The role of hexokinases from grape berries (Vitis vinifera L.) in regulating the expression of cell wall invertase and sucrose synthase genes

Fruit Photosynthesis: More to Know about Where, How and Why

Not only leaves but also other plant organs and structures typically considered as carbon sinks, including stems, roots, flowers, fruits and seeds, may exhibit photosynthetic activity. There is still a lack of a coherent and systematized body of knowledge and consensus on the role(s) of photosynthesis in these "sink" organs. With regard to fruits, their actual photosynthetic activity is influenced by a range of properties, including fruit anatomy, histology, physiology, development and the surrounding microclimate. At early stages of development fruits generally contain high levels of chlorophylls, a high density of functional stomata and thin cuticles. While some plant species retain functional chloroplasts in their fruits upon subsequent development or ripening, most species undergo a disintegration of the fruit chloroplast grana and reduction in stomata functionality, thus limiting gas exchange. In addition, the increase in fruit volume hinders light penetration and access to CO₂, also reducing photosynthetic activity. This review aimed to compile information on aspects related to fruit photosynthesis, from fruit characteristics to ecological drivers, and to address the following challenging biological questions: why does a fruit show photosynthetic activity and what could be its functions? Overall, there is a body of evidence to support the hypothesis that photosynthesis in fruits is key to locally providing: ATP and NADPH, which are both fundamental for several demanding biosynthetic pathways (e.g., synthesis of fatty acids); O₂, to prevent hypoxia in its inner tissues including seeds; and carbon skeletons, which can fuel the biosynthesis of primary and secondary metabolites important for the growth of fruits and for spreading, survival and germination of their seed (e.g., sugars, flavonoids, tannins, lipids). At the same time, both primary and secondary metabolites present in fruits and seeds are key to human life, for instance as sources for nutrition, bioactives, oils and other economically important compounds or components. Understanding the functions of photosynthesis in fruits is pivotal to crop management, providing a rationale for manipulating microenvironmental conditions and the expression of key photosynthetic genes, which may help growers or breeders to optimize development, composition, yield or other economically important fruit quality aspects.

Characterization of hexokinase gene family members in Glycine max and functional analysis of GmHXK2 under salt stress

Hexokinase (HXK) is a bifunctional enzyme involved in carbohydrate metabolism and sugar signal sensing. HXK gene family has been extensively discussed in many species, while the detailed investigations of the family in Glycine max have yet to be reported. In this study, 17 GmHXK genes (GmHXKs) were identified in the G. max genome and the features of their encoded proteins, conserved domains, gene structures, and cis-acting elements were systematically characterized. The GmHXK2 gene isolated from G. max was firstly constructed into plant expression vector pMDC83 and then transformed with Agrobacterium tumefaciens into Arabidopsis thaliana. The expression of integrated protein was analyzed by Western Blotting. Subcellular localization analysis showed that the GmHXK2 was located on both vacuolar and cell membrane. Under salt stress, seedlings growth was significantly improved in Arabidopsis overexpressing GmHXK2 gene. Furthermore, physiological indicators and expression of salt stress responsive genes involved in K⁺ and Na⁺ homeostasis were significantly lower in GmHXK2-silenced soybean seedlings obtained by virus-induced gene silencing (VIGS) technique under salt stress compared with the control plants. Our study showed that GmHXK2 gene played an important role in resisting salt stress, which suggested potential value for the genetic improvement of abiotic resistant crops.

Transcriptional profiling of two muscadine grape cultivars "Carlos" and "Noble" to reveal new genes, gene regulatory networks, and pathways that involved in grape berry ripening

In commercial fruit production, synchronized ripening and stable shelf life are important properties. The loosely clustered or non-bunching muscadine grape has unrealized potential as a disease-resistant cash crop, but requires repeated hand harvesting due to its unsynchronized or long or heterogeneous maturation period. Genomic research can be used to identify the developmental and environmental factors that control fruit ripening and postharvest quality. This study coupled the morphological, biochemical, and genetic variations between "Carlos" and "Noble" muscadine grape cultivars with RNA-sequencing analysis during berry maturation. The levels of antioxidants, anthocyanins, and titratable acids varied between the two cultivars during the ripening process. We also identified new genes, pathways, and regulatory networks that modulated berry ripening in muscadine grape. These findings may help develop a large-scale database of the genetic factors of muscadine grape ripening and postharvest profiles and allow the discovery of the factors underlying the ripeness heterogeneity at harvest. These genetic resources may allow us to combine applied and basic research methods in breeding to improve table and wine grape ripening uniformity, quality, stress tolerance, and postharvest handling and storage.

Screening cellulose synthesis related genes of EgrEXP and EgrHEX in Eucalyptus grandis

Eucalyptus (including Eucalyptus grandis) is an excellent wood forest tree species that provides a large number of plant fiber raw materials for the paper and timber industries. Cellulose, an essential structural component in plant cell walls, is a renewable biomass resource that plays a very important role in nature. There is still a lack of research on the role of gene regulation in cellulose synthesis. To study the genes of cellulose synthesis, the wood chemical indexes of Eucalyptus grandis were analyzed by taking three different parts from the main stem of Eucalyptus grandis as raw materials. The results showed that the cellulose content in the middle of the trunk was significantly higher than that at the chest diameter and at the upper part of the trunk. A total of 296 differentially expressed genes (DEGs) were obtained from the three site by transcriptome, and 19 key candidate genes were related to the synthesis of cellulose in Eucalyptus grandis. EgrEXP1 and EgrHEX4 were overexpressed in 84 K poplar, the content of cellulose and lignin in genetically modified plants was significantly higher than that of wild type 84 K poplar. Also, the average plant height and average root count were significantly higher than those of control plants, and the average diameter of the middle and stem bases were significantly larger than those of control plants. In this study, the genes related to cellulose synthesis in Eucalyptus grandis are studied, which serve as a strong foundation for understanding the molecular regulation of cellulose synthesis in plants.

Quantitative Ubiquitylomic Analysis of the Dynamic Changes and Extensive Modulation of Ubiquitylation in Papaya During the Fruit Ripening Process

Lysine ubiquitination is a highly conserved post-translational modification with diverse biological functions. However, there is little available information on lysine ubiquitination of non-histone proteins in papaya (Carica papaya L.). In total, 3,090 ubiquitination sites on 1,249 proteins with diverse localizations and functions were identified. Five conserved ubiquitinated K motifs were identified. Enrichment analysis showed that many Hsps were differentially ubiquitinated proteins (DUPs), suggesting an essential role of ubiquitination in degradation of molecular chaperone. Furthermore, 12 sugar metabolism-related enzymes were identified as DUPs, including an involvement of ubiquitination in nutrimental changes during the papaya ripening process. The ubiquitination levels of five fruit ripening-related DUPs, including one ethylene-inducible protein, two 1-aminocyclopropane-1-carboxylic acid oxidases, one endochitinase, and one cell wall invertase, were significantly changed during the ripening process. Our study extends the understanding of diverse functions for lysine ubiquitination in regulation of the papaya fruit ripening process.

N-Acetyl-d-glucosamine Inhibition of Hexokinase Results in Downregulation of the Phenylpropanoid Metabolic Pathway and Decreased Resistance to Brown Rot in Peach Fruit

To explore the role of hexokinase (HXK) on disease resistance in peach fruit, peaches were treated with N-acetyl-d-glucosamine (NAG), a known HXK inhibitor, and then inoculated with Monilinia fructicola. We demonstrate that NAG significantly inhibits HXK activity, which in turn results in significantly reduced resistance to M. fructicola infection. In the HXK-inhibited fruit, the sucrose content was higher and the glucose and fructose contents were lower than in the control fruit. By transcriptome analysis, we found 347 differentially expressed genes (DEGs) between NAG-treated and control peaches, most of which were involved in the mitogen-activated protein kinase signaling pathway in plants, plant-pathogen interaction, plant hormone signal transduction, and the phenylpropanoid biosynthesis pathway. In particular, the DEGs related to phenylpropanoid metabolism, such as peroxidase, flavonoid, and isoflavonoid biosynthesis were significantly downregulated. Nontargeted metabolomic analysis revealed 44 differential metabolites, 9 of which were increased and 35 of which were decreased in the NAG-treated fruit. The decreased metabolites were secondary metabolites, including polyphenols, flavonoids, terpenoids, and glycosides. The relationship between HXK and phenylpropanoid metabolism was further investigated, and we found that in HXK-inhibited fruits the activities of phenylalanine ammonia-lyase, 4-coumarate-CoA ligase, and cinnamate 4-hydroxylase were significantly decreased over the control fruit, as well as the total phenol and total flavone contents were also significantly decreased. These results demonstrate that the inhibition of HXK activity decreases the disease resistance of peach fruits by affecting sugar metabolism and the phenylpropanoid pathway.

Insights into sugar metabolism during bilberry (Vaccinium myrtillus L.) fruit development

Bilberry fruit is regarded as one of the best natural sources of anthocyanins and is widely explored for its health-beneficial compounds. Besides anthocyanins, one of the major attributes that determine the berry quality is the accumulation of sugars that provide sweetness and flavor to ripening fruit. In this study, we have identified 25 sugar metabolism-related genes in bilberry, including invertases (INVs), hexokinases (HKs), fructokinases (FKs), sucrose synthases (SSs), sucrose phosphate synthases (SPSs), and sucrose phosphate phosphatases (SPPs). The results indicate that isoforms of the identified genes are expressed differentially during berry development, suggesting specialized functions. The highest sugar content was found in ripe berries, with fructose and glucose dominating accompanied by low sucrose amount. The related enzyme activities during berry development and ripening were further analyzed to understand the molecular mechanism of sugar accumulation. The activity of INVs in the cell wall and vacuole increased toward ripe berries. Amylase activity involved in starch metabolism was not detected in unripe berries but was found in ripe berries. Sucrose resynthesizing SS enzyme activity was detected upon early ripening and had the highest activity in ripe berries. Interestingly, our transcriptome data showed that supplemental irradiation with red and blue light triggered upregulation of several sugar metabolism-related genes, including α- and β-amylases. Also, differential expression patterns in responses to red and blue light were found across sucrose, galactose, and sugar-alcohol metabolism. Our enzymological and transcriptional data provide new understanding of the bilberry fruit sugar metabolism having major effect on fruit quality.

Putative regulatory role of hexokinase and fructokinase in terpenoid aroma biosynthesis in Lilium 'Siberia'

Lily is one of the most economically important flowers worldwide due to its elegant appearance and appealing scent, which is mainly composed of monoterpene ocimene, linalool and benzenoids. Sugars are the primary products of plants, with fructose and hexose sugars being the substrate material for most organic compounds and metabolic pathways in plants. Herein, we isolated and functionally characterized hexokinase (LoHXK) and fructokinase (LoFRK) from Lilium 'Siberia' flower, which indicated their potential roles in floral aroma production. Real-time PCR analysis showed that LoHXK and LoFRK were highly expressed in the flower filament. Overexpression and virus-induced gene silencing (VIGS) assays revealed that LoHXK and LoFRK significantly modified the emission of β-ocimene and linalool contents via regulation of expression of key structural volatile synthesis genes (LoTPS1 and LoTPS3). Under exogenous glucose and fructose application, the volatile contents of β-ocimene and linalool were increased and the expression levels of key structural genes were upregulated. The emission of β-ocimene and linalool followed a diurnal circadian rhythm. Determination of carbon fluxes via ¹³C-labeled glucose and ¹³C-labeled fructose experiments showed that the mass spectra of ocimene and linalool significantly increased, however, the m/z ratio of ethyl benzoate did not change. Furthermore, yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that LoFRK interacted with LoMYB1 and LoMYB2 proteins. Together, these results suggest that hexokinase and fructokinase may play significant roles in the regulation of ocimene and linalool biosynthesis in Lilium 'Siberia'.

Light Microclimate-Driven Changes at Transcriptional Level in Photosynthetic Grape Berry Tissues

Viticulture practices that change the light distribution in the grapevine canopy can interfere with several physiological mechanisms, such as grape berry photosynthesis and other metabolic pathways, and consequently impact the berry biochemical composition, which is key to the final wine quality. We previously showed that the photosynthetic activity of exocarp and seed tissues from a white cultivar (Alvarinho) was in fact responsive to the light microclimate in the canopy (low and high light, LL and HL, respectively), and that these different light microclimates also led to distinct metabolite profiles, suggesting a berry tissue-specific interlink between photosynthesis and metabolism. In the present work, we analyzed the transcript levels of key genes in exocarps and seed integuments of berries from the same cultivar collected from HL and LL microclimates at three developmental stages, using real-time qPCR. In exocarp, the expression levels of genes involved in carbohydrate metabolism (VvSuSy1), phenylpropanoid (VvPAL1), stilbenoid (VvSTS1), and flavan-3-ol synthesis (VvDFR, VvLAR2, and VvANR) were highest at the green stage. In seeds, the expression of several genes associated with both phenylpropanoid (VvCHS1 and VvCHS3) and flavan-3-ol synthesis (VvDFR and VvLAR2) showed a peak at the véraison stage, whereas that of RuBisCO was maintained up to the mature stage. Overall, the HL microclimate, compared to that of LL, resulted in a higher expression of genes encoding elements associated with both photosynthesis (VvChlSyn and VvRuBisCO), carbohydrate metabolism (VvSPS1), and photoprotection (carotenoid pathways genes) in both tissues. HL also induced the expression of the VvFLS1 gene, which was translated into a higher activity of the FLS enzyme producing flavonol-type flavonoids, whereas the expression of several other flavonoid pathway genes (e.g., VvCHS3, VvSTS1, VvDFR, and VvLDOX) was reduced, suggesting a specific role of flavonols in photoprotection of berries growing in the HL microclimate. This work suggests a possible link at the transcriptional level between berry photosynthesis and pathways of primary and secondary metabolism, and provides relevant information for improving the management of the light microenvironment at canopy level of the grapes.

VvWRKY22 transcription factor interacts with VvSnRK1.1/VvSnRK1.2 and regulates sugar accumulation in grape

WRKY transcription factors are one of the largest transcriptional regulator families, involved in various signaling networks in plants. However, only limited functional exploration of the sugar signaling of Vitis vinifera WRKY22 transcription factor (VvWRKY22) has been conducted. In this study, the roles played by VvWRKY22 in sugar accumulation in grapes were investigated. VvWRKY22 was co-expressed with 16 sugar-related genes, and the expression of VvWRKY22 in grape suspension cells was inhibited by sucrose, and induced by fructose and abscisic acid (ABA). Results showed that over-expression of VvWRKY22 decreased the sucrose, glucose and fructose content, and regulated the expression levels of sugar and ABA-related genes. Moreover, it was found that VvWRKY22 interacted with VvSnRK1.1 or VvSnRK1.2 proteins (Sucrose non-fermenting-1-related protein kinase 1), which are important kinases related to sugar metabolism. These results, thus, provide new genetic evidences to support the view that VvWRKY22 functions in regulating sugar metabolism in grapes.

Comparative transcriptome analysis of Rheum australe, an endangered medicinal herb, growing in its natural habitat and those grown in controlled growth chambers

Rheum australe is an endangered medicinal herb of high altitude alpine region of Himalayas and is known to possess anti-cancerous properties. Unlike many herbs of the region, R. australe has broad leaves. The species thrives well under the environmental extremes in its niche habitat, therefore an understanding of transcriptome of R. australe to environmental cues was of significance. Since, temperature is one of the major environmental variables in the niche of R. australe, transcriptome was studied in the species growing in natural habitat and those grown in growth chambers maintained at 4 °C and 25 °C to understand genes associated with different temperatures. A total of 39,136 primarily assembled transcripts were obtained from 10,17,74,336 clean read, and 21,303 unigenes could match to public databases. An analysis of transcriptome by fragments per kilobase of transcript per million, followed by validation through qRT-PCR showed 22.4% up- and 22.5% down-regulated common differentially expressed genes in the species growing under natural habitat and at 4 °C as compared to those at 25 °C. These genes largely belonged to signaling pathway, transporters, secondary metabolites, phytohormones, and those associated with cellular protection, suggesting their importance in imparting adaptive advantage to R. australe in its niche.

Sucrose transportation control mediates the fresh-keeping effects of burdock fructooligosaccharide in 'Crimson Seedless' grapes

In 'Crimson Seedless' grapes, the appearance of senescence caused by abnormal dark red color, the loss of crisp taste caused by the decrease in firmness, and the fading of sweetness caused by the decrease in total soluble sugar (TSS) are the main problems affecting its edible qualities after storage. In the mesocarp, burdock fructooligosaccharide (BFO) restricted sucrose export; therefore, more carbohydrates were retained directly leading to higher TSS and sweetness, and cell osmotic pressure and firmness were retained indirectly. In the exocarp, BFO restricted sucrose import; therefore, the signal molecule sucrose was reduced and the senescence-related processes were inhibited. The downregulation of SUC12 and SUC27 by BFO may play an important role in restricting sucrose transportation. The opposing effects exhibited by exogenous sucrose treatments compared to those of BFO further verified these mechanisms. Based on the above mechanisms, sucrose transportation mediates the fresh-keeping effects of BFO in 'Crimson Seedless' grapes.

Early-stage sugar beet taproot development is characterized by three distinct physiological phases

Despite the agronomic importance of sugar beet (Beta vulgaris L.), the early-stage development of its taproot has only been poorly investigated. Thus, the mechanisms that determine growth and sugar accumulation in sugar beet are largely unknown. In the presented study, a physiological characterization of early-stage sugar beet taproot development was conducted. Activities were analyzed for fourteen key enzymes of carbohydrate metabolism in developing taproots over the first 80 days after sowing. In addition, we performed in situ localizations of selected carbohydrate-metabolic enzyme activities, anatomical investigations, and quantifications of soluble carbohydrates, hexose phosphates, and phytohormones. Based on the accumulation dynamics of biomass and sucrose, as well as on anatomical parameters, the early phase of taproot development could be subdivided into three stages-prestorage, transition, secondary growth and sucrose accumulation stage-each of which was characterized by distinct metabolic and phytohormonal signatures. The enzyme activity signatures corresponding to these stages were also shown to be robustly reproducible in experiments conducted in two additional locations. The results from this physiological phenotyping approach contribute to the identification of the key regulators of sugar beet taproot development and open up new perspectives for sugar beet crop improvement concerning both physiological marker-based breeding and biotechnological approaches.

The Molecular Regulation of Carbon Sink Strength in Grapevine (Vitis vinifera L.)

Sink organs, the net receivers of resources from source tissues, provide food and energy for humans. Crops yield and quality are improved by increased sink strength and source activity, which are affected by many factors, including sugars and hormones. With the growing global population, it is necessary to increase photosynthesis into crop biomass and yield on a per plant basis by enhancing sink strength. Sugar translocation and accumulation are the major determinants of sink strength, so understanding molecular mechanisms and sugar allocation regulation are conducive to develop biotechnology to enhance sink strength. Grapevine (Vitis vinifera L.) is an excellent model to study the sink strength mechanism and regulation for perennial fruit crops, which export sucrose from leaves and accumulates high concentrations of hexoses in the vacuoles of fruit mesocarp cells. Here recent advances of this topic in grape are updated and discussed, including the molecular biology of sink strength, including sugar transportation and accumulation, the genes involved in sugar mobilization and their regulation of sugar and other regulators, and the effects of hormones on sink size and sink activity. Finally, a molecular basis model of the regulation of sugar accumulation in the grape is proposed.

Polysaccharide biosynthetic pathway profiling and putative gene mining of Dendrobium moniliforme using RNA-Seq in different tissues

BACKGROUND

Dendrobium moniliforme (Linnaeus) Swartz is a well-known plant used in traditional Chinese medicine due to bioactive constituents. Polysaccharides are the main medicinal ingredients, yet no studies have been published on polysaccharide biosynthesis in D. moniliforme. To comprehensively investigate the polysaccharide at the transcription level, we performed de novo transcriptome sequencing for the first time to produce a comprehensive transcriptome of D. moniliforme.

RESULTS

In our study, a database of 562,580 unigenes (average length = 1115.67 bases) was generated by performing transcriptome sequencing. Based on the gene annotation of the transcriptome, we identified 1204 carbohydrate-active related unigenes against CAZy database, including 417 glycosyltransferase genes (GTs), 780 glycoside hydrolases (GHs), 19 carbohydrate esterases (CEs), 75 carbohydrate-binding modules (CBMs), and 44 polysaccharide lyases (PLs). In the cellulose synthase family, 21 differential expression genes (DEGs) related to polysaccharide were identified. Subsequently, the tissue-specific expression patterns of the genes involved in polysaccharide pathway were investigated, which provide understanding of the biosynthesis and regulation of DMP at the molecular level. The two key enzyme genes (Susy and SPS) involved in the polysaccharide pathway were identified, and their expression patterns in different tissues were further analyzed using quantitative real-time PCR.

CONCLUSIONS

We determined the content of polysaccharides from Dendrobium moniliforme under different tissues, and we obtained a large number of differential genes by transcriptome sequencing. This database provides a pool of candidate genes involved in biosynthesis of polysaccharides in D. moniliforme. Furthermore, the comprehensive analysis and characterization of the significant pathways are expected to give a better insight regarding the diversity of chemical composition, synthetic characteristics, and the regulatory mechanism which operate in this medical herb.

Isolation and characterization of hexokinase genes PsHXK1 and PsHXK2 from tree peony (Paeonia suffruticosa Andrews)

Hexokinase (HXK) plays important roles in hexose phosphorylation and sugar signaling. HXK regulates the glucose-induced accumulation of anthocyanin in many species. Little is known about the biological function of the HXK gene family in Paeonia suffruticosa. cDNA sequences of two hexokinase genes PsHXK1 and PsHXK2 were isolated using RACE-PCR and RT-PCR from P. suffruticosa. PsHXK1 encodes 498 amino acids with a 1497-bp open reading frame (ORF), and PsHXK2 contains 493 amino acids with a 1482-bp ORF. Sequence and phylogenetic analyses suggest that PsHXK1 and PsHXK2 belong to type-B HXK and may function as glucose sensors. PsHXK1 and PsHXK2 mRNA were detected in all tested tissues. PsHXK1 is highly expressed in petals and stamens, while PsHXK2 is highly expressed in stamens. At the former stages of flower opening, PsHXK1 and PsHXK2 show higher expression levels in on-tree flowers compared with cut flowers. Overexpressing PsHXK1 and PsHXK2 in Arabidopsis enhances glucose sensitivity, inhibits plant growth in response to glucose, and induces anthocyanin accumulation in response to the high level of glucose. Overall, our results primarily reveal the biological function of PsHXK1 and PsHXK2, especially their involvement in glucose-induced anthocyanin accumulation.

Arabidopsis GSM1 is involved in ABI4-regulated ABA signaling under high-glucose condition in early seedling growth

In plants, sugar acts as an essential signaling molecule that modulates various aspects of metabolism, growth and development, which are also controlled by phytohormones. However, the molecular mechanism of cross-talk between sugar and phytohormones still remains to be elucidated. We have identified gsm1 (glucose-hypersensitive mutant 1) as a mutant with impaired cotyledon development that shows sensitivity to exogenous abscisic acid (ABA). The addition of fluridone can reverse the glucose (Glc) inhibitory effect in gsm1, implying that endogenous ABA is involved in the Glc response of gsm1. In 4.5% Glc, the expression of Glc-induced ABA-responsive genes in gsm1-1 was nearly two times higher than that in the wild type. Compared to gsm1-1, the gsm1-1 abi4-1 double mutant exhibited reduced sensitivity to Glc and ABA, which was similar to the Glc and ABA insensitive phenotype of abi4-1, suggesting that ABI4 is epistatic to GSM1. In the treatment with 4.5% Glc, the GSM1 transcript level was greatly increased in abi4-1 by almost 4-fold of that in the wild type. These data suggest that GSM1 plays an important role in the ABI4-regulated Glc-ABA signaling cascade during Arabidopsis early seedling growth.

Infection of post-harvest peaches by Monilinia fructicola accelerates sucrose decomposition and stimulates the Embden-Meyerhof-Parnas pathway

To study the changes in sugar metabolism caused by fungal infection in post-harvest peaches, fruit from two cultivars ('Baifeng' and 'Yulu') was inoculated with Monilinia fructicola and stored at 10 °C. During disease development, soluble sugar content was monitored, as well as the activities and expression of selected enzymes. Disease progression was accompanied by a decrease in sucrose content and increases in reducing sugars and soluble solids, consistent with higher enzyme activities for acid invertase, neutral invertase and sucrose synthase-cleavage, and lower activities for sucrose synthase-synthesis and sucrose phosphate synthase. Activities of phosphofructokinase, hexokinase, and pyruvate kinase, which are related to hexose metabolism, also increased. These changes stimulate the Embden-Meyerhof-Parnas (EMP) pathway. We conclude that the fungal disease in peach fruit accelerates the decomposition of sucrose, thereby providing more glucose as a substrate to the EMP pathway.

Isolation and Characterization of Ftsz Genes in Cassava

The filamenting temperature-sensitive Z proteins (FtsZs) play an important role in plastid division. In this study, three FtsZ genes were isolated from the cassava genome, and named MeFtsZ1, MeFtsZ2-1, and MeFtsZ2-2, respectively. Based on phylogeny, the MeFtsZs were classified into two groups (FtsZ1 and FtsZ2). MeFtsZ1 with a putative signal peptide at N-terminal, has six exons, and is classed to FtsZ1 clade. MeFtsZ2-1 and MeFtsZ2-2 without a putative signal peptide, have seven exons, and are classed to FtsZ2 clade. Subcellular localization found that all the three MeFtsZs could locate in chloroplasts and form a ring in chloroplastids. Structure analysis found that all MeFtsZ proteins contain a conserved guanosine triphosphatase (GTPase) domain in favor of generate contractile force for cassava plastid division. The expression profiles of MeFtsZ genes by quantitative reverse transcription-PCR (qRT-PCR) analysis in photosynthetic and non-photosynthetic tissues found that all of the MeFtsZ genes had higher expression levels in photosynthetic tissues, especially in younger leaves, and lower expression levels in the non-photosynthetic tissues. During cassava storage root development, the expressions of MeFtsZ2-1 and MeFtsZ2-2 were comparatively higher than MeFtsZ1. The transformed Arabidopsis of MeFtsZ2-1 and MeFtsZ2-2 contained abnormally shape, fewer number, and larger volume chloroplasts. Phytohormones were involved in regulating the expressions of MeFtsZ genes. Therefore, we deduced that all of the MeFtsZs play an important role in chloroplast division, and that MeFtsZ2 (2-1, 2-2) might be involved in amyloplast division and regulated by phytohormones during cassava storage root development.

Identification of candidate genes involved in the sugar metabolism and accumulation during pear fruit post-harvest ripening of 'Red Clapp's Favorite' (Pyrus communis L.) by transcriptome analysis

Background

Pear (Pyrus spp.) is a popular fruit that is commercially cultivated in most temperate regions. In fruits, sugar metabolism and accumulation are important factors for fruit organoleptic quality. Post-harvest ripening is a special feature of ‘Red Clapp’s Favorite’.

Results

In this study, transcriptome sequencing based on the Illumina platform generated 23.8 - 35.8 million unigenes of nine cDNA libraries constructed using RNAs from the ‘Red Clapp’s Favorite’ pear variety with different treatments, in which 2629 new genes were discovered, and 2121 of them were annotated. A total of 2146 DEGs, 3650 DEGs, 1830 DEGs from each comparison were assembled. Moreover, the gene expression patterns of 8 unigenes related to sugar metabolism revealed by qPCR. The main constituents of soluble sugars were fructose and glucose after pear fruit post-harvest ripening, and five unigenes involved in sugar metabolism were discovered.

Conclusions

Our study not only provides a large-scale assessment of transcriptome resources of ‘Red Clapp’s Favorite’ but also lays the foundation for further research into genes correlated with sugar metabolism.

Electronic supplementary material

The online version of this article (10.1186/s41065-017-0046-0) contains supplementary material, which is available to authorized users.

Plant Hexokinases are Multifaceted Proteins

Sugars are the main carbon and energy source in cells, but they can also act as signaling molecules that affect the whole plant life cycle. Certain tissues can produce sugars and supply them to others, and this plant tissue heterogeneity makes sugar signaling a highly complex process that requires elements capable of perceiving changes in sugar concentrations among different tissues, cell compartments and developmental stages. In plants, the regulatory effects of glucose (Glc) have been the most studied to date. The first Glc sensor identified in plants was hexokinase (HXK), which is currently recognized as a dual-function protein. In addition to its catalytic activity, this enzyme can also repress the expression of some photosynthetic genes in response to high internal Glc concentrations. Additionally, the catalytic activity of HXKs has a profound impact on cell metabolism and other sugar signaling pathways that depend on phosphorylated hexoses and intermediate glycolytic products. HXKs are the only proteins that are able to phosphorylate Glc in plants, since no evidence has been provided to date concerning the existence of a glucokinase. Moreover, the intracellular localization of HXKs seems to be crucial to their activity and sensor functions. Recently, two new and surprising functions have been described for HXKs. In this review, we discuss the versatility of HXKs in regard to their catalytic and glucose sensor activities, intracellular location, protein-protein and hormone interactions, as well as how these HXK characteristics influence plant growth and development, in an effort to understand this enzyme's role in improving plant productivity.

Structure, Expression, and Functional Analysis of the Hexokinase Gene Family in Cassava

Hexokinase (HXK) proteins play important roles in catalyzing hexose phosphorylation and sugar sensing and signaling. To investigate the roles of HXKs in cassava tuber root development, seven HXK genes (MeHXK1-7) were isolated and analyzed. A phylogenetic analysis revealed that the MeHXK family can be divided into five subfamilies of plant HXKs. MeHXKs were clearly divided into type A (MeHXK1) and type B (MeHXK2-7) based on their N-terminal sequences. MeHXK1-5 all had typical conserved regions and similar protein structures to the HXKs of other plants; while MeHXK6-7 lacked some of the conserved regions. An expression analysis of the MeHXK genes in cassava organs or tissues demonstrated that MeHXK2 is the dominant HXK in all the examined tissues (leaves, stems, fruits, tuber phloems, and tuber xylems). Notably, the expression of MeHXK2 and the enzymatic activity of HXK were higher at the initial and expanding tuber stages, and lower at the mature tuber stage. Furthermore, the HXK activity of MeHXK2 was identified by functional complementation of the HXK-deficient yeast strain YSH7.4-3C (hxk1, hxk2, glk1). The gene expression and enzymatic activity of MeHXK2 suggest that it might be the main enzyme for hexose phosphorylation during cassava tuber root development, which is involved in sucrose metabolism to regulate the accumulation of starch.

Grape hexokinases are involved in the expression regulation of sucrose synthase- and cell wall invertase-encoding genes by glucose and ABA

Hexokinase (HXK, EC 2.7.1.1) is a multifunctional protein that both is involved in catalyzing the first step of glycolysis and plays an important role in sugar signaling. However, the supporting genetic evidence on hexokinases (CsHXKs) from grape (Vitis vinifera L. cv. Cabernet Sauvignon) berries has been lacking. Here, to investigate the role of CsHXK isoforms as glucose (Glc) and abscisic acid (ABA) sensors, we cloned two hexokinase isozymes, CsHXK1 and CsHXK2 with highly conserved genomic structure of nine exons and eight introns. We also found adenosine phosphate binding, substrate recognition and connection sites in their putative proteins. During grape berry development, the expression profiles of two CsHXK isoforms, sucrose synthases (SuSys) and cell wall invertase (CWINV) genes increased concomitantly with high levels of endogenous Glc and ABA. Furthermore, we showed that in wild type grape berry calli (WT), glucose repressed the expression levels of sucrose synthase (SuSy) and cell wall invertase (CWINV) genes, while ABA increased their expression levels. ABA could not only effectively improve the expression levels of SuSy and CWINV, but also block the repression induced by glucose on the expression of both genes. However, after silencing CsHXK1 or CsHXK2 in grape calli, SuSy and CWINV expression were enhanced, and the expressions of the two genes are insensitive in response to Glc treatment. Interestingly, exogenous ABA alone could not or less increase SuSy and CWINV expression in silencing CsHXK1 or CsHXK2 grape calli compared to WT. Meantime, ABA could not block the repression induced by glucose on the expression of SuSy and CWINV in CsHXK1 or CsHXK2 mutants. Therefore, Glc signal transduction depends on the regulation of CsHXK1 or CsHXK2. ABA signal was also disturbed by CsHXK1 or CsHXK2 silencing. The present results provide new insights into the regulatory role of Glc and ABA on the enzymes related to sugar metabolism in grape berry.

Molecular candidates for early-stage flower-to-fruit transition in stenospermocarpic table grape (Vitis vinifera L.) inflorescences ascribed by differential transcriptome and metabolome profiles

Flower-to-fruit transition depends of nutrient availability and regulation at the molecular level by sugar and hormone signalling crosstalk. However, in most species, the identities of fruit initiation regulators and their targets are largely unknown. To ascertain the main pathways involved in stenospermocarpic table grape fruit set, comprehensive transcriptional and metabolomic analyses were conducted specifically targeting the early phase of this developmental stage in 'Thompson Seedless'. The high-throughput analyses performed disclosed the involvement of 496 differentially expressed genes and 28 differently accumulated metabolites in the sampled inflorescences. Our data show broad transcriptome reprogramming of molecule transporters, globally down-regulating gene expression, and suggest that regulation of sugar- and hormone-mediated pathways determines the downstream activation of berry development. The most affected gene was the SWEET14 sugar transporter. Hormone-related transcription changes were observed associated with increased indole-3-acetic acid, stimulation of ethylene and gibberellin metabolisms and cytokinin degradation, and regulation of MADS-box and AP2-like ethylene-responsive transcription factor expression. Secondary metabolism, the most representative biological process at transcriptome level, was predominantly repressed. The results add to the knowledge of molecular events occurring in grapevine inflorescence fruit set and provide a list of candidates, paving the way for genetic manipulation aimed at model research and plant breeding.

Brassinosteroids are involved in controlling sugar unloading in Vitis vinifera 'Cabernet Sauvignon' berries during véraison

Sugar unloading in grape berries is a crucial step in the long-distance transport of carbohydrates from grapevine leaves to berries. Brassinosteroids (BRs) mediate many physiological processes in plants including carbohydrate metabolism. Here, 'Cabernet Sauvignon' (Vitis vinifera L.) grape berries cultivated in clay loam fields were treated with an exogenous BR (24-epibrassinolide; EBR), a BR synthesis inhibitor (brassinazole; Brz), Brz + EBR (sprayed with EBR 24 h after a Brz treatment), and deionized water (control) at the onset of véraison. The EBR treatment sharply increased the soluble sugars content in the berries, but decreased it in the skins. The EBR and Brz + EBR treatments significantly promoted the activities of both invertases (acidic and neutral) and sucrose synthase (sucrolytic) at various stages of ripening. The mRNA levels of genes encoding sucrose metabolic invertase (VvcwINV), and monosaccharide (VvHT3, 4, 5 and 6) and disaccharide (VvSUC12 and 27) transporters were increased by the EBR and/or Brz + EBR treatments. Generally, the effects of the Brz treatment on the measured targets contrasted with the effects of the EBR treatments. The EBR and Brz treatments inhibited the biosynthesis of the endogenous BRs 6-deoxocastastarone and castasterone. Both EBR and Brz + EBR treatments increased the brassinolide contents, down-regulated the expression of genes encoding BRs biosynthetic enzymes BRASSINOSTEROID-6-OXIDASE and DWARF1, (VvBR6OX1 and VvDWF1) and induced BR receptor gene BRASSINOSTEROID INSENSITIVE 1 (VvBRI1) expression in deseeded berries. Together, these results show that BRs are involved in controlling sugar unloading in grape berries during véraison.

The involvement of hexokinase in the coordinated regulation of glucose and gibberellin on cell wall invertase and sucrose synthesis in grape berry

In plants, hexokinase (HXK, EC 2.7.1.1), an enzyme normally involved in hexose phosphorylation, plays an important role in sugar sensing and signaling. The hexokinase activity of grape HXKs was confirmed by functional complementation of the hexokinase-deficient yeast strain YSH7.4-3C (hxk1, hxk2, glk1). HXK1 and HXK2 were able to complement this mutant. The subcellular localization of HXK1 and HXK2, observed with green fluorescent protein fusion constructs, indicated that HXK1 localized to the cytosol while HXK2 was a nuclear-targeted hexokinase. Gibberellin (GA3) control various processes across plant life and has been involved in sugar accumulation. The coordinated regulation of exogenous GA3 with Glc on CWINV, SuSy1, or SuSy2 expressions indicated that GA3 can relieve the repression of Glc on CWINV or SuSy1 expression, and the repression of GA3 on SuSy2 expression overrides the Glc-inductive effect, resulting in the down-regulation of SuSy2 expression. It was concluded that GA3 negatively interfere with Glc signal transduction depending on hexokinase phosphorylation. GA3 might regulate CWINV, SuSy1 or SuSy2 expression to in order to maintain an intracellular sugar levels and normal cell metabolism. Our results provide new insights into the crosstalk mechanism of GA3 and Glc signaling depending on hexokinase in grape berry sugar accumulation.