakoid grana in leaf senescence. Phylogenetic analysis further revealed the existence of NYC1-LIKE (NOL) as the most closely related protein to NYC1. In the present paper, the nol mutant in rice was also found to show a stay-green phenotype very similar to that of the nyc1 mutant, i.e. the degradation of chlorophyll b was severely inhibited and light-harvesting complex II was selectively retained during senescence, resulting in the retention of thylakoid grana even at a late stage of senescence. The nyc1 nol double mutant did not show prominent enhancement of inhibition of chlorophyll degradation. NOL was localized on the stromal side of the thylakoid membrane despite the lack of a transmembrane domain. Immunoprecipitation analysis revealed that NOL and NYC1 interact physically in vitro. These observations suggest that NOL and NYC1 are co-localized in the thylakoid membrane and act in the form of a complex as a chlorophyll b reductase in rice.

Overexpression of Arabidopsis sorting nexin AtSNX2b inhibits endocytic trafficking to the vacuole

Sorting nexins are conserved proteins that function in vesicular trafficking and contain a characteristic phox homology (PX) domain. Here, we characterize the ubiquitously expressed Arabidopsis thaliana sorting nexin AtSNX2b. Sub-cellular fractionation studies indicate that AtSNX2b is peripherally associated with membranes. The AtSNX2b PX domain binds to phosphatidylinositol 3-phosphate in vitro and this association is required for the localization of GFP-AtSNX2b to punctate structures in vivo, identified as the trans-Golgi network, prevacuolar compartment and endosomes. Overexpression of GFP-tagged AtSNX2b produces enlarged GFP-labeled compartments that can also be labeled by the endocytic tracer FM4-64. Endocytic trafficking of FM4-64 to the vacuole is arrested in these GFP-AtSNX2b compartments, and similar FM4-64-accumulating compartments are seen upon overexpression of untagged AtSNX2b. This suggests that exit of membrane components from these enlarged or aggregated endosomes is inhibited. Vacuolar proteins containing an N-terminal propeptide, but not those with a C-terminal propeptide, are also present in these enlarged compartments. We hypothesize that AtSNX2b is involved in vesicular trafficking from endosomes to the vacuole.

Vascular-related NAC-DOMAIN7 is involved in the differentiation of all types of xylem vessels in Arabidopsis roots and shoots

SUMMARY

The Arabidopsis thaliana NAC domain transcription factor, vascular-related NAC-DOMAIN7 (VND7), plays a pivotal role in regulating the differentiation of root protoxylem vessels. In order to understand the mechanisms underscoring the function of VND7 in vessel differentiation in more detail, we conducted extensive molecular analyses in yeast (Saccharomyces cerevisiae), Arabidopsis, and Nicotiana tabacum L. cv. Bright Yellow 2 (tobacco BY-2) cells. The transcriptional activation activity of VND7 was confirmed in yeast and Arabidopsis, and the C-terminal region was shown to be required for VND7 transcriptional activation. Expression of the C-terminus-truncated VND7 protein under the control of the native VND7 promoter resulted in inhibition of the normal development of metaxylem vessels in roots and vessels in aerial organs, as well as protoxylem vessels in roots. The expression pattern of VND7 overlapped that of VND2 to VND5 in most of the differentiating vessels. Furthermore, a yeast two-hybrid assay revealed the ability of VND7 to form homodimers and heterodimers with other VND proteins via their N-termini, which include the NAC domain. The heterologous expression of VND7 in tobacco BY-2 cells demonstrated that the stability of VND7 could be regulated by proteasome-mediated degradation. Together these data suggest that VND7 regulates the differentiation of all types of vessels in roots and shoots, possibly in cooperation with VND2 to VND5 and other regulatory proteins.

Identification of a host 14-3-3 Protein that Interacts with Xanthomonas effector AvrRxv

AvrRxv is a member of a family of pathogen effectors present in pathogens of both plant and mammalian species. Xanthomonas campestris pv. vesicatoria strains carrying AvrRxv induce a hypersensitive response (HR) in the tomato cultivar Hawaii 7998. Using a yeast two-hybrid screen, we identified a 14-3-3 protein from tomato that interacts with AvrRxv called AvrRxv Interactor 1 (ARI1). The interaction was confirmed in vitro with affinity chromatography. Using mutagenesis, we identified a 14-3-3-binding domain in AvrRxv and demonstrated that a mutant in that domain showed concomitant loss of interaction with ARI1 and HR-inducing activity in tomato. These results demonstrate that the AvrRxv bacterial effector recruits 14-3-3 proteins for its function within host cells. AvrRxv homologues YopP and YopJ from Yersinia do not have AvrRxv-specific HR-inducing activity when delivered into tomato host cells by Agrobacterium. Although YopP itself cannot induce HR, its C-terminal domain containing the catalytic residues can replace that of AvrRxv in an AvrRxv-YopP chimera for HR-inducing activity. Phylogenetic analysis indicates that the sequences encoding the C-termini of family members are evolving independently from those encoding the N-termini. Our results support a model in which there are three functional domains in proteins of the family, translocation, interaction, and catalytic.

Hormone- and light-regulated nucleocytoplasmic transport in plants: current status

The gene regulation mechanisms underlying hormone- and light-induced signal transduction in plants rely not only on post-translational modification and protein degradation, but also on selective inclusion and exclusion of proteins from the nucleus. For example, plant cells treated with light or hormones actively transport many signalling regulatory proteins, transcription factors, and even photoreceptors and hormone receptors into the nucleus, while actively excluding other proteins. The nuclear envelope (NE) is the physical and functional barrier that mediates this selective partitioning, and nuclear transport regulators transduce hormone- or light-initiated signalling pathways across the membrane to mediate nuclear activities. Recent reports revealed that mutating the proteins regulating nuclear transport through the pores, such as nucleoporins, alters the plant's response to a stimulus. In this review, recent works are introduced that have revealed the importance of regulated nucleocytoplasmic partitioning. These important findings deepen our understanding about how co-ordinated plant hormone and light signal transduction pathways facilitate communication between the cytoplasm and the nucleus. The roles of nucleoporin components within the nuclear pore complex (NPC) are also emphasized, as well as nuclear transport cargo, such as Ran/TC4 and its binding proteins (RanBPs), in this process. Recent findings concerning these proteins may provide a possible direction by which to characterize the regulatory potential of hormone- or light-triggered nuclear transport.

Molecular cloning, expression profile and promoter analysis of a novel ethylene responsive transcription factor gene GhERF4 from cotton (Gossypium hirstum)

Ethylene-responsive element binding factors (ERFs) are plant-specific transcription factors, many of which have been linked to stress responses. A novel ERF gene, designated GhERF4, was isolated by RACE-PCR from Gossypium hirsutum. The GhERF4 cDNA has a total length of 1061bp with an open reading frame of 669bp, encoding a protein of 222 amino acids with a molecular weight of 23.5kDa and a calculated pI of 9.03. Sequence alignment shows that GhERF4 contains a 58 amino acid long AP2/ERF domain and a RKRP nuclear localization signal, and belongs to a group II protein in the ERF subfamily as typified by the C-terminal ERF-associated Amphiphilic Repression (EAR) motif. Southern blot analysis indicates that GhERF4 is a single copy gene in cotton genome. Using green fluorescent protein fusion, we demonstrate that GhERF4 accumulates specifically in the nucleus of onion epidermis cells. Semi-quantitative RT-PCR reveals that GhERF4 is constitutively expressed in true leaves, roots, seeds and stems. The transcripts of GhERF4 accumulate highly and rapidly when plants are treated with exogenous ethylene, salt, cold, drought stresses and exogenous abscisic acid (ABA) treatment, suggesting that GhERF4 is regulated by certain components of the stress signaling pathway. Promoter analysis indicates that the 5' upstream region of GhERF4 possesses some elements induced by physiological and environmental factors. These results indicate that GhERF4 may play an important role in response to ethylene, ABA and environmental stresses.

Gibberellin metabolism and its regulation

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.

Molecular analysis and expression patterns of the 14-3-3 gene family from Oryza sativa

The ubiquitous family of 14-3-3 proteins functions as regulators in a variety of physiological processes. Eight rice 14-3-3 genes, designated OsGF14a through h, were identified from an exhaustive search of the genome database. Comparisons of deduced amino acid sequences reveal a high degree of identity among members of the OsGF14 family and reported Arabidopsis 14-3-3 proteins. A phylogenetic study indicates that OsGF14s contain both epsilon and non-epsilon forms, which is also confirmed by a structural analysis of OsGF14 genes. Furthermore, transcripts of OsGF14b, OsGF14c, OsGF14d, OsGF14e, OsGF14f and OsGF14g were detected in rice tissues. Their different expression patterns, the different effects of environmental stresses and plant hormones on their transcription levels, and the different complementary phenotypes in yeast 14-3-3 mutants not only indicates that OsGF14s are responsive to various stress conditions and regulated by multiple signaling pathways, but also suggests that functional similarity and diversity coexist among the members of OsGF14 family.

An essential role for 14-3-3 proteins in brassinosteroid signal transduction in Arabidopsis

Brassinosteroids (BRs) are essential hormones for plant growth and development. BRs regulate gene expression by inducing dephosphorylation of two key transcription factors, BZR1 and BZR2/BES1, through a signal transduction pathway that involves cell-surface receptors (BRI1 and BAK1) and a GSK3 kinase (BIN2). How BR-regulated phosphorylation controls the activities of BZR1/BZR2 is not fully understood. Here, we show that BIN2-catalyzed phosphorylation of BZR1/BZR2 not only inhibits DNA binding, but also promotes binding to the 14-3-3 proteins. Mutations of a BIN2-phosphorylation site in BZR1 abolish 14-3-3 binding and lead to increased nuclear localization of BZR1 protein and enhanced BR responses in transgenic plants. Further, BR deficiency increases cytoplasmic localization, and BR treatment induces rapid nuclear localization of BZR1/BZR2. Thus, 14-3-3 binding is required for efficient inhibition of phosphorylated BR transcription factors, largely through cytoplasmic retention. This study demonstrates that multiple mechanisms are required for BR regulation of gene expression and plant growth.

Protein phosphorylation and binding of a 14-3-3 protein in Vicia guard cells in response to ABA

Under drought stress, ABA promotes stomatal closure to prevent water loss. Although protein phosphorylation plays an important role in ABA signaling, little is known about these processes at the biochemical level. In this study, we searched for substrates of protein kinases in ABA signaling through the binding of a 14-3-3 protein to phosphorylated proteins using Vicia guard cell protoplasts. ABA induced binding of a 14-3-3 protein to proteins with molecular masses of 61, 43 and 39 kDa, with the most remarkable signal for the 61 kDa protein. The ABA-induced binding to the 61 kDa protein occurred only in guard cells, and reached a maximum within 3 min at 1 microM ABA. The 61 kDa protein localized in the cytosol. ABA induced the binding of endogenous vf14-3-3a to the 61 kDa protein in guard cells. Autophosphorylation of ABA-activated protein kinase (AAPK), which mediates anion channel activation, and ABA-induced phosphorylation of the 61 kDa protein showed similar time courses and similar sensitivities to the protein kinase inhibitor K-252a. AAPK elicits the binding of the 14-3-3 protein to the 61 kDa protein in vitro when AAPK in guard cells was activated by ABA. The phosphorylation of the 61 kDa protein by ABA was not affected by the NADPH oxidase inhibitor, H(2)O(2), W-7 or EGTA. From these results, we conclude that the 61 kDa protein may be a substrate for AAPK and that the 61 kDa protein is located upstream of H(2)O(2) and Ca(2+), or on Ca(2+)-independent signaling pathways in guard cells.

Gene suppression in a tolerant tomato-vascular pathogen interaction

A plant can respond to the threat of a pathogen through resistance defenses or through tolerance. Resistance has been widely studied in many host pathogen systems but little is known about genetic changes which underlie a tolerant interaction. A recently developed model system for a tolerant tomato (Lycopersicon esculentum Mill) interaction with a fungal wilt pathogen, Verticillium dahliae Kleb, is examined with respect to changes in gene expression and compared to a susceptible infection. The results indicate that genetic changes can be dramatically different and some genes that are strongly elevated in the susceptible interaction are actually down-regulated in tolerance. Similar levels of fungal DNA and an up-regulation of many pathogenesis related genes indicate that in both types of interaction the presence of fungus is clearly recognized by the plant but other changes correlate with the absence of symptoms in the tolerant interaction. For example, a gene encoding a known 14-3-3 regulatory protein and a number of genes normally affected by this protein are down-regulated. Furthermore, genes which may contribute to foliar necrosis and cell death in the susceptible interaction also appear to be suppressed in the tolerant interaction, raising the possibility that the wilt symptoms, chlorosis and necrosis which are observed in the susceptible interaction, are actually programmed to further limit the growth of the fungal pathogen, and protect the general tomato population.

Gibberellins and heterosis of plant height in wheat (Triticum aestivum L.)

Background

Heterosis in internode elongation and plant height are commonly observed in hybrid plants, and higher GAs contents were found to be correlated with the heterosis in plant height. However, the molecular basis for the increased internode elongation in hybrids is unknown.

Results

In this study, heterosis in plant height was determined in two wheat hybrids, and it was found that the increased elongation of the uppermost internode contributed mostly to the heterosis in plant height. Higher GA₄ level was also observed in a wheat hybrid. By using the uppermost internode tissues of wheat, we examined expression patterns of genes participating in both GA biosynthesis and GA response pathways between a hybrid and its parental inbreds. Our results indicated that among the 18 genes analyzed, genes encoding enzymes that promote synthesis of bioactive GAs, and genes that act as positive components in the GA response pathways were up-regulated in hybrid, whereas genes encoding enzymes that deactivate bioactive GAs, and genes that act as negative components of GA response pathways were down-regulated in hybrid. Moreover, the putative wheat GA receptor gene TaGID1, and two GA responsive genes participating in internode elongation, GIP and XET, were also up-regulated in hybrid. A model for GA and heterosis in wheat plant height was proposed.

Conclusion

Our results provided molecular evidences not only for the higher GA levels and more active GA biosynthesis in hybrid, but also for the heterosis in plant height of wheat and possibly other cereal crops.

bZIP10-LSD1 antagonism modulates basal defense and cell death in Arabidopsis following infection

Plants use sophisticated strategies to balance responses to oxidative stress. Programmed cell death, including the hypersensitive response (HR) associated with successful pathogen recognition, is one cellular response regulated by reactive oxygen in various cellular contexts. The Arabidopsis basic leucine zipper (bZIP) transcription factor AtbZIP10 shuttles between the nucleus and the cytoplasm and binds consensus G- and C-box DNA sequences. Surprisingly, AtbZIP10 can be retained outside the nucleus by LSD1, a protein that protects Arabidopsis cells from death in the face of oxidative stress signals. We demonstrate that AtbZIP10 is a positive mediator of the uncontrolled cell death observed in lsd1 mutants. AtbZIP10 and LSD1 act antagonistically in both pathogen-induced HR and basal defense responses. LSD1 likely functions as a cellular hub, where its interaction with AtbZIP10 and additional, as yet unidentified, proteins contributes significantly to plant oxidative stress responses.

Systematic analysis of Arabidopsis organelles and a protein localization database for facilitating fluorescent tagging of full-length Arabidopsis proteins

Cells are organized into a complex network of subcellular compartments that are specialized for various biological functions. Subcellular location is an important attribute of protein function. To facilitate systematic elucidation of protein subcellular location, we analyzed experimentally verified protein localization data of 1,300 Arabidopsis (Arabidopsis thaliana) proteins. The 1,300 experimentally verified proteins are distributed among 40 different compartments, with most of the proteins localized to four compartments: mitochondria (36%), nucleus (28%), plastid (17%), and cytosol (13.3%). About 19% of the proteins are found in multiple compartments, in which a high proportion (36.4%) is localized to both cytosol and nucleus. Characterization of the overrepresented Gene Ontology molecular functions and biological processes suggests that the Golgi apparatus and peroxisome may play more diverse functions but are involved in more specialized processes than other compartments. To support systematic empirical determination of protein subcellular localization using a technology called fluorescent tagging of full-length proteins, we developed a database and Web application to provide preselected green fluorescent protein insertion position and primer sequences for all Arabidopsis proteins to study their subcellular localization and to store experimentally verified protein localization images, videos, and their annotations of proteins generated using the fluorescent tagging of full-length proteins technology. The database can be searched, browsed, and downloaded using a Web browser at http://aztec.stanford.edu/gfp/. The software can also be downloaded from the same Web site for local installation.

ZmMPK6, a novel maize MAP kinase that interacts with 14-3-3 proteins

Although an increasing body of evidence indicates that plant MAP kinases are involved in a number of cellular processes, such as cell cycle regulation and cellular response to abiotic stresses, hormones and pathogen attack, very little is known about their biochemical properties and regulation mechanism. In this paper we report on the identification and characterization of a novel member of the MAP kinase family from maize, ZmMPK6. The amino acid sequence reveals a high degree of identity with group D plant MAP kinases. Recombinant ZmMPK6, expressed in Escherichia coli, is an active enzyme able to autophosphorylate. Remarkably, ZmMPK6 interacts in vitro with GF14-6, a maize 14-3-3 protein and the interaction is dependent on autophosphorylation. The interacting domain of ZmMPK6 is on the C-terminus and is comprised between amino acid 337 and amino acid 467. Our results represent the first evidence of an interaction between a plant MAP kinase and a 14-3-3 protein. Possible functional roles of this association in vivo are discussed.

Isolation and functional characterisation of two new bZIP maize regulators of the ABA responsive gene rab28

The plant hormone abscisic acid regulates gene expression in response to growth stimuli and abiotic stress. Previous studies have implicated members of the bZIP family of transcription factors as mediators of abscisic acid dependent gene expression through the ABRE cis-element. Here, we identify two new maize bZIP transcription factors, EmBP-2 and ZmBZ-1 related to EmBP-1 and OsBZ-8 families. They are differentially expressed during embryo development; EmBP-2 is constitutive, whereas ZmBZ-1 is abscisic acid-inducible and accumulates during late embryogenesis. Both factors are nuclear proteins that bind to ABREs and activate transcription of the abscisic acid-inducible gene rab28 from maize. EmBP-2 and ZmBZ-1 are phosphorylated by protein kinase CK2 and phosphorylation alters their DNA binding properties. Our data suggest that EmBP-2 and ZmBZ-1 are involved in the expression of abscisic acid inducible genes such as rab28 and their activity is modulated by ABA and by phosphorylation.

Visualization of autophagy in Arabidopsis using the fluorescent dye monodansylcadaverine and a GFP-AtATG8e fusion protein

Autophagy is a process that is thought to occur in all eukaryotes in which cells recycle cytoplasmic contents when subjected to environmental stress conditions or during certain stages of development. Upon induction of autophagy, double membrane-bound structures called autophagosomes engulf portions of the cytoplasm and transfer them to the vacuole or lysosome for degradation. In this study, we have characterized two potential markers for autophagy in plants, the fluorescent dye monodansylcadaverine (MDC) and a green fluorescent protein (GFP)-AtATG8e fusion protein, and propose that they both label autophagosomes in Arabidopsis. Both markers label the same small, apparently membrane-bound structures found in cells under conditions that are known to induce autophagy such as starvation and senescence. They are usually seen in the cytoplasm, but occasionally can be observed within the vacuole, consistent with a function in the transfer of cytoplasmic material into the vacuole for degradation. MDC-staining and the GFP-AtATG8e fusion protein can now be used as very effective tools to complement biochemical and genetic approaches to the study of autophagy in plant systems.

The variable C-terminus of 14-3-3 proteins mediates isoform-specific interaction with sucrose-phosphate synthase in the yeast two-hybrid system

Sucrose-6-phosphate synthase (SPS) is a target for 14-3-3 protein binding in plants. Because several isoforms of the 14-3-3 protein are expressed in plants, I investigated which isoforms have the ability to bind SPS. Two 14-3-3 isoforms (T14-3d and a novel isoform designated T14-3 g) were found to interact with SPS from tobacco (Nicotiana tabacum L.) in a two-hybrid screen. To further address the question of isoform specificity of 14-3-3s, four additional isoforms were tested for their ability to interact with SPS in the yeast two-hybrid system. The results clearly revealed large differences in affinity between individual 14-3-3 isoforms toward SPS. Deletion analysis suggested that these differences were mediated by the variable C-terminus of 14-3-3s. Site-directed mutagenesis of candidate 14-3-3 binding sites on SPS demonstrated that interaction could be independent of a phosphorylated serine residue within conserved binding motifs in the yeast system. These findings suggest that the large number of 14-3-3 isoforms present in plants reflects functional specificity.

Role of 14–3–3 Proteins in Eukaryotic Signaling and Development

14-3-3 genes encode a ubiquitous family of highly conserved eukaryotic proteins from fungi to humans and plants with several molecular and cellular functions. Most notably, 14-3-3 proteins bind to phosphoserine/phosphothreonine motifs in a sequence-specific manner. More than 100 14-3-3 binding partners involved in signal transduction, cell cycle regulation, apoptosis, stress responses, and malignant transformation have been identified. The 14-3-3 proteins form homodimers and heterodimers, and there is redundancy of the binding specificity and function of different 14-3-3 proteins because of their highly similar amino acid sequence and tertiary structure. 14-3-3 proteins can regulate target protein function by several mechanisms. Although the molecular and cellular functions of 14-3-3 proteins have been well studied, there have been fewer studies addressing the in vivo role of 14-3-3s. Here we review what is known about 14-3-3 proteins during eukaryotic development.

Gibberellin metabolism and signaling

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

Nucleocytoplasmic Trafficking in Plant Cells

In a eukaryotic cell, the nuclear envelope (NE) separates genetic information from the environment of biosynthesis and metabolism. Transfer of macromolecules across the NE involves the nuclear pores--large multisubunit protein complexes--and machinery that facilitates rapid, directional, and selective transport. While core elements of the transport process are conserved between kingdoms, different solutions to similar problems have also evolved. Although the structure and composition of the yeast and mammalian nuclear pore have been unraveled recently, the plant nuclear pore remains largely enigmatic. Like any other process, nucleocytoplasmic transport can be regulated. Several examples from plants are discussed that promise insights into the regulation of signaling pathways. While controlling the partitioning of cellular components, the nuclear envelope also presents an obstacle to viruses and transforming agents that need access to the genome, and different mechanisms have evolved to overcome this obstacle. Finally, the recent recognition of the importance of small RNAs for gene regulation emphasizes the need to understand small RNA nuclear export and the levels of its regulation. This review attempts to wed our molecular-mechanistic understanding of nucleocytoplasmic trafficking drawn from all model systems with the intriguing examples of regulated nucleocytoplasmic partitioning in plants.

Functional dissection of a small anaerobically induced bZIP transcription factor from tomato

A small anaerobically induced tomato transcription factor was isolated from a subtractive library. This factor, designated ABZ1 (anaerobic basic leucine zipper), is anaerobically induced in fruits, leaves and roots and encodes a nuclear localized protein. ABZ1 shares close structural and sequence homology with the S-family of small basic leucine zipper (bZIP) transcription factors that are implicated in stress response. Nuclear localization of ABZ1 is mediated by the basic region and occurs under normoxic conditions. ABZ1 binds to G-box-like target sites as a dimer. Binding can be abolished by heterodimerization with a truncated protein retaining the leucine zipper but lacking the DNA binding domain. The protein binds in a sequence specific manner to the CaMV 35S promoter which is down regulated when ABZ1 is coexpressed. This correlates with the anaerobic down regulation of the 35S promoter in tomato and tobacco. These results may suggest that small bZIP proteins are involved in the negative regulation of gene expression under anaerobic conditions.

Nuclear localization and interaction of RolB with plant 14-3-3 proteins correlates with induction of adventitious roots by the oncogene rolB

The rooting-locus gene B (rolB) on the T-DNA of the root-inducing (Ri) plasmid in Agrobacterium rhizogenes is responsible for the induction of transformed adventitious roots, although the root induction mechanism is unknown. We report here that the RolB protein of pRi1724 (1724RolB) is associated with Nicotianatabacum14-3-3-like protein omegaII (Nt14-3-3 omegaII) in tobacco bright yellow (BY)-2 cells. Nt14-3-3 omegaII directly interacts with 1724RolB protein. Green fluorescent protein (GFP)-fused 1724RolB is localized to the nucleus. GFP-fused mutant 1724RolB proteins having a deletion or amino acid substitution are unable to interact with Nt14-3-3 omegaII and also show impaired nuclear localization. Moreover, these 1724RolB mutants show decreased capacity for adventitious root induction. These results suggest that adventitious root induction by 1724RolB protein correlates with its interaction with Nt14-3-3 omegaII and the nuclear localization of 1724RolB protein.

Gibberellin signalling pathway

Recent molecular biological and genetical studies have identified several positive and negative regulators of gibberellin (GA) signalling pathways in higher plants. The DELLA protein functions as a negative regulator of GA signalling; its degradation through the ubiquitin/proteasome pathway is a key event in the regulation of GA-stimulated processes.

The C-terminal tail of Arabidopsis 14-3-3omega functions as an autoinhibitor and may contain a tenth alpha-helix

The eukaryotic regulatory protein 14-3-3 is involved in many important plant cellular processes including regulation of nitrate assimilation through inhibition of phosphorylated nitrate reductase (pNR) in darkened leaves. Divalent metal cations (Me2+) and some polyamines interact with the loop 8 region of the 14-3-3 proteins and allow them to bind and inhibit pNR in vitro. The role of the highly variant C-terminal regions of the 14-3-3 isoforms in regulation by polycations is not clear. In this study, we carried out structural analyses on the C-terminal tail of the Arabidopsis 14-3-3omega isoform and evaluated its contributions to the inhibition of pNR. Nested C-terminal truncations of the recombinant 14-3-3omega protein revealed that the removal of the C-terminal tail renders the protein partially Mg2+-independent in both pNR binding and inhibition of activity, suggesting that the C-terminus functions as an autoinhibitor. The C-terminus of 14-3-3omega appears to undergo a conformational change in the presence of polycations as demonstrated by its increased trypsin cleavage at Lys-247. C-terminal truncation of 14-3-3omega at Thr-255 increased its interaction with antibodies to the C-terminus of 14-3-3omega in non-denaturing conditions, but not in denaturing conditions, suggesting that the C-terminal tail contains ordered structures that might be disrupted by the truncation. Circular dichroism (CD) analysis of a C-terminal peptide, from Trp-234 to Lys-249, revealed that the C-terminal tail might contain a tenth alpha-helix, in agreement with the in silico predictions. The function of the putative tenth alpha-helix is not clear because substituting two prolyl residues within the predicted helix (E245P/I246P mutant), which prevented the corresponding peptide from adopting a helical conformation, did not affect the inhibition of pNR activity in the presence or absence of Mg2+. We propose that in the absence of polycations, access of target proteins to their binding groove in the 14-3-3 protein is restricted by the C-terminus, which acts as part of a gate that opens with the binding of polycations to loop 8.

14-3-3 proteins find new partners in plant cell signalling

14-3-3 proteins are phosphoserine-binding proteins that regulate the activities of a wide array of targets via direct protein-protein interactions. In animal cells, the majority of their known targets are involved in signal transduction and transcription. In plants, we know about them primarily through their regulation of the plasma membrane H(+)-ATPase and enzymes of carbon and nitrogen metabolism. Nevertheless, an increasing number of plant signalling proteins are now being recognized as 14-3-3-interacting proteins. Plant 14-3-3 proteins bind a range of transcription factors and other signalling proteins, and have roles regulating plant development and stress responses. Important mechanisms of regulation by 14-3-3 include shuttling proteins between different cellular locations and acting as scaffolds for the assembly of larger signalling complexes.

Vitamin D receptor and retinoid X receptor interactions in motion

Vitamin D receptor (VDR) and retinoid X receptor (RXR) are members of the nuclear receptor superfamily and they bind target DNA sequences as heterodimers to regulate transcription. This article surveys the latest findings regarding the roles of dimerizing RXR in VDR function and emphasizes potential areas for future developments. We first highlight the importance of dimerization with RXR for both the ligand-independent (hair growth) and ligand-dependent functions of VDR (calcium homeostasis, bone development and mineralization, control of cell growth and differentiation). Emerging information regarding the regulatory control of dimerization based on biochemical, structural, and genetic studies is then presented. Finally, the main focus of this article is a new dynamic perspective of dimerization functions, based on recent research with fluorescent protein chimeras in living cells by microscopy. These studies revealed that both VDR and RXR constantly shuttle between the cytoplasm and the nucleus and between subnuclear compartments, and showed the transient nature of receptor--DNA and receptor--coregulator interactions. Because RXR dimerizes with most of the nuclear receptors, regulation of receptor dynamics by RXR has a broad significance.

bZIP transcription factors in Arabidopsis

In plants, basic region/leucine zipper motif (bZIP) transcription factors regulate processes including pathogen defence, light and stress signalling, seed maturation and flower development. The Arabidopsis genome sequence contains 75 distinct members of the bZIP family, of which approximately 50 are not described in the literature. Using common domains, the AtbZIP family can be subdivided into ten groups. Here, we review the available data on bZIP functions in the context of subgroup membership and discuss the interacting proteins. This integration is essential for a complete functional characterization of bZIP transcription factors in plants, and to identify functional redundancies among AtbZIP factors.