Conserved cis-regulatory elements for DNA-binding-with-one-finger and homeo-domain-leucine-zipper transcription factors regulate companion cell-specific expression of the Arabidopsis thaliana SUCROSE TRANSPORTER 2 gene

Current perspectives on the regulatory mechanisms of sucrose accumulation in sugarcane

Sugars transported from leaves (source) to stems (sink) energize cell growth, elongation, and maintenance. which are regulated by a variety of genes. This review reflects progress and prospects in the regulatory mechanism for maximum sucrose accumulation, including the role of sucrose metabolizing enzymes, sugar transporters and the elucidation of post-transcriptional control of sucrose-induced regulation of translation (SIRT) in the accumulation of sucrose. The current review suggests that SIRT is emerging as a significant mechanism controlling Scbzip44 activities in response to endogenous sugar signals (via the negative feedback mechanism). Sucrose-controlled upstream open reading frame (SC-uORF) exists at the 5' leader region of Scbzip44's main ORF, which inhibits sucrose accumulation through post-transcriptional regulatory mechanisms. Sucrose transporters (SWEET1a/4a/4b/13c, TST, SUT1, SUT4 and SUT5) are crucial for sucrose translocation from source to sink. Particularly, SWEET13c was found to be a major contributor to the efflux in the transportation of stems. Tonoplast sugar transporters (TSTs), which import sucrose into the vacuole, suggest their tissue-specific role from source to sink. Sucrose cleavage has generally been linked with invertase isozymes, whereas sucrose synthase (SuSy)-catalyzed metabolism has been associated with biosynthetic processes such as UDP-Glc, cellulose, hemicellulose and other polymers. However, other two key sucrose-metabolizing enzymes, such as sucrose-6-phosphate phosphohydrolase (S6PP) and sucrose phosphate synthase (SPS) isoforms, have been linked with sucrose biosynthesis. These findings suggest that manipulation of genes, such as overexpression of SPS genes and sucrose transporter genes, silencing of the SC-uORF of Scbzip44 (removing the 5' leader region of the main ORF that is called SIRT-Insensitive) and downregulation of the invertase genes, may lead to maximum sucrose accumulation. This review provides an overview of sugarcane sucrose-regulating systems and baseline information for the development of cultivars with higher sucrose accumulation.

How pollen tubes fight for food: the impact of sucrose carriers and invertases of Arabidopsis thaliana on pollen development and pollen tube growth

Pollen tubes of higher plants grow very rapidly until they reach the ovules to fertilize the female gametes. This growth process is energy demanding, however, the nutrition strategies of pollen are largely unexplored. Here, we studied the function of sucrose transporters and invertases during pollen germination and pollen tube growth. RT-PCR analyses, reporter lines and knockout mutants were used to study gene expression and protein function in pollen. The genome of Arabidopsis thaliana contains eight genes that encode functional sucrose/H⁺ symporters. Apart from AtSUC2, which is companion cell specific, all other AtSUC genes are expressed in pollen tubes. AtSUC1 is present in developing pollen and seems to be the most important sucrose transporter during the fertilization process. Pollen of an Atsuc1 knockout plant contain less sucrose and have defects in pollen germination and pollen tube growth. The loss of other sucrose carriers affects neither pollen germination nor pollen tube growth. A multiple knockout line Atsuc1Atsuc3Atsuc8Atsuc9 shows a phenotype that is comparable to the Atsuc1 mutant line. Loss of AtSUC1 can`t be complemented by AtSUC9, suggesting a special function of AtSUC1. Besides sucrose carriers, pollen tubes also synthesize monosaccharide carriers of the AtSTP family as well as invertases. We could show that AtcwINV2 and AtcwINV4 are expressed in pollen, AtcwINV1 in the transmitting tissue and AtcwINV5 in the funiculi of the ovary. The vacuolar invertase AtVI2 is also expressed in pollen, and a knockout of AtVI2 leads to a severe reduction in pollen germination. Our data indicate that AtSUC1 mediated sucrose accumulation during late stages of pollen development and cleavage of vacuolar sucrose into monosaccharides is important for the process of pollen germination.

Two evolutionarily duplicated domains individually and post-transcriptionally control SWEET expression for phloem transport

Cell type-specific gene expression is critical for the specialized functions within multicellular organisms. In Arabidopsis, SWEET11 and SWEET12 sugar transporters are specifically expressed in phloem parenchyma (PP) cells and are responsible for sucrose efflux from the PP, the first step of a two-step apoplasmic phloem-loading strategy that initiates the long-distance transport of sugar from leaves to nonphotosynthetic sink tissues. However, we know nothing about what determines the PP cell-specific expression of these SWEETs. Sequence deletions, histochemical β-glucuronidase (GUS) analysis, cross-sectioning, live-cell imaging, and evolutionary analysis were used to elucidate domains responsible for PP specificity, while a Förster resonance energy transfer (FRET) sensor-based transport assay was used to determine whether substrate specificity coevolved with PP specificity. We identified two domains in the Arabidopsis SWEET11 coding sequence that, along with its promoter (including 5' UTR), regulate PP-specific expression at the post-transcriptional level, probably involving RNA-binding proteins. This mechanism is conserved among vascular plants but independent of transport substrate specificity. We conclude that two evolutionarily duplicated coding sequence domains are essential and individually sufficient for PP-specific expression of SWEET11. We also provide a crucial experimental tool to study PP physiology and development.

RabA2b Overexpression Alters the Plasma-Membrane Proteome and Improves Drought Tolerance in Arabidopsis

Rab proteins are small GTPases that are important in the regulation of vesicle trafficking. Through data mining, we identified RabA2b to be stress responsive, though little is known about the involvement of RabA in plant responses to abiotic stresses. Analysis of the RabA2b native promoter showed strong activity during osmotic stress, which required the stress hormone Abscisic acid (ABA) and was restricted to the vasculature. Sequence analysis of the promoter region identified predicted binding motifs for several ABA-responsive transcription factors. We cloned RabA2b and overexpressed it in Arabidopsis. The resulting transgenic plants were strikingly drought resistant. The reduced water loss observed in detached leaves of the transgenic plants could not be explained by stomatal aperture or density, which was similar in all the genotypes. Subcellular localization studies detected strong colocalization between RabA2b and the plasma membrane (PM) marker PIP2. Further studies of the PM showed, for the first time, a distinguished alteration in the PM proteome as a result of RabA2b overexpression. Proteomic analysis of isolated PM fractions showed enrichment of stress-coping proteins as well as cell wall/cuticle modifiers in the transgenic lines. Finally, the cuticle permeability of transgenic leaves was significantly reduced compared to the wild type, suggesting that it plays a role in its drought resistant properties. Overall, these data provide new insights into the roles and modes of action of RabA2b during water stresses, and indicate that increased RabA2b mediated PM trafficking can affect the PM proteome and increase drought tolerance.

Ethylene response factors regulate expression of HbSUT3, the sucrose influx carrier in laticifers of Hevea brasiliensis

Natural rubber is an important industrial raw material and is commercially produced by rubber trees (Hevea brasiliensis). The sucrose transporter HbSUT3 plays an essential role in rubber production. Its expression in latex (cytoplasm of rubber-producing laticifers) is induced by bark treatment with Ethrel, an ethylene releaser, and the inducing effect correlates well with Ethrel-stimulated rubber yield increase. However, the mechanisms of ethylene induction on HbSUT3 expression are not known. Here, five Ethylene Response Factor (ERF) genes were identified from the cDNA library of Hevea latex by yeast one-hybrid screening with the promoter of HbSUT3 gene as bait. As revealed in a tobacco (Nicotiana tabacum) protoplast transient expression system, these HbERFs were mainly localized in the nucleus and four of them exhibited apparent transactivation activity. Of the five HbERF genes, HbERF-IXc4 was the most frequently screened in yeast one-hybrid, accounting for 65% of the ERF clones obtained. Moreover, among the five HbERFs, HbERF-IXc4 showed the strongest transactivation capacity when expressed in tobacco protoplast, the highest transcript abundance in latex and a close expressional correlation with its target gene, HbSUT3, in response to the Ethrel treatment. Taken together, our results indicate that ERFs, especially HbERF-IXc4, are critically involved in the activation of HbSUT3 expression in latex after Ethrel treatment on Hevea bark, and thus the stimulated latex yield.

Comprehensive Analysis of Evolutionary Characterization and Expression for Monosaccharide Transporter Family Genes in Nelumbo nucifera

Sugar transporters, an important class of transporters for sugar function, regulate many processes associated with growth, maturation, and senescence processes in plants. In this study, a total of 35 NuMSTs were identified in the Nelumbo nucifera genome and grouped by conserved domains and phylogenetic analysis. Additionally, we identified 316 MST genes in 10 other representative plants and performed a comparative analysis with Nelumbo nucifera genes, including evolutionary trajectory, gene duplication, and expression pattern. A large number of analyses across plants and algae indicated that the MST family could have originated from STP and Glct, expanding to form STP and SFP by dispersed duplication. Finally, a quantitative real-time polymerase chain reaction and cis-element analysis showed that some of them may be regulated by plant hormones (e.g., abscisic acid), biotic stress factors, and abiotic factors (e.g., drought, excessive cold, and light). We found that under the four abiotic stress conditions, only NuSTP5 expression was upregulated, generating a stress response, and ARBE and LTR were present in NuSTP5. In summary, our findings are significant for understanding and exploring the molecular evolution and mechanisms of NuMSTs in plants.

Different regulatory modules of two mango ERS1 promoters modulate specific gene expression in response to phytohormones in transgenic model plants

Ethylene is a key element of plant physiology, thus ethylene research is important for both, fundamental research and agriculture. Previous work on ethylene receptors focused on expression level and protein interaction, but knowledge on regulation of gene transcription is scarce. Promoters of mango ethylene receptor genes (pMiERS1a, pMiERS1b) were analysed particularly regarding responsiveness to hormones. The promoter sequences reveal some variation and they were characterized by identifying functional regulatory candidate modules via truncated-promoter approach. Based on ectopic gene expression studies in transgenic Arabidopsis and Nicotiana it is demonstrated that both promoters are positively responsive to ethylene. For pMiERS1a the AHBP/DOFF1 module is linked to ethylene responsiveness, while for pMiERS1b it is the module MYBL/OPAQ1. A negative gene regulation in response to abscisic acid (ABA) is linked to MYBL/DOFF2. A positive response to indole-3-acetic acid (IAA) was found for GTBX/MYCL1, containing the motifs IBOX/IDDF/TEFB, which are present in this combination only in pMiERS1b, but not in pMiERS1a. Conclusively, the general response of the ethylene receptor genes is conserved, but similar regulation can be linked to different modules. Further, a minor variation in a transcription factor binding site (TFBS) motif within an overall conserved module type can lead to a different expression.

The AtSUC2 Promoter: A Powerful Tool to Study Phloem Physiology and Development

The sucrose carrier AtSUC2 of Arabidopsis thaliana is localized in the phloem, where it catalyzes the uptake of sucrose from the apoplast into companion cells. Imported sucrose moves passively via plasmodesmata from the companion cells into the neighboring sieve elements that distribute this disaccharide to the different sink organs. Phloem loading of sucrose by the AtSUC2 protein is an essential process, and mutants lacking this protein stay tiny, develop no or only few flowers, and have a strongly reduced root system. The promoter of the AtSUC2 gene is active exclusively in companion cells of the phloem. Moreover, it drives very strong expression not only in Arabidopsis, but also in all plant species tested so far, including monocot species. Due to these features, the AtSUC2 promoter has become an important tool in diverse areas of plant research during the last two decades. It was used to study phloem development and function including phloem loading and unloading. Furthermore, it was helpful in analyzing the pathways of posttranscriptional silencing by RNA interference, the regulation of flowering, mechanisms of nutrient withdrawal by phloem-feeding pathogens, and other physiological functions that are related to long distance transport. The present paper gives an overview of different approaches in plant research that utilized the strong and companion cell-specific expression of own or foreign genes driven by the AtSUC2 promoter.

Genome-wide Characterization and Expression Analysis of Sugar Transporter Family Genes in Woodland Strawberry

In higher plants, sugars are nutrients and important signal molecules. Sugar transporters (STs) facilitate sugar transport across membranes and are associated with loading and unloading of the conducting complex. Strawberry ( Duchesne ex Rozier) is one of the most economically important and widely cultivated fruit crop and a model plant among fleshy fruits worldwide. In this study, 66 woodland strawberry ( L.) ST (FvST) genes were identified and further classified into eight distinct subfamilies in the woodland strawberry genome based on the phylogenetic analysis. In the promoter sequences of FvST gene families, a search for -regulatory elements suggested that some of them might probably be regulated by plant hormones (e.g., salicylic acid, abscisic acid, and auxin), abiotic (e.g., drought, excessive cold, and light), and biotic stress factors. Exon-intron analysis showed that each subfamily manifested closely associated gene architectural features based on similar number or length of exons. Moreover, to comprehend the potential evolution mechanism of FvST gene family, the analysis of genome duplication events was performed. The segmental and tandem duplication analysis elucidated that some of ST genes arose through whole-genome duplication (WGD) or segmental duplication, accompanied by tandem duplications. The expression analysis of 24 FvST genes in vegetative and during fruit development has shown that the expression of several ST genes was tissue and developmental stage specific. Generally, our findings are important in understanding of the allocation of photo assimilates from source to sink cell and provide insights into the genomic organization and expression profiling of FvST gene families in woodland strawberry.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

Strength, Stability, and cis-Motifs of In silico Identified Phloem-Specific Promoters in Brassica juncea (L.)

Aphids, a hemipteran group of insects pose a serious threat to many of the major crop species including Brassica oilseeds. Transgenic strategies for developing aphid-resistant plant types necessitate phloem-bound expression of the insecticidal genes. A few known phloem-specific promoters, in spite of tissue-specific activity fail to confer high level gene-expression. Here, we identified seven orthologues of phloem-specific promoters in B. juncea (Indian mustard), and experimentally validated their strength of expression in phloem exudates. Significant cis-motifs, globally occurring in phloem-specific promoters showed variable distribution frequencies in these putative phloem-specific promoters of B. juncea. In RT-qPCR based gene-expression study promoter of Glutamine synthetase 3A (GS3A) showed multifold higher activity compared to others, across the different growth stages of B. juncea plants. A statistical method employing four softwares was devised for rapidly analysing stability of the promoter-activities across the plant developmental stages. Different statistical softwares ranked these B. juncea promoters differently in terms of their stability in promoter-activity. Nevertheless, the consensus in output empirically suggested consistency in promoter-activity of the six B. juncea phloem- specific promoters including GS3A. The study identified suitable endogenous promoters for high level and consistent gene-expression in B. juncea phloem exudate. The study also demonstrated a rapid method of assessing species-specific strength and stability in expression of the endogenous promoters.

Genome-Wide Function, Evolutionary Characterization and Expression Analysis of Sugar Transporter Family Genes in Pear (Pyrus bretschneideri Rehd)

The sugar transporter (ST) plays an important role in plant growth, development and fruit quality. In this study, a total of 75 ST genes were identified in the pear (Pyrus bretschneideri Rehd) genome based on systematic analysis. Furthermore, all ST genes identified were grouped into eight subfamilies according to conserved domains and phylogenetic analysis. Analysis of cis-regulatory element sequences of all ST genes identified the MYBCOREATCYCB1 promoter in sucrose transporter (SUT) and monosaccharide transporter (MST) genes of pear, while in grape it is exclusively found in SUT subfamily members, indicating divergent transcriptional regulation in different species. Gene duplication event analysis indicated that whole-genome duplication (WGD) and segmental duplication play key roles in ST gene amplification, followed by tandem duplication. Estimation of positive selection at codon sites of ST paralog pairs indicated that all plastidic glucose translocator (pGlcT) subfamily members have evolved under positive selection. In addition, the evolutionary history of ST gene duplications indicated that the ST genes have experienced significant expansion in the whole ST gene family after the second WGD, especially after apple and pear divergence. According to the global RNA sequencing results of pear fruit development, gene expression profiling showed the expression of 53 STs. Combined with quantitative real-time PCR (qRT-PCR) analysis, two polyol/monosaccharide transporter (PLT) and three tonoplast monosaccharide transporter (tMT) members were identified as candidate genes, which may play important roles in sugar accumulation during pear fruit development and ripening. Identification of highly expressed STs in fruit is important for finding novel genes contributing to enhanced levels of sugar content in pear fruit.

Transcriptional repression caused by Dof5.8 is involved in proper vein network formation in Arabidopsis thaliana leaves

Vascular plants have a network of vasculature in their leaves, which supplies water and nutrients and exports photoassimilates to other tissues. The vascular network is patterned during the development of leaf primordia through the induction of provascular differentiation by auxin. Arabidopsis thaliana Dof5.8, encoding a Dof-type transcription factor, is expressed early in provascular cells under the control of the MONOPTEROS transcription factor, also known as auxin response factor 5 (ARF5). Here, we report the effect of overexpressing Dof5.8 in provascular cells on the formation of the vascular network. Overexpression of Dof5.8 inhibited the formation of higher-order veins in cotyledons and leaves, probably through transcriptional repression by Dof5.8. The expression of auxin-associated transcription factor genes, DORNRöSCHEN and SHI-RELATED SEQUENCE 5, was downregulated in the Dof5.8 overexpressors, and overexpression of these genes partially rescued the impaired formation of higher-order veins in Dof5.8-overexpressing lines, suggesting that the overexpression of Dof5.8 modulates the auxin response and leads to impaired vein formation in A. thaliana.

Cloning and functional characterization of the promoter of PsSEOF1 gene from Pisum sativum under different stress conditions using Agrobacterium-mediated transient assay

PsSEOF1, a SEO (sieve element occlusion) gene family protein (forisome) is calcium powered motor protein and is located close to plasma membrane of sieve element. In sieve element (SE) it senses the calcium ion levels and undergoes ATP-independent conformational shifts. Forisome, meaning gate-bodies (Latin foris: wing of a gate; Greek soma: body). Recent reports show that SEO gene family protein can prevent the loss of nutrient rich photoassimilate upon wound injury. The regulation of SEO protein forisome under abiotic/ biotic stress is still unknown. The analysis of cis-regulatory element present in the upstream region is not well understood. Tissue specific promoters guarantee correct expression when it perceives particular stimuli. Here we report isolation of tissue specific promoter of PsSEOF1 was isolated by gene walking PCR from P. sativum (pea) genomic DNA library constructed by BD genome walker kit. In silico analysis revealed several putative cis element within this promoter sequence like wound response, cold, dehydration. Putative elements which might be required for its vascular tissue specificity has also been identified. The GUS activities of PsSEOF1 promoter-GUS chimeric construct in the agroinfiltrated leaves under different environmental stress abiotic and biotic like wound, cold, salt and phytohormones has shown high level of GUS activity. To identify the activity of PsSEOF1 promoter under different stress condition an Agrobacterium-mediated transient expression of tobacco plants were subjected to histochemical GUS staining. Stress-inducible nature of PsSEOF1 promoter opens possibility for the study of the PsSEOF1 gene regulation under stress condition. The isolated promoter sequence could serve as an important candidate for tissue specific promoter in genetic engineering of plant under stress conditions.

Identification of MAIN, a factor involved in genome stability in the meristems of Arabidopsis thaliana

Stem cells in the root and shoot apical meristem provide the descendant cells required for growth and development throughout the lifecycle of a plant. We found that mutations in the Arabidopsis MAINTENANCE OF MERISTEMS (MAIN) gene led to plants with distorted stem cell niches in which stem cells are not maintained and undergo premature differentiation or cell death. The malfunction of main meristems leads to short roots, mis-shaped leaves, reduced fertility and partial fasciation of stems. MAIN encodes a nuclear-localized protein and is a member of a so far uncharacterized plant-specific gene family. As main mutant plants are hypersensitive to DNA-damaging agents, expression of genes involved in DNA repair is induced and dead cells with damaged DNA accumulate in the mutant meristems, we propose that MAIN is required for meristem maintenance by sustaining genome integrity in stem cells and their descendants cells.

The role of the DNA-binding One Zinc Finger (DOF) transcription factor family in plants

The DOF (DNA-binding One Zinc Finger) family of transcription factors is involved in many fundamental processes in higher plants, including responses to light and phytohormones as well as roles in seed maturation and germination. DOF transcription factor genes are restricted in their distribution to plants, where they are in many copies in both gymnosperms and angiosperms and also present in lower plants such as the moss Physcomitrella patens and in the alga Chlamydomonas reinhardtii which possesses a single DOF gene. DOF transcription factors bind to their promoter targets at the consensus sequence AAAG. This binding depends upon the presence of the highly conserved DOF domain in the protein. Depending on the target gene, DOF factor binding may activate or repress transcription. DOF factors are expressed in most if not all tissues of higher plants, but frequently appear to be functionally redundant. Recent next-generation sequencing data provide a more comprehensive survey of the distribution of DOF sequence classes among plant species and within tissue types, and clues as to the evolution of functions assumed by this transcription factor family. DOFs do not appear to be implicated in the initial differentiation of the plant body plan into organs via the resolution of meristematic zones, in contrast to MADS-box and homeobox transcription factors, which are found in other non-plant eukaryotes, and this may reflect a more recent evolutionary origin.

Vascular gene expression: a hypothesis

The phloem is the conduit through which photoassimilates are distributed from autotrophic to heterotrophic tissues and is involved in the distribution of signaling molecules that coordinate plant growth and responses to the environment. Phloem function depends on the coordinate expression of a large array of genes. We have previously identified conserved motifs in upstream regions of the Arabidopsis genes, encoding the homologs of pumpkin phloem sap mRNAs, displaying expression in vascular tissues. This tissue-specific expression in Arabidopsis is predicted by the overrepresentation of GA/CT-rich motifs in gene promoters. In this work we have searched for common motifs in upstream regions of the homologous genes from plants considered to possess a "primitive" vascular tissue (a lycophyte), as well as from others that lack a true vascular tissue (a bryophyte), and finally from chlorophytes. Both lycophyte and bryophyte display motifs similar to those found in Arabidopsis with a significantly low E-value, while the chlorophytes showed either a different conserved motif or no conserved motif at all. These results suggest that these same genes are expressed coordinately in non-vascular plants; this coordinate expression may have been one of the prerequisites for the development of conducting tissues in plants. We have also analyzed the phylogeny of conserved proteins that may be involved in phloem function and development. The presence of CmPP16, APL, FT, and YDA in chlorophytes suggests the recruitment of ancient regulatory networks for the development of the vascular tissue during evolution while OPS is a novel protein specific to vascular plants.

Functional characterization of cis-elements conferring vascular vein expression of At4g34880 amidase family protein gene in Arabidopsis

The expression of At4g34880 gene encoding amidase in Arabidopsis was characterized in this study. A promoter region of 1.5 kb on the upstream of the start codon of the gene (referred as AmidP) was fused with uidA (GUS) reporter gene, and transformed into Arabidopsis plant for determining its spatial expression. The results indicated that AmidP drived GUS expression in vascular system, predominately in leaves. Truncation analysis of AmidP demonstrated that VASCULAR VEIN ELEMENT (VVE) motif with a region of 176 bp sequence (−1500 to −1324) was necessary and sufficient to direct the vascular vein specific GUS expression in the transgenic plant. Tandem copy of VVE increased vascular system expression, and 5′- and 3′- deletions of VVE motif in combination with a truncated −65 CaMV 35S minimal promoter showed that 11bp cis-acting element, naming DOF2 domain, played an essential role for the vascular vein specific expression. Meanwhile, it was also observed that the other cis-acting elements among the VVE region are also associated with specificity or strength of GUS activities in vascular system.

LuFLA1PRO and LuBGAL1PRO promote gene expression in the phloem fibres of flax (Linum usitatissimum)

Cell type-specific promoters were identified that drive gene expression in an industrially important product. To identify flax (Linum usitatissimum) gene promoters, we analyzed the genomic regions upstream of a fasciclin-like arabinogalactan protein (LuFLA1) and a beta-galactosidase (LuBGAL1). Both of these genes encode transcripts that have been found to be highly enriched in tissues bearing phloem fibres. Using a beta-glucuronidase (GUS) reporter construct, we found that a 908-bp genomic sequence upstream of LuFLA1 (LuFLA1PRO) directed GUS expression with high specificity to phloem fibres undergoing secondary cell wall development. The DNA sequence upstream of LuBGAL1 (LuBGAL1PRO) likewise produced GUS staining in phloem fibres with developing secondary walls, as well as in tissues of developing flowers and seed bolls. These data provide further evidence of a specific role for LuFLA1 in phloem fibre development, and demonstrate the utility of LuFLA1PRO and LuBGAL1PRO as tools for biotechnology and further investigations of phloem fibre development.

The plant-specific dof transcription factors family: new players involved in vascular system development and functioning in Arabidopsis

In higher plants phloem and xylem are responsible for long-distance transport of water, nutrients, and signals that act systemically at short or long-distance to coordinate developmental processes. The formation of the plant vascular system is a complex process that integrates signaling events and gene regulation at transcriptional and posttranscriptional levels. Thanks to transcriptomic and proteomic analysis we start to better understand the mechanisms underlying the formation and the functioning of the vascular system. The role of the DNA-binding with one finger (Dof TFs), a group of plant-specific transcription factors, recently emerged as part of the transcriptional regulatory networks acting on the formation and functioning of the vascular tissues. More than half of the members of this TF family are expressed in the vascular system. In addition some of them have been proposed to be mobile proteins, suggesting a possible role in the control of short- or long-distance signaling as well. This review summarizes the current knowledge on Dof TFs family in Arabidopsis with a special focus on their role in vascular development and functioning.

Phloem development in nematode-induced feeding sites: the implications of auxin and cytokinin

Sedentary plant parasitic nematodes such as root-knot nematodes and cyst nematodes induce giant cells or syncytia, respectively, in their host plant's roots. These highly specialized structures serve as feeding sites from which exclusively the nematodes withdraw nutrients. While giant cells are symplastically isolated and obtain assimilates by transporter-mediated processes syncytia are massively connected to the phloem by plasmodesmata. To support the feeding sites and the nematode during their development, phloem is induced around syncytia and giant cells. In the case of syncytia the unloading phloem consists of sieve elements and companion cells and in the case of root knots it consists exclusively of sieve elements. We applied immunohistochemistry to identify the cells within the developing phloem that responded to auxin and cytokinin. Both feeding sites themselves did not respond to either hormone. We were able to show that in root knots an auxin response precedes the differentiation of these auxin responsive cells into phloem elements. This process appears to be independent of B-type Arabidopsis response regulators. Using additional markers for tissue identity we provide evidence that around giant cells protophloem is formed and proliferates dramatically. In contrast, the phloem around syncytia responded to both hormones. The presence of companion cells as well as hormone-responsive sieve elements suggests that metaphloem development occurs. The implication of auxin and cytokinin in the further development of the metaphloem is discussed.

Vascular expression and C-terminal sequence divergence of cytokinin response factors in flowering plants

Cytokinin response factors (CRFs) are important transcription factors that form a side branch of the cytokinin signaling pathway and have been linked to cytokinin-regulated processes during development. CRF proteins are defined as belonging to a specific transcription factor family by the presence of an AP2/ERF DNA-binding domain and can be distinguished within this family by a group-specific CRF domain involved in protein-protein interactions. Here we further delimit CRFs into five distinct clades (I-V) represented across all major angiosperm lineages. Protein sequences within each clade contain a clade-specific C-terminal region distinct from other CRFs, suggesting ancient evolutionary divergence and specialization within this gene family. Conserved patterns of transcriptional regulation support these clade divisions. Despite these important differences, CRFs appear to show preferential localization or targeting to vascular tissue in quantitative real-time PCR and reporter line analyses of Arabidopsis thaliana and Solanum lycopersicum (tomato). Phloem tissue expression within the vasculature often appears the strongest in CRF reporter lines, and an analysis of CRF promoter sequences revealed conservation and significant enrichment of phloem targeting cis-elements, suggesting a potential role for CRFs in this tissue. An examination of CRF loss-of-function mutants from cytokinin-regulated clades revealed alterations in higher order vein patterning. This supports both the general link of CRFs to vascular tissue and clade-specific differences between CRFs, since alterations in vascular patterning appear to be clade specific. Together these findings indicate that CRFs are potential regulators of developmental processes associated with vascular tissues.

Multiple cis-regulatory elements are involved in the complex regulation of the sieve element-specific MtSEO-F1 promoter from Medicago truncatula

The sieve element occlusion (SEO) gene family includes several members that are expressed specifically in immature sieve elements (SEs) in the developing phloem of dicotyledonous plants. To determine how this restricted expression profile is achieved, we analysed the SE-specific Medicago truncatula SEO-F1 promoter (PMtSEO-F1) by constructing deletion, substitution and hybrid constructs and testing them in transgenic tobacco plants using green fluorescent protein as a reporter. This revealed four promoter regions, each containing cis-regulatory elements that activate transcription in SEs. One of these segments also contained sufficient information to suppress PMtSEO-F1 transcription in the phloem companion cells (CCs). Subsequent in silico analysis revealed several candidate cis-regulatory elements that PMtSEO-F1 shares with other SEO promoters. These putative sieve element boxes (PSE boxes) are promising candidates for cis-regulatory elements controlling the SE-specific expression of PMtSEO-F1.

Vacuoles release sucrose via tonoplast-localised SUC4-type transporters

Arabidopsis thaliana has seven genes for functionally active sucrose transporters. Together with sucrose transporters from other dicot and monocot plants, these proteins form four separate phylogenetic groups. Group-IV includes the Arabidopsis protein SUC4 (synonym SUT4) and related proteins from monocots and dicots. These Group-IV sucrose transporters were reported to be either tonoplast- or plasma membrane-localised, and in heterologous expression systems were shown to act as sucrose/H(+) symporters. Here, we present comparative analyses of the subcellular localisation of the Arabidopsis SUC4 protein and of several other Group-IV sucrose transporters, studies on tissue specificity of the Arabidopsis SUC4 promoter, phenotypic characterisations of Atsuc4.1 mutants and AtSUC4 overexpressing (AtSUC4-OX) plants, and functional comparisons of Atsuc4.1 and AtSUC4-OX vacuoles. Our data show that SUC4-type sucrose transporters from different plant families (Brassicaceae, Cucurbitaceae and Solanaceae) localise exclusively to the tonoplast, demonstrating that vacuolar sucrose transport is a common theme of all SUC4-type proteins. AtSUC4 expression is confined to the stele of Arabidopsis roots, developing anthers and meristematic tissues in all aerial parts. Analyses of the carbohydrate content of WT and mutant seedlings revealed reduced sucrose content in AtSUC4-OX seedlings. This is in line with patch-clamp analyses of AtSUC4-OX vacuoles that characterise AtSUC4 as a sucrose/H(+) symporter directly in the tonoplast membrane.

Characterization of the tomato prosystemin promoter: organ-specific expression, hormone specificity and methyl jasmonate responsiveness by deletion analysis in transgenic tobacco plants

Tomato systemin is a bioactive peptide that regulates the systemic activation of wound-responsive genes. It is released from its 200 amino acid precursor called prosystemin. Initial tissue-localization and hormone-induced expression assays indicated that the tomato prosystemin gene (SlPS) accumulates mainly in floral tissues and in response to exogenous abscisic acid and methyl jasmonate (MeJA) treatments, respectively. Later, the promoter regions of the PS gene in tomato (Solanum lycopersicum L. cv. Castlemart), pepper (Capsicum annuum) and potato (Solanum tuberosum) were isolated and an in silico analysis of the SlPS promoter revealed an over-representation of stress- and MeJA-responsive motifs. A subsequent 5' deletion analysis of the SlPS promoter fused to the β-glucuronidase reporter (GUS) gene showed that the -221 to +40 bp proximal SlPS promoter region was sufficient to direct the stigma, vascular bundle-specific and MeJA-responsive expression of GUS in transgenic tobacco plants. Important vascular-tissue-specific, light- and MeJA-responsive cis-elements were also present in this region. These findings provide relevant information regarding the transcriptional regulation mechanisms of the SlPS promoter operating in transgenic tobacco plants. They also suggest that its tissue-specificity and inducible nature could have wide applicability in plant biotechnology.

The Arabidopsis CORI3 promoter contains two cis-acting regulatory regions required for transcriptional activity in companion cells

Companion cells are metabolically active and functionally specialized cells that behave as terminals for the transport of materials between phloem and the surrounding tissue. Although previous research has clarified the distinct function of companion cells, it is still largely unknown how plants establish and maintain the special identity of these cells. To shed further light on this issue, we carried out expressed sequence tag (EST) analysis. To minimize the difficulty of dissociating and gathering intact companion cells, vascular strings with an abundant content of companion cells were excised from the petioles of Brassica napus. By random sequencing with a string-specific cDNA library derived by suppression subtractive hybridization between the string itself and the petiole from which it had been removed, we identified 377 ESTs and assembled them into 247 EST groups. The most frequent EST was ExBdl-102 (15 of 377 ESTs), which showed the highest sequence similarity to the Arabidopsis CORI3 (CORONATINE INDUCED 3) gene. The CORI3 promoter:GUS showed predominant expression in the vascular tissue of Arabidopsis. Through transient expression assay using Brassica vasculature and gene-gun-mediated transient assay, we found two integrated cis-regulatory regions of the CORI3 promoter. This work has provided not only string-specific EST information and shown that two novel cis-regulatory regions sustain transcriptional activity in companion cells, but also a series of procedures for efficiently examining the transcriptional framework of companion cells by exploiting the histochemical advantage of B. napus as an experimental material.

Vascular expression in Arabidopsis is predicted by the frequency of CT/GA-rich repeats in gene promoters

Phloem-transported signals play an important role in regulating plant development and in orchestrating responses to environmental stimuli. Among such signals, phloem-mobile RNAs have been shown to play an important role as long-distance signaling agents. At maturity, angiosperm sieve elements are enucleate, and thus transcripts in the phloem translocation stream probably originate from the nucleate companion cells. In the present study, a pumpkin (Cucurbita maxima) phloem transcriptome was used to test for the presence of common motifs within the promoters of this unique set of genes, which may function to coordinate expression in cells of the vascular system. A bioinformatics analysis of the upstream sequences from 150 Arabidopsis genes homologous to members of the pumpkin phloem transcriptome identified degenerate sequences containing CT/GA- and GT/CA-rich motifs that were common to many of these promoters. Parallel studies performed on genes shown previously to be expressed in phloem tissues identified similar motifs. An expanded analysis, based on homologs of the pumpkin phloem transcriptome from cucumber (Cucumis sativus), identified similar sets of common motifs within the promoters of these genes. Promoter analysis offered support for the hypothesis that these motifs regulate expression within the vascular system. Our findings are discussed in terms of a role for these motifs in coordinating gene expression within the companion cell/sieve element system. These motifs could provide a useful bioinformatics tool for genome-wide screens on plants for which phloem tissues cannot readily be obtained.

Modulation of sugar metabolism by an INDETERMINATE DOMAIN transcription factor contributes to photoperiodic flowering in Arabidopsis

There has been a long-standing interest in the role played by sugars in flowering. Of particular interest is how sugar-related signals are integrated into flowering genetic pathways. Here, we demonstrate that the INDETERMINATE DOMAIN transcription factor AtIDD8 regulates photoperiodic flowering by modulating sugar transport and metabolism. We found that whereas AtIDD8-deficient idd8 mutants exhibit delayed flowering under long days, AtIDD8-overexpressing plants (35S:IDD8) show early flowering. In addition, the sucrose synthase genes SUS1 and SUS4 were upregulated in 35S:IDD8 plants but downregulated in idd8 mutants, in which endogenous sugar levels were altered. AtIDD8 activates the SUS4 gene by binding directly to its promoter, resulting in promoted flowering in SUS4-overexpressing plants. SUS4 expression also responds to photoperiodic signals. Notably, the AtIDD8 gene is suppressed by sugar deprivation. Therefore, we conclude that AtIDD8 regulation of sugar transport and metabolism is linked to photoperiodic flowering.

The promoters of 3 celery salt-induced phloem-specific genes as new tools for monitoring salt stress responses

Genes induced by a progressive 3 week salt stress (final NaCl concentration 300 mM) were identified in the phloem of celery (Apium graveolens L., cv Vert d'Elne). A subtractive library was constructed and screened for salt-induced, phloem-specific genes. Work was focused on phloem due to its central role in inter-organ exchanges. Three genes were studied in more details, 2 coding for metallothioneins (AgMT2 and AgMT3) and one for a new mannitol transporter (AgMaT3). Expression of a reporter gene in transgenic Arabidopsis under control of promoter of each gene was located in the phloem. pAgMT2 has a typical phloem pattern with slight induction by salt stress. pAgMT3 and pAgMaT3 expression was induced by salt stress, except in minor veins. pAgMaT3 was highly active in stressed roots. The promoters described here could be regarded as new tools for engineering salt-resistant plants.

Potassium (K+) gradients serve as a mobile energy source in plant vascular tissues

The essential mineral nutrient potassium (K(+)) is the most important inorganic cation for plants and is recognized as a limiting factor for crop yield and quality. Nonetheless, it is only partially understood how K(+) contributes to plant productivity. K(+) is used as a major active solute to maintain turgor and to drive irreversible and reversible changes in cell volume. K(+) also plays an important role in numerous metabolic processes, for example, by serving as an essential cofactor of enzymes. Here, we provide evidence for an additional, previously unrecognized role of K(+) in plant growth. By combining diverse experimental approaches with computational cell simulation, we show that K(+) circulating in the phloem serves as a decentralized energy storage that can be used to overcome local energy limitations. Posttranslational modification of the phloem-expressed Arabidopsis K(+) channel AKT2 taps this "potassium battery," which then efficiently assists the plasma membrane H(+)-ATPase in energizing the transmembrane phloem (re)loading processes.

The Vitis vinifera sugar transporter gene family: phylogenetic overview and macroarray expression profiling

BACKGROUND

In higher plants, sugars are not only nutrients but also important signal molecules. They are distributed through the plant via sugar transporters, which are involved not only in sugar long-distance transport via the loading and the unloading of the conducting complex, but also in sugar allocation into source and sink cells. The availability of the recently released grapevine genome sequence offers the opportunity to identify sucrose and monosaccharide transporter gene families in a woody species and to compare them with those of the herbaceous Arabidopsis thaliana using a phylogenetic analysis.

RESULTS

In grapevine, one of the most economically important fruit crop in the world, it appeared that sucrose and monosaccharide transporter genes are present in 4 and 59 loci, respectively and that the monosaccharide transporter family can be divided into 7 subfamilies. Phylogenetic analysis of protein sequences has indicated that orthologs exist between Vitis and Arabidospis. A search for cis-regulatory elements in the promoter sequences of the most characterized transporter gene families (sucrose, hexoses and polyols transporters), has revealed that some of them might probably be regulated by sugars. To profile several genes simultaneously, we created a macroarray bearing cDNA fragments specific to 20 sugar transporter genes. This macroarray analysis has revealed that two hexose (VvHT1, VvHT3), one polyol (VvPMT5) and one sucrose (VvSUC27) transporter genes, are highly expressed in most vegetative organs. The expression of one hexose transporter (VvHT2) and two tonoplastic monosaccharide transporter (VvTMT1, VvTMT2) genes are regulated during berry development. Finally, three putative hexose transporter genes show a preferential organ specificity being highly expressed in seeds (VvHT3, VvHT5), in roots (VvHT2) or in mature leaves (VvHT5).

CONCLUSIONS

This study provides an exhaustive survey of sugar transporter genes in Vitis vinifera and revealed that sugar transporter gene families in this woody plant are strongly comparable to those of herbaceous species. Dedicated macroarrays have provided a Vitis sugar transporter genes expression profiling, which will likely contribute to understand their physiological functions in plant and berry development. The present results might also have a significant impact on our knowledge on plant sugar transporters.

An ABA-responsive element in the AtSUC1 promoter is involved in the regulation of AtSUC1 expression

Abscisic acid (ABA) and sugars regulate many aspects of plant growth and development, and we are only just beginning to understand the complex interactions between ABA and sugar signaling networks. Here, we show that ABA-dependent transcription factors bind to the promoter of the Arabidopsis thaliana AtSUC1 (At1g71880) sucrose transporter gene in vitro. We present the characterization of a cis-regulatory element by truncation of the AtSUC1 promoter and by electrophoretic mobility shift assays that is identical to a previously characterized ABA-responsive element (ABRE). In yeast 1-hybrid analyses we identified ABI5 (AtbZIP39; At2g36270) and AREB3 (AtbZIP66; At3g56850) as potential interactors. Analyses of plants expressing the beta-glucuronidase reporter gene under the control of ABI5 or AREB3 promoter sequences demonstrated that both transcription factor genes are co-expressed with AtSUC1 in pollen and seedlings, the primary sites of AtSUC1 action. Mutational analyses of the identified cis-regulatory element verified its importance for AtSUC1 expression in young seedlings. In abi5-4 seedlings, we observed an increase of sucrose-dependent anthocyanin accumulation and AtSUC1 mRNA levels. This suggests that ABI5 prevents an overshoot of sucrose-induced AtSUC1 expression and confirmed a novel cross-link between sugar and ABA signaling.

Identification of a cis-regulatory element that acts in companion cell-specific expression of AtMT2B promoter through the use of Brassica vasculature and gene-gun-mediated transient assay

The molecular basis underlying the development, maintenance and function of companion cells in plants is largely unknown. The identification of several genes expressed specifically in companion cells implies the contribution of specific transcriptional elements to the identity of companion cells. However, less is known about the companion cell-specific transcriptional regulation of promoters. We established a novel assay method using gene-gun delivery of partially deleted promoters to string-containing vascular bundles excised from the petiole of Brassica napus for the rapid identification of cis-elements. To test this system, we analyzed the Arabidopsis METALLOTHIONEIN 2B (MT2B) gene, which is expressed in companion cells. The assay revealed a 49-bp region possessing two predicted cis-regulatory elements: a G-box and an evening element-related sequence (EEr), and EEr showed higher activity. We confirmed the reliability of the result with stable transformants harboring a deleted MT2B promoter:GUS transgene. The lack of EEr completely eliminated the MT2B-like expression, but the lack of G-box did not eliminate it. We conclude that EEr is a major cis-regulatory element of the MT2B promoter. Our method will help to explain the transcriptional background of companion cells through the rapid identification of cis-regulatory regions.