Overexpression of a proton-coupled vacuolar glucose exporter impairs freezing tolerance and seed germination

Comprehensive identification and expression analyses of sugar transporter genes reveal the role of GmSTP22 in salt stress resistance in soybean

The transport, compartmentation and allocation of sugar are critical for plant growth and development, as well as for stress resistance, but sugar transporter genes have not been comprehensively characterized in soybean. Here, we performed a genome-wide identification and expression analyses of sugar transporter genes in soybean in order to reveal their putative functions. A total of 122 genes encoding sucrose transporters (SUTs) and monosaccharide transporters (MSTs) were identified in soybean. They were classified into 8 subfamilies according to their phylogenetic relationships and their conserved motifs. Comparative genomics analysis indicated that whole genome duplication/segmental duplication and tandem duplication contributed to the expansion of sugar transporter genes in soybean. Expression analysis by retrieving transcriptome datasets suggested that most of these sugar transporter genes were expressed in various tissues, and a number of genes exhibited tissue-specific expression patterns. Several genes including GmSTP21, GmSFP8, and GmPLT5/6/7/8/9 were predominantly expressed in nodules, and GmPLT8 was significantly induced by rhizobia inoculation in root hairs. Transcript profiling and qRT-PCR analyses suggested that half of these sugar transporter genes were significantly induced or repressed under stresses like salt, drought, and cold. In addition, GmSTP22 was found to be localized in the plasma membrane, and its overexpression promoted plant growth and salt tolerance in transgenic Arabidopsis under the supplement with glucose or sucrose. This study provides insights into the evolutionary expansion, expression pattern and functional divergence of sugar transporter gene family, and will enable further understanding of their biological functions in the regulation of growth, yield formation and stress resistance of soybean.

The transcription factor ERF110 promotes cold tolerance by directly regulating sugar and sterol biosynthesis in citrus

ERFs (ethylene-responsive factors) are known to play a key role in orchestrating cold stress signal transduction. However, the regulatory mechanisms and target genes of most ERFs are far from being well deciphered. In this study, we identified a cold-induced ERF, designated as PtrERF110, from trifoliate orange (Poncirus trifoliata L. Raf., also known as Citrus trifoliata L.), an elite cold-hardy plant. PtrERF110 is a nuclear protein with transcriptional activation activity. Overexpression of PtrERF110 remarkably enhanced cold tolerance in lemon (Citrus limon) and tobacco (Nicotiana tabacum), whereas VIGS (virus-induced gene silencing)-mediated knockdown of PtrERF110 drastically impaired the cold tolerance. RNA sequence analysis revealed that PtrERF110 overexpression resulted in global transcriptional reprogramming of a range of stress-responsive genes. Three of the genes, including PtrERD6L16 (early responsive dehydration 6-like transporters), PtrSPS4 (sucrose phosphate synthase 4), and PtrUGT80B1 (UDP-glucose: sterol glycosyltransferases 80B1), were confirmed as direct targets of PtrERF110. Consistently, PtrERF110-overexpressing plants exhibited higher levels of sugars and sterols compared to their wild type counterparts, whereas the VIGS plants had an opposite trend. Exogenous supply of sucrose restored the cold tolerance of PtrERF110-silencing plants. In addition, knockdown of PtrSPS4, PtrERD6L16, and PtrUGT80B1 substantially impaired the cold tolerance of P. trifoliata. Taken together, our findings indicate that PtrERF110 positively modulates cold tolerance by directly regulating sugar and sterol synthesis through transcriptionally activating PtrERD6L16, PtrSPS4, and PtrUGT80B1. The regulatory modules (ERF110-ERD6L16/SPS4/UGT80B1) unraveled in this study advance our understanding of the molecular mechanisms underlying sugar and sterol accumulation in plants subjected to cold stress.

Grapevine gray mold disease: infection, defense and management

Grapevine (Vitis vinifera L.,) is among the world's leading fruit crops. The production of grapes is severely affected by many diseases including gray mold, caused by the necrotrophic fungus Botrytis cinerea. Although all Vitis species can be hosts for B. cinerea, V. vinifera are particularly susceptible. Accordingly, this disease poses a significant threat to the grape industry and causes substantial economic losses. Development of resistant V. vinifera cultivars has progressed from incidental selection by farmers, to targeted selection through the use of statistics and experimental design, to the employment of genetic and genomic data. Emerging technologies such as marker-assisted selection and genetic engineering have facilitated the development of cultivars that possess resistance to B. cinerea. A promising method involves using the CRISPR/Cas9 system to induce targeted mutagenesis and develop genetically modified non-transgenic crops. Hence, scientists are now engaged in the active pursuit of identifying genes associated with susceptibility and resistance. This review focuses on the known mechanisms of interaction between the B. cinerea pathogen and its grapevine host. It also explores innate immune systems that have evolved in V. vinifera, with the objective of facilitating the rapid development of resistant grapevine cultivars.

Carbohydrate distribution via SWEET17 is critical for Arabidopsis inflorescence branching under drought

Sugars Will Eventually be Exported Transporters (SWEETs) are the most recently discovered family of plant sugar transporters. By acting as uniporters, SWEETs facilitate the diffusion of sugars across cell membranes and play an important role in various physiological processes such as abiotic stress adaptation. AtSWEET17, a vacuolar fructose facilitator, was shown to be involved in the modulation of the root system during drought. In addition, previous studies have shown that overexpression of an apple homolog leads to increased drought tolerance in tomato plants. Therefore, SWEET17 might be a molecular element involved in plant responses to drought. However, the role and function of SWEET17 in above-ground tissues of Arabidopsis under drought stress remain elusive. By combining gene expression analysis and stem architecture with the sugar profiles of different above-ground tissues, we uncovered a putative role for SWEET17 in carbohydrate supply and thus cauline branch elongation, especially during periods of carbon limitation, as occurs under drought stress. Thus, SWEET17 seems to be involved in maintaining efficient plant reproduction under drought stress conditions.

Sugar beet PMT5a and STP13 carriers suitable for proton-driven plasma membrane sucrose and glucose import in taproots

Sugar beet (Beta vulgaris) is the major sugar-producing crop in Europe and Northern America, as the taproot stores sucrose at a concentration of around 20%. Genome sequence analysis together with biochemical and electrophysiological approaches led to the identification and characterization of the TST sucrose transporter driving vacuolar sugar accumulation in the taproot. However, the sugar transporters mediating sucrose uptake across the plasma membrane of taproot parenchyma cells remained unknown. As with glucose, sucrose stimulation of taproot parenchyma cells caused inward proton fluxes and plasma membrane depolarization, indicating a sugar/proton symport mechanism. To decipher the nature of the corresponding proton-driven sugar transporters, we performed taproot transcriptomic profiling and identified the cold-induced PMT5a and STP13 transporters. When expressed in Xenopus laevis oocytes, BvPMT5a was characterized as a voltage- and H+-driven low-affinity glucose transporter, which does not transport sucrose. In contrast, BvSTP13 operated as a high-affinity H+/sugar symporter, transporting glucose better than sucrose, and being more cold-tolerant than BvPMT5a. Modeling of the BvSTP13 structure with bound mono- and disaccharides suggests plasticity of the binding cleft to accommodate the different saccharides. The identification of BvPMT5a and BvSTP13 as taproot sugar transporters could improve breeding of sugar beet to provide a sustainable energy crop.

The MdCBF1/2-MdTST1/2 module regulates sugar accumulation in response to low temperature in apple

Soluble sugar content is a key component in controlling fruit flavor, and its accumulation in fruit is largely determined by sugar metabolism and transportation. When the diurnal temperature range is greater, the fleshy fruits accumulated more soluble sugars and become more sweeter. However, the molecular mechanism underlying this response remains largely unknown. In this study, we verified that low-temperature treatment promoted soluble sugar accumulation in apple fruit and found that this was due to the upregulation of the Tonoplast Sugar Transporter genes MdTST1/2. A combined strategy using assay for transposase-accessible chromatin (ATAC) sequencing and gene expression and cis-acting elements analyses, we identified two C-repeat Binding Factors, MdCBF1 and MdCBF2, that were induced by low temperature and that might be upstream transcription factors of MdTST1/2. Further studies established that MdCBF1/2 could bind to the promoters of MdTST1/2 and activate their expression. Overexpression of MdCBF1 or MdCBF2 in apple calli and fruit significantly upregulated MdTST1/2 expression and increased the concentrations of glucose, fructose, and sucrose. Suppression of MdTST1 and/or MdTST2 in an MdCBF1/2-overexpression background abolished the positive effect of MdCBF1/2 on sugar accumulation. In addition, simultaneous silencing of MdCBF1/2 downregulated MdTST1/2 expression and apple fruits failed to accumulate more sugars under low-temperature conditions, indicating that MdCBF1/2-mediated sugar accumulation was dependent on MdTST1/2 expression. Hence, we concluded that the MdCBF1/2-MdTST1/2 module is crucial for sugar accumulation in apples in response to low temperatures. Our findings provide mechanistic components coordinating the relationship between low temperature and sugar accumulation as well as new avenues to improve fruit quality.

Types of Membrane Transporters and the Mechanisms of Interaction between Them and Reactive Oxygen Species in Plants

Membrane transporters are proteins that mediate the entry and exit of substances through the plasma membrane and organellar membranes and are capable of recognizing and binding to specific substances, thereby facilitating substance transport. Membrane transporters are divided into different types, e.g., ion transporters, sugar transporters, amino acid transporters, and aquaporins, based on the substances they transport. These membrane transporters inhibit reactive oxygen species (ROS) generation through ion regulation, sugar and amino acid transport, hormone induction, and other mechanisms. They can also promote enzymatic and nonenzymatic reactions in plants, activate antioxidant enzyme activity, and promote ROS scavenging. Moreover, membrane transporters can transport plant growth regulators, solute proteins, redox potential regulators, and other substances involved in ROS metabolism through corresponding metabolic pathways, ultimately achieving ROS homeostasis in plants. In turn, ROS, as signaling molecules, can affect the activity of membrane transporters under abiotic stress through collaboration with ions and involvement in hormone metabolic pathways. The research described in this review provides a theoretical basis for improving plant stress resistance, promoting plant growth and development, and breeding high-quality plant varieties.

Superior glucose metabolism supports NH4+ assimilation in wheat to improve ammonium tolerance

The use of slow-release fertilizers and seed-fertilizers cause localized high-ammonium (NH4 +) environments in agricultural fields, adversely affecting wheat growth and development and delaying its yield. Thus, it is important to investigate the physiological responses of wheat and its tolerance to NH4 + stress to improve the adaptation of wheat to high NH4 + environments. In this study, the physiological mechanisms of ammonium tolerance in wheat (Triticum aestivum) were investigated in depth by comparative analysis of two cultivars: NH4 +-tolerant Xumai25 and NH4 +-sensitive Yangmai20. Cultivation under hydroponic conditions with high NH4 + (5 mM NH4 +, AN) and nitrate (5 mM NO3 -, NN), as control, provided insights into the nuanced responses of both cultivars. Compared to Yangmai20, Xumai25 displayed a comparatively lesser sensitivity to NH4 + stress, as evident by a less pronounced reduction in dry plant biomass and a milder adverse impact on root morphology. Despite similarities in NH4 + efflux and the expression levels of TaAMT1.1 and TaAMT1.2 between the two cultivars, Xumai25 exhibited higher NH4 + influx, while maintaining a lower free NH4 + concentration in the roots. Furthermore, Xumai25 showed a more pronounced increase in the levels of free amino acids, including asparagine, glutamine, and aspartate, suggesting a superior NH4 + assimilation capacity under NH4 + stress compared to Yangmai20. Additionally, the enhanced transcriptional regulation of vacuolar glucose transporter and glucose metabolism under NH4 + stress in Xumai25 contributed to an enhanced carbon skeleton supply, particularly of 2-oxoglutarate and pyruvate. Taken together, our results demonstrate that the NH4 + tolerance of Xumai25 is intricately linked to enhanced glucose metabolism and optimized glucose transport, which contributes to the robust NH4 + assimilation capacity.

Genome-wide analysis of sugar transporter genes in maize (Zea mays L.): identification, characterization and their expression profiles during kernel development

Sugar transporters (STs) play a crucial role in the development of maize kernels. However, very limited information about STs in maize is known. In this study, sixty-eight ZmST genes were identified from the maize genome and classified into eight major groups based on phylogenetic relationship. Gene structure analysis revealed that members within the same group shared similar exon numbers. Synteny analysis indicated that ZmSTs underwent 15 segmental duplication events under purifying selection. Three-dimensional structure of ZmSTs demonstrated the formation of a compact helix bundle composed of 8-13 trans-membrane domains. Various development-related cis-acting elements, enriched in promoter regions, were correlated with the transcriptional response of ZmSTs during kernel development. Transcriptional expression profiles exhibited expression diversity of various ZmST genes in roots, stems, leaves, tassels, cobs, embryos, endosperms and seeds tissues. During kernel development, the expression of 24 ZmST genes was significantly upregulated in the early stage of grain filling. This upregulation coincided with the sharply increased grain-filling rate observed in the early stage. Overall, our findings shed light on the characteristics of ZmST genes in maize and provide a foundation for further functional studies.

Expression of the plastocyanin gene PETE2 in Camelina sativa improves seed yield and salt tolerance

Plastocyanin functions as an electron carrier in the photosynthetic electron transport chain, located at the thylakoid membrane. In several species, endogenous plastocyanin levels are correlated with the photosynthetic electron transport rate. Overexpression of plastocyanin genes in Arabidopsis thaliana increases plant size, but this phenomenon has not been observed in crop species. Here, we investigated the effects of heterologous expression of a gene encoding a plastocyanin isoform from Arabidopsis, AtPETE2, in the oil seed crop Camelina sativa under standard growth conditions and under salt stress. AtPETE2 heterologous expression enhanced photosynthetic activity in Camelina, accelerating plant development and improving seed yield under standard growth conditions. Additionally, CsPETE2 from Camelina was induced by salt stress and AtPETE2 expression lines had larger primary roots and more lateral roots than the wild type. AtPETE2 expression lines also had larger seeds and higher total seed yield under long-term salt stress compared with non-transgenic Camelina. Our results demonstrate that increased plastocyanin levels in Camelina can enhance photosynthesis and productivity, as well as tolerance to osmotic and salt stresses. Heterologous expression of plastocyanin may be a useful strategy to mitigate crop stress in saline soils.

Vacuolar Sugar Transporter TMT2 Plays Crucial Roles in Germination and Seedling Development in Arabidopsis

Vacuolar sugar transporters transport sugar across the tonoplast, are major players in maintaining sugar homeostasis, and therefore play vital roles in plant growth, development, and biomass yield. In this study, we analyzed the physiological roles of the tonoplast monosaccharide transporter 2 (TMT2) in Arabidopsis. In contrast to the wild type (WT) that produced uniform seedlings, the tmt2 mutant produced three types of offspring: un-germinated seeds (UnG), seedlings that cannot form true leaves (tmt2-S), and seedlings that develop normally (tmt2-L). Sucrose, glucose, and fructose can substantially, but not completely, rescue the abnormal phenotypes of the tmt2 mutant. Abnormal cotyledon development, arrested true leaf development, and abnormal development of shoot apical meristem (SAM) were observed in tmt2-S seedlings. Cotyledons from the WT and tmt2-L seedlings restored the growth of tmt2-S seedlings through micrografting. Moreover, exogenous sugar sustained normal growth of tmt2-S seedlings with cotyledon removed. Finally, we found that the TMT2 deficiency resulted in growth defects, most likely via changing auxin signaling, target of rapamycin (TOR) pathways, and cellular nutrients. This study unveiled the essential functions of TMT2 for seed germination and initial seedling development, ensuring cotyledon function and mobilizing sugars from cotyledons to seedlings. It also expanded the current knowledge on sugar metabolism and signaling. These findings have fundamental implications for enhancing plant biomass production or seed yield in future agriculture.

Vacuolar sugar transporter EARLY RESPONSE TO DEHYDRATION6-LIKE4 affects fructose signaling and plant growth

Regulation of intracellular sugar homeostasis is maintained by regulation of activities of sugar import and export proteins residing at the tonoplast. We show here that the EARLY RESPONSE TO DEHYDRATION6-LIKE4 (ERDL4) protein, a member of the monosaccharide transporter family, resides in the vacuolar membrane in Arabidopsis (Arabidopsis thaliana). Gene expression and subcellular fractionation studies indicated that ERDL4 participates in fructose allocation across the tonoplast. Overexpression of ERDL4 increased total sugar levels in leaves due to a concomitantly induced stimulation of TONOPLAST SUGAR TRANSPORTER 2 (TST2) expression, coding for the major vacuolar sugar loader. This conclusion is supported by the finding that tst1-2 knockout lines overexpressing ERDL4 lack increased cellular sugar levels. ERDL4 activity contributing to the coordination of cellular sugar homeostasis is also indicated by 2 further observations. First, ERDL4 and TST genes exhibit an opposite regulation during a diurnal rhythm, and second, the ERDL4 gene is markedly expressed during cold acclimation, representing a situation in which TST activity needs to be upregulated. Moreover, ERDL4-overexpressing plants show larger rosettes and roots, a delayed flowering time, and increased total seed yield. Consistently, erdl4 knockout plants show impaired cold acclimation and freezing tolerance along with reduced plant biomass. In summary, we show that modification of cytosolic fructose levels influences plant organ development and stress tolerance.

Carbohydrate dynamics in Populus trees under drought: An expression atlas of genes related to sensing, translocation, and metabolism across organs

In trees, nonstructural carbohydrates (NSCs) serve as long-term carbon storage and long-distance carbon transport from source to sink. NSC management in response to drought stress is key to our understanding of drought acclimation. However, the molecular mechanisms underlying these processes remain unclear. By combining a transcriptomic approach with NSC quantification in the leaves, stems, and roots of Populus alba under drought stress, we analyzed genes from 29 gene families related to NSC signaling, translocation, and metabolism. We found starch depletion across organs and accumulation of soluble sugars (SS) in the leaves. Activation of the trehalose-6-phosphate/SNF1-related protein kinase (SnRK1) signaling pathway across organs via the suppression of class I TREHALOSE-PHOSPHATE SYNTHASE (TPS) and the expression of class II TPS genes suggested an active response to drought. The expression of SnRK1α and β subunits, and SUCROSE SYNTHASE6 supported SS accumulation in leaves. The upregulation of active transporters and the downregulation of most passive transporters implied a shift toward active sugar transport and enhanced regulation over partitioning. SS accumulation in vacuoles supports osmoregulation in leaves. The increased expression of sucrose synthesis genes and reduced expression of sucrose degradation genes in the roots did not coincide with sucrose levels, implying local sucrose production for energy. Moreover, the downregulation of invertases in the roots suggests limited sucrose allocation from the aboveground organs. This study provides an expression atlas of NSC-related genes that respond to drought in poplar trees, and can be tested in tree improvement programs for adaptation to drought conditions.

The novel chloroplast glucose transporter pGlcT2 affects adaptation to extended light periods

Intracellular sugar compartmentation is critical in plant development and acclimation to challenging environmental conditions. Sugar transport proteins are present in plasma membranes and in membranes of organelles such as vacuoles, the Golgi apparatus, and plastids. However, there may exist other transport proteins with uncharacterized roles in sugar compartmentation. Here we report one such, a novel transporter of the Monosaccharide Transporter Family (MSF), the closest phylogenetic homolog of which is the chloroplast-localized glucose transporter pGlcT and that we therefore term plastidic glucose transporter 2 (pGlcT2). We show, using gene-complemented glucose uptake deficiency of an Escherichia coli ptsG/manXYZ mutant strain and biochemical characterization, that this protein specifically facilitates glucose transport, whereas other sugars do not serve as substrates. In addition, we demonstrate pGlcT2-GFP localized to the chloroplast envelope, and that pGlcT2 is mainly produced in seedlings and in the rosette center of mature Arabidopsis plants. Therefore, in conjunction with molecular and metabolic data, we propose pGlcT2 acts as a glucose importer that can limit cytosolic glucose availability in developing pGlcT2-overexpressing seedlings. Finally, we show both overexpression and deletion of pGlcT2 resulted in impaired growth efficiency under long day and continuous light conditions, suggesting pGlcT2 contributes to a release of glucose derived from starch mobilization late in the light phase. Together, these data indicate the facilitator pGlcT2 changes the direction in which it transports glucose during plant development and suggest the activity of pGlcT2 must be controlled spatially and temporarily in order to prevent developmental defects during adaptation to periods of extended light.

Transcriptome analysis of the pulp of citrus fruitlets suggests that domestication enhanced growth processes and reduced chemical defenses increasing palatability

To identify key traits brought about by citrus domestication, we have analyzed the transcriptomes of the pulp of developing fruitlets of inedible wild Ichang papeda (Citrus ichangensis), acidic Sun Chu Sha Kat mandarin (C. reticulata) and three palatable segregants of a cross between commercial Clementine (C. x clementina) and W. Murcott (C. x reticulata) mandarins, two pummelo/mandarin admixtures of worldwide distribution. RNA-seq comparison between the wild citrus and the ancestral sour mandarin identified 7267 differentially expressed genes, out of which 2342 were mapped to 117 KEGG pathways. From the remaining genes, a set of 2832 genes was functionally annotated and grouped into 45 user-defined categories. The data suggest that domestication promoted fundamental growth processes to the detriment of the production of chemical defenses, namely, alkaloids, terpenoids, phenylpropanoids, flavonoids, glucosinolates and cyanogenic glucosides. In the papeda, the generation of energy to support a more active secondary metabolism appears to be dependent upon upregulation of glycolysis, fatty acid degradation, Calvin cycle, oxidative phosphorylation, and ATP-citrate lyase and GABA pathways. In the acidic mandarin, downregulation of cytosolic citrate degradation was concomitant with vacuolar citrate accumulation. These changes affected nitrogen and carbon allocation in both species leading to major differences in organoleptic properties since the reduction of unpleasant secondary metabolites increases palatability while acidity reduces acceptability. The comparison between the segregants and the acidic mandarin identified 357 transcripts characterized by the occurrence in the three segregants of additional downregulation of secondary metabolites and basic structural cell wall components. The segregants also showed upregulation of genes involved in the synthesis of methyl anthranilate and furaneol, key substances of pleasant fruity aroma and flavor, and of sugar transporters relevant for sugar accumulation. Transcriptome and qPCR analysis in developing and ripe fruit of a set of genes previously associated with citric acid accumulation, demonstrated that lower acidity is linked to downregulation of these regulatory genes in the segregants. The results suggest that the transition of inedible papeda to sour mandarin implicated drastic gene expression reprograming of pivotal pathways of the primary and secondary metabolism, while palatable mandarins evolved through progressive refining of palatability properties, especially acidity.

Genome-wide identification and expression profiling of sugar transporter genes in tobacco

Sugars are both nutrients and important signal molecules in higher plants. Sugar transporters (STs) are involved in sugar loading and unloading and facilitate sugar transport across membranes. Tobacco (Nicotiana tabacum) is a model plant and one of the most significant plants economically. In our research, 92 N. tabacum ST (NtST) genes were identified and classified into eight distinct subfamilies in the tobacco genome based on phylogenetic analysis. Exon-intron analysis revealed that each subfamily manifested closely associated gene architectural features based on a comparable number or length of exons. Tandem repetition and purifying selection were the main factors of NtST gene evolution. A search for cis-regulatory elements in the promoter sequences of the NtST gene families suggested that they are probably regulated by light, plant hormones, and abiotic stress factors. We performed a comprehensive expression study in different tissues, viarious abiotic and phytohormone stresses. The results revealed different expression patterns and the functional diversification of NtST genes. The resulting data showed that NtSFP1 was highly expressed all measured five tobacco tissues, and also regulated by the MeJA, and temperature stress. In addition, the virus-induced NibenSFP1 silencing in tobacco and detected dramatically enhanced glucose content, indicating the NtSFP1 might regulate the glucose content and involved in MeJA signaling way to response the temperature stress. In general, our findings provide useful information on understanding the roles of STs in phytohormone signaling way and abiotic stresses in N. tabacum.

Overexpression of the vacuolar sugar importer BvTST1 from sugar beet in Camelina improves seed properties and leads to altered root characteristics

Overexpression of the vacuolar sugar transporter TST1 in Arabidopsis leads to higher seed lipid levels and higher total seed yield per plant. However, effects on fruit biomass have not been observed in crop plants like melon, strawberry, cotton, apple, or tomato with increased tonoplast sugar transporter (TST) activity. Thus, it was unclear whether overexpression of TST in selected crops might lead to increased fruit yield, as observed in Arabidopsis. Here, we report that constitutive overexpression of TST1 from sugar beet in the important crop species Camelina sativa (false flax) resembles the seed characteristics observed for Arabidopsis upon increased TST activity. These effects go along with a stimulation of sugar export from source leaves and not only provoke optimised seed properties like higher lipid levels and increased overall seed yield per plant, but also modify the root architecture of BvTST1 overexpressing Camelina lines. Such mutants grew longer primary roots and showed an increased number of lateral roots, especially when developed under conditions of limited water supply. These changes in root properties result in a stabilisation of total seed yield under drought conditions. In summary, we demonstrate that increased vacuolar TST activity may lead to optimised yield of an oil-seed crop species with high levels of healthy ω3 fatty acids in storage lipids. Moreover, since BvTST1 overexpressing Camelina mutants, in addition, exhibit optimised yield under limited water availability, we might devise a strategy to create crops with improved tolerance against drought, representing one of the most challenging environmental cues today and in future.

A vacuolar hexose transport is required for xylem development in the inflorescence stem

In Angiosperms, the development of the vascular system is controlled by a complex network of transcription factors. However, how nutrient availability in the vascular cells affects their development remains to be addressed. At the cellular level, cytosolic sugar availability is regulated mainly by sugar exchanges at the tonoplast through active and/or facilitated transport. In Arabidopsis (Arabidopsis thaliana), among the genes encoding tonoplastic transporters, SUGAR WILL EVENTUALLY BE EXPORTED TRANSPORTER 16 (SWEET16) and SWEET17 expression has been previously detected in the vascular system. Here, using a reverse genetics approach, we propose that sugar exchanges at the tonoplast, regulated by SWEET16, are important for xylem cell division as revealed in particular by the decreased number of xylem cells in the swt16 mutant and the accumulation of SWEET16 at the procambium-xylem boundary. In addition, we demonstrate that transport of hexoses mediated by SWEET16 and/or SWEET17 is required to sustain the formation of the xylem secondary cell wall. This result is in line with a defect in the xylem cell wall composition as measured by Fourier-transformed infrared spectroscopy in the swt16swt17 double mutant and by upregulation of several genes involved in secondary cell wall synthesis. Our work therefore supports a model in which xylem development partially depends on the exchange of hexoses at the tonoplast of xylem-forming cells.

An Insight into the Abiotic Stress Responses of Cultivated Beets (Beta vulgaris L.)

Cultivated beets (sugar beets, fodder beets, leaf beets, and garden beets) belonging to the species Beta vulgaris L. are important sources for many products such as sugar, bioethanol, animal feed, human nutrition, pulp residue, pectin extract, and molasses. Beta maritima L. (sea beet or wild beet) is a halophytic wild ancestor of all cultivated beets. With a requirement of less water and having shorter growth period than sugarcane, cultivated beets are preferentially spreading from temperate regions to subtropical countries. The beet cultivars display tolerance to several abiotic stresses such as salt, drought, cold, heat, and heavy metals. However, many environmental factors adversely influence growth, yield, and quality of beets. Hence, selection of stress-tolerant beet varieties and knowledge on the response mechanisms of beet cultivars to different abiotic stress factors are most required. The present review discusses morpho-physiological, biochemical, and molecular responses of cultivated beets (B. vulgaris L.) to different abiotic stresses including alkaline, cold, heat, heavy metals, and UV radiation. Additionally, we describe the beet genes reported for their involvement in response to these stress conditions.

Vacuolar fructose transporter SWEET17 is critical for root development and drought tolerance

Root growth and architecture are markedly influenced by both developmental and environmental cues. Sugars integrate different stimuli and are essential building blocks and signaling molecules for modulating the root system. Members from the SUGAR WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET) family facilitate the transport of different sugars over cellular membranes and steer both inter and intracellular distribution of sugars. SWEET17 represents a fructose-specific sugar porter localized to the vacuolar membrane, the tonoplast. Here, we analyzed how SWEET17-dependent fructose released from vacuoles affects root growth during drought stress in Arabidopsis (Arabidopsis thaliana). We found that the SWEET17 gene was predominantly expressed in the root vasculature and in meristematic cells of the root tip. SWEET17 expression appeared markedly induced during lateral root (LR) outgrowth and under drought. Moreover, fructose repressed primary root growth but induced density and length of first order LRs. Consistently, sweet17 knock-out mutants exhibited reduced LR growth and a diminished expression of LR-development-related transcription factors during drought stress, resulting in impaired drought tolerance of sweet17 mutants. We discuss how SWEET17 activity integrates drought-induced cellular responses into fructose signaling necessary for modulation of the root system and maximal drought tolerance.

Responsiveness of Early Response to Dehydration Six-Like Transporter Genes to Water Deficit in Arabidopsis thaliana Leaves

Drought is one of the main abiotic stresses, which affects plant growth, development, and crop yield. Plant response to drought implies carbon allocation to sink organs and sugar partitioning between different cell compartments, and thereby requires the involvement of sugar transporters (SUTs). Among them, the early response to dehydration six-like (ESL), with 19 members in Arabidopsis thaliana, form the largest subfamily of monosaccharide transporters (MSTs) still poorly characterized. A common feature of these genes is their involvement in plant response to abiotic stresses, including water deficit. In this context, we carried out morphological and physiological phenotyping of A. thaliana plants grown under well-watered (WW) and water-deprived (WD) conditions, together with the expression profiling of 17 AtESL genes in rosette leaves. The drought responsiveness of 12 ESL genes, 4 upregulated and 8 downregulated, was correlated to different water statuses of rosette leaves. The differential expression of each of the tandem duplicated AtESL genes in response to water stress is in favor of their plausible functional diversity. Furthermore, transfer DNA (T-DNA) insertional mutants for each of the four upregulated ESLs in response to water deprivation were identified and characterized under WW and WD conditions. To gain insights into global sugar exchanges between vacuole and cytosol under water deficit, the gene expression of other vacuolar SUTs and invertases (AtTMT, AtSUC, AtSWEET, and AtβFRUCT) was analyzed and discussed.

Phytochromes control metabolic flux, and their action at the seedling stage determines adult plant biomass

Phytochrome photoreceptors are known to regulate plastic growth responses to vegetation shade. However, recent reports also suggest an important role for phytochromes in carbon resource management, metabolism, and growth. Here, we use 13CO2 labelling patterns in multiallele phy mutants to investigate the role of phytochrome in the control of metabolic fluxes. We also combine quantitative data of 13C incorporation into protein and cell wall polymers, gas exchange measurements, and system modelling to investigate why biomass is decreased in adult multiallele phy mutants. Phytochrome influences the synthesis of stress metabolites such as raffinose and proline, and the accumulation of sugars, possibly through regulating vacuolar sugar transport. Remarkably, despite their modified metabolism and vastly altered architecture, growth rates in adult phy mutants resemble those of wild-type plants. Our results point to delayed seedling growth and smaller cotyledon size as the cause of the adult-stage phy mutant biomass defect. Our data signify a role for phytochrome in metabolic stress physiology and carbon partitioning, and illustrate that phytochrome action at the seedling stage sets the trajectory for adult biomass production.

Plant sugars: Homeostasis and transport under abiotic stress in plants

The sessile nature of plants' life is endowed with a highly evolved defense system to adapt and survive under environmental extremes. To combat such stresses, plants have developed complex and well-coordinated molecular and metabolic networks encompassing genes, metabolites, and acclimation responses. These modulate growth, photosynthesis, osmotic maintenance, and carbohydrate homeostasis. Under a given stress condition, sugars act as key players in stress perception, signaling, and are a regulatory hub for stress-mediated gene expression ensuring responses of osmotic adjustment, scavenging of reactive oxygen species, and maintaining the cellular energy status through carbon partitioning. Several sugar transporters are known to regulate carbohydrate partitioning and key signal transduction steps involved in the perception of biotic and abiotic stresses. Sugar transporters such as SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEETs), SUCROSE TRANSPORTERS (SUTs), and MONOSACCHARIDE TRANSPORTERS (MSTs) are involved in sugar loading and unloading as well as long-distance transport (source to sink) besides orchestrating oxidative and osmotic stress tolerance. It is thus necessary to understand the structure-function relationship of these sugar transporters to fine-tune the abiotic stress-modulated responses. Advances in genomics have unraveled many sugars signaling components playing a key role in cross-talk in abiotic stress pathways. An integrated omics approach may aid in the identification and characterization of sugar transporters that could become targets for developing stress tolerance plants to mitigate climate change effects and improve crop yield. In this review, we have presented an up-to-date analysis of the sugar homeostasis under abiotic stresses as well as describe the structure and functions of sugar transporters under abiotic stresses.

Improved resource allocation and stabilization of yield under abiotic stress

Sugars are the main building blocks for carbohydrate storage, but also serve as signaling molecules and protective compounds during abiotic stress responses. Accordingly, sugar transport proteins fulfill multiple roles as they mediate long distance sugar allocation, but also shape the subcellular and tissue-specific carbohydrate profiles by balancing the levels of these molecules in various compartments. Accordingly, transporter activity represents a target by classical or directed breeding approaches, to either, directly increase phloem loading or to increase sink strength in crop species. The relative subcellular distribution of sugars is critical for molecular signaling affecting yield-relevant processes like photosynthesis, onset of flowering and stress responses, while controlled long-distance sugar transport directly impacts development and productivity of plants. However, long-distance transport is prone to become unbalanced upon adverse environmental conditions. Therefore, we highlight the influence of stress stimuli on sucrose transport in the phloem and include the role of stress induced cellular carbohydrate sinks, like raffinose or fructans, which possess important roles to build up tolerance against challenging environmental conditions. In addition, we report on recent breeding approaches that resulted in altered source and sink capacities, leading to increased phloem sucrose shuttling in crops. Finally, we present strategies integrating the need of cellular stress-protection into the general picture of long-distance transport under abiotic stress, and point to possible approaches improving plant performance and resource allocation under adverse environmental conditions, leading to stabilized or even increased crop yield.

Evolutionary expansion and functional divergence of sugar transporters in Saccharum (S. spontaneum and S. officinarum)

The sugar transporter (ST) family is considered to be the most important gene family for sugar accumulation, but limited information about the ST family in the important sugar-yielding crop Saccharum is available due to its complex genetic background. Here, 105 ST genes were identified and clustered into eight subfamilies in Saccharum spontaneum. Comparative genomics revealed that tandem duplication events contributed to ST gene expansions of two subfamilies, PLT and STP, in S. spontaneum, indicating an early evolutionary step towards high sugar content in Saccharum. The analyses of expression patterns were based on four large datasets with a total of 226 RNA sequencing samples from S. spontaneum and Saccharum officinarum. The results clearly demonstrated 50 ST genes had different spatiotemporal expression patterns in leaf tissues, 10 STs were specifically expressed in the stem, and 10 STs responded to the diurnal rhythm. Heterologous expression experiments in the defective yeast strain EBY.VW4000 indicated STP13, pGlcT2, VGT3, and TMT4 are the STs with most affinity for glucose/fructose and SUT1_T1 has the highest affinity to sucrose. Furthermore, metabolomics analysis suggested STP7 is a sugar starvation-induced gene and STP13 has a function in retrieving sugar in senescent tissues. PLT11, PLT11_T1, TMT3, and TMT4 contributed to breaking the limitations of the storage sink. SUT1, SUT1_T1, PLT11, TMT4, pGlcT2, and VGT3 responded for different functions in these two Saccharum species. This study demonstrated the evolutionary expansion and functional divergence of the ST gene family and will enable the further investigation of the molecular mechanism of sugar metabolism in Saccharum.

Early Response to Dehydration Six-Like Transporter Family: Early Origin in Streptophytes and Evolution in Land Plants

Carbon management by plants involves the activity of many sugar transporters, which play roles in sugar subcellular partitioning and reallocation at the whole organism scale. Among these transporters, the early response to dehydration six-like (ESL) monosaccharide transporters (MSTs) are still poorly characterized although they represent one of the largest sugar transporter subfamilies. In this study, we used an evolutionary genomic approach to infer the evolutionary history of this multigenic family. No ESL could be identified in the genomes of rhodophytes, chlorophytes, and the brown algae Ectocarpus siliculosus, whereas one ESL was identified in the genome of Klebsormidium nitens providing evidence for the early emergence of these transporters in Streptophytes. A phylogenetic analysis using the 519 putative ESL proteins identified in the genomes of 47 Embryophyta species and being representative of the plant kingdom has revealed that ESL protein sequences can be divided into three major groups. The first and second groups originated in the common ancestor of all spermaphytes [ζ: 340 million years ago (MYA)] and of angiosperms (ε: 170-235 MYA), respectively, and the third group originated before the divergence of rosids and asterids (γ/1R: 117 MYA). In some eudicots (Vitales, Malpighiales, Myrtales, Sapindales, Brassicales, Malvales, and Solanales), the ESL family presents remarkable expansions of gene copies associated with tandem duplications. The analysis of non-synonymous and synonymous substitutions for the dN/dS ratio of the ESL copies of the genus Arabidopsis has revealed that ESL genes are evolved under a purifying selection even though the progressive increase of dN/dS ratios in the three groups suggests subdiversification phenomena. To further explore the possible acquisition of novel functions by ESL MSTs, we identified the gene structure and promoter cis-acting elements for Arabidopsis thaliana ESL genes. The expression profiling of Arabidopsis ESL unraveled some gene copies that are almost constitutively expressed, whereas other gene copies display organ-preferential expression patterns. This study provides an evolving framework to better understand the roles of ESL transporters in plant development and response to environmental constraints.

Cold-Triggered Induction of ROS- and Raffinose Metabolism in Freezing-Sensitive Taproot Tissue of Sugar Beet

Sugar beet (Beta vulgaris subsp. vulgaris) is the exclusive source of sugar in the form of sucrose in temperate climate zones. Sugar beet is grown there as an annual crop from spring to autumn because of the damaging effect of freezing temperatures to taproot tissue. A collection of hybrid and non-hybrid sugar beet cultivars was tested for winter survival rates and freezing tolerance. Three genotypes with either low or high winter survival rates were selected for detailed study of their response to frost. These genotypes differed in the severity of frost injury in a defined inner region in the upper part of the taproot, the so-called pith. We aimed to elucidate genotype- and tissue-dependent molecular processes during freezing and combined analyses of sugar beet anatomy and physiology with transcriptomic and metabolite profiles of leaf and taproot tissues at low temperatures. Freezing temperatures induced strong downregulation of photosynthesis in leaves, generation of reactive oxygen species (ROS), and ROS-related gene expression in taproots. Simultaneously, expression of genes involved in raffinose metabolism, as well as concentrations of raffinose and its intermediates, increased markedly in both leaf and taproot tissue at low temperatures. The accumulation of raffinose in the pith tissue correlated with freezing tolerance of the three genotypes. We discuss a protective role for raffinose and its precursors against freezing damage of sugar beet taproot tissue.

MdERDL6-mediated glucose efflux to the cytosol promotes sugar accumulation in the vacuole through up-regulating TSTs in apple and tomato

Sugar transport across membranes is essential for maintaining cellular sugar homeostasis and metabolic balance in plant cells. However, it remains unclear how this process is regulated among different classes of sugar transporters. Here, we identified an apple tonoplast H⁺/glucose symporter, MdERDL6-1, that exports glucose to cytosols to up-regulate the expression of H⁺/sugar antiporter genes TST1 and TST2 to import sugars from cytosol to vacuole for accumulation to high concentrations in apples and tomatoes. The findings provide insights into the regulatory mechanism underlying sugar exchange between cytosol and vacuole.

Sugar transport across tonoplasts is essential for maintaining cellular sugar homeostasis and metabolic balance in plant cells. It remains unclear, however, how this process is regulated among different classes of sugar transporters. Here, we identified a tonoplast H⁺/glucose symporter, MdERDL6-1, from apples, which was highly expressed in fruits and exhibited expression patterns similar to those of the tonoplast H⁺/sugar antiporters MdTST1 and MdTST2. Overexpression of MdERDL6-1 unexpectedly increased not only glucose (Glc) concentration but also that of fructose (Fru) and sucrose (Suc) in transgenic apple and tomato leaves and fruits. RNA sequencing (RNA-seq) and expression analyses showed an up-regulation of TST1 and TST2 in the transgenic apple and tomato lines overexpressing MdERDL6-1. Further studies established that the increased sugar concentration in the transgenic lines correlated with up-regulation of TST1 and TST2 expression. Suppression or knockout of SlTST1 and SlTST2 in the MdERDL6-1–overexpressed tomato background reduced or abolished the positive effect of MdERDL6-1 on sugar accumulation, respectively. The findings demonstrate a regulation of TST1 and TST2 by MdERDL6-1, in which Glc exported by MdERDL6-1 from vacuole up-regulates TST1 and TST2 to import sugars from cytosol to vacuole for accumulation to high concentrations. The results provide insight into the regulatory mechanism of sugar accumulation in vacuoles mediated by the coordinated action of two classes of tonoplast sugar transporters.

Identification of key gene networks controlling organic acid and sugar metabolism during watermelon fruit development by integrating metabolic phenotypes and gene expression profiles

The organoleptic qualities of watermelon fruit are defined by the sugar and organic acid contents, which undergo considerable variations during development and maturation. The molecular mechanisms underlying these variations remain unclear. In this study, we used transcriptome profiles to investigate the coexpression patterns of gene networks associated with sugar and organic acid metabolism. We identified 3 gene networks/modules containing 2443 genes highly correlated with sugars and organic acids. Within these modules, based on intramodular significance and Reverse Transcription Quantitative polymerase chain reaction (RT-qPCR), we identified 7 genes involved in the metabolism of sugars and organic acids. Among these genes, Cla97C01G000640, Cla97C05G087120 and Cla97C01G018840 (r² = 0.83 with glucose content) were identified as sugar transporters (SWEET, EDR6 and STP) and Cla97C03G064990 (r² = 0.92 with sucrose content) was identified as a sucrose synthase from information available for other crops. Similarly, Cla97C07G128420, Cla97C03G068240 and Cla97C01G008870, having strong correlations with malic (r² = 0.75) and citric acid (r² = 0.85), were annotated as malate and citrate transporters (ALMT7, CS, and ICDH). The expression profiles of these 7 genes in diverse watermelon genotypes revealed consistent patterns of expression variation in various types of watermelon. These findings add significantly to our existing knowledge of sugar and organic acid metabolism in watermelon.

Potassium Application Boosts Photosynthesis and Sorbitol Biosynthesis and Accelerates Cold Acclimation of Common Plantain (Plantago major L.)

Potassium (K) is essential for the processes critical for plant performance, including photosynthesis, carbon assimilation, and response to stress. K also influences translocation of sugars in the phloem and regulates sucrose metabolism. Several plant species synthesize polyols and transport these sugar alcohols from source to sink tissues. Limited knowledge exists about the involvement of K in the above processes in polyol-translocating plants. We, therefore, studied K effects in Plantago major, a species that accumulates the polyol sorbitol to high concentrations. We grew P. major plants on soil substrate adjusted to low-, medium-, or high-potassium conditions. We found that biomass, seed yield, and leaf tissue K contents increased in a soil K-dependent manner. K gradually increased the photosynthetic efficiency and decreased the non-photochemical quenching. Concomitantly, sorbitol levels and sorbitol to sucrose ratio in leaves and phloem sap increased in a K-dependent manner. K supply also fostered plant cold acclimation. High soil K levels mitigated loss of water from leaves in the cold and supported cold-dependent sugar and sorbitol accumulation. We hypothesize that with increased K nutrition, P. major preferentially channels photosynthesis-derived electrons into sorbitol biosynthesis and that this increased sorbitol is supportive for sink development and as a protective solute, during abiotic stress.

VvERD6l13 is a grapevine sucrose transporter highly up-regulated in response to infection by Botrytis cinerea and Erysiphe necator

The Early-Response to Dehydration six-like (ERD6l) is one of the largest families of sugar transporters in plants, however, is also one of the less studied with very few members characterized. In this work, we identified 18 members of the grapevine ERD6l family, analyzed their promoters and putative topology and additionally functionally characterized the member VvERD6l13. VvERD6l13 was strongly up-regulated in grape berries infected with Botrytis cinerea and Erysiphe necator in cv. Trincadeira and Carignan, respectively, suggesting an important role in grape berry-pathogen interaction, as we had hypothesized. In Cabernet Sauvignon Berry suspension cultured cells, VvERD6l13 was also up-regulated, by 4-fold, 48 h after elicitation with mycelium extract of B. cinerea. Besides being expressed in grape berries from various developmental stages, VvERD6l13 is also expressed in leaves, canes, flowers and, noticeably, in roots. Using tobacco and an hxt-null Saccharomyces cerevisiae strain as heterologous expression models, we showed that VvERD6l13 is localized at the plasma membrane and mediates the H⁺-dependent transport of sucrose (K_m = 33 mM) thus confirming VvERD6l13 as a bona fide sugar transporter involved in sugar mobilization in grapevine and transcriptionally induced in response to biotic stress.

Vacuolar sucrose homeostasis is critical for plant development, seed properties, and night-time survival in Arabidopsis

Most cellular sucrose is present in the cytosol and vacuoles of plant cells; however, little is known about the effect of this sucrose compartmentation on plant properties. Here, we examined the effects of altered intracellular sucrose compartmentation in Arabidopsis thaliana leaves by heterologously expressing the sugar beet (Beta vulgaris) vacuolar sucrose loader BvTST2.1 and by generating lines with reduced vacuolar invertase activity (amiR vi1-2). Heterologous expression of BvTST2.1 led to increased monosaccharide levels in leaves, whereas sucrose levels remained constant, indicating that vacuolar invertase activity in mesophyll vacuoles exceeds sucrose uptake. This notion was supported by analysis of tobacco (Nicotiana benthamiana) leaves transiently expressing BvTST2.1 and the invertase inhibitor NbVIF. However, sucrose levels were strongly elevated in leaf extracts from amiR vi1-2 lines, and experiments confirmed that sucrose accumulated in the corresponding vacuoles. The amiR vi1-2 lines exhibited impaired early development and reduced seed weight. When germinated in the dark, amiR vi1-2 seedlings were less able to convert sucrose into monosaccharides than the wild type. Cold temperatures strongly down-regulated both VI genes, but the amiR vi1-2 lines showed normal frost tolerance. These observations indicate that increased vacuolar sucrose levels fully compensate for the effects of low monosaccharide concentrations on frost tolerance.

The sugar transporter system of strawberry: genome-wide identification and expression correlation with fruit soluble sugar-related traits in a Fragaria × ananassa germplasm collection

Sugar from plant photosynthesis is a basic requirement for life activities. Sugar transporters are the proteins that mediate sugar allocation among or within source/sink organs. The transporters of the major facilitator superfamily (MFS) targeting carbohydrates represent the largest family of sugar transporters in many plants. Strawberry (Fragaria × ananassa Duchesne) is an important crop appreciated worldwide for its unique fruit flavor. The involvement of MFS sugar transporters (STs) in cultivated strawberry fruit sugar accumulation is largely unknown. In this work, we characterized the genetic variation associated with fruit soluble sugars in a collection including 154 varieties. Then, a total of 67 ST genes were identified in the v4.0 genome integrated with the v4.0.a2 protein database of F. vesca, the dominant subgenome provider for modern cultivated strawberry. Phylogenetic analysis updated the nomenclature of strawberry ST homoeologs. Both the chromosomal distribution and structural characteristics of the ST family were improved. Semi-RT-PCR analysis in nine tissues from cv. Benihoppe screened 34 highly expressed ST genes in fruits. In three varieties with dramatically differing fruit sugar levels, qPCR integrated with correlation analysis between ST transcript abundance and sugar content identified 13 sugar-correlated genes. The correlations were re-evaluated across 19 varieties, including major commercial cultivars grown in China. Finally, a model of the contribution of the sugar transporter system to subcellular sugar allocation in strawberry fruits was proposed. Our work highlights the involvement of STs in controlling strawberry fruit soluble sugars and provides candidates for the future functional study of STs in strawberry development and responses and a new approach for strawberry genetic engineering and molecular breeding.

Overexpression of Melon Tonoplast Sugar Transporter CmTST1 Improved Root Growth under High Sugar Content

Sugar allocation is based on the source-to-sink and intracellular transport between different organelles, and sugar transporters are usually involved in these processes. Tonoplast sugar transporters (TST) are responsible for transporting sugar into vacuoles; however, the role of TSTs in root growth and the response to abiotic stress is poorly studied. Here, RNA analysis and promoter-β-glucuronidase staining revealed that a melon TST1 gene (CmTST1) is highly expressed in the roots. The sugar feeding experiment results showed that the expression of CmTST1 in the roots was induced by a relatively high level of sucrose (6%), glucose (3%), and fructose (3%). The ectopic overexpression of CmTST1 in Arabidopsis improved the root and shoot growth of seedlings under high exogenous sugar stress. Furthermore, the ectopic expression of CmTST1 promoted the expression of plasma membrane-located sugar transporters. We proposed that CmTST1 plays a key role in importing sugar transport into the vacuoles of roots in response to metabolic demands to maintain cytosolic sugar homeostasis.

Mechanisms of Sugar Beet Response to Biotic and Abiotic Stresses

Sugar beet is used not only in the sugar production, but also in a wide range of industries including the production of bioethanol as a source of renewable energy, extraction of pectin and production of molasses. The red beetroot has attracted much attention as health-promoting and disease-preventing functional food. The negative effects of environmental stresses, including abiotic and biotic ones, significantly decrease the cash crop sugar beet productivity. In this paper, we outline the mechanisms of sugar beet response to biotic and abiotic stresses at the levels of physiological change, the genes' functions, transcription and translation. Regarding the physiological changes, most research has been carried out on salt and drought stress. The functions of genes from sugar beet in response to salt, cold and heavy metal stresses were mainly investigated by transgenic technologies. At the transcriptional level, the transcriptome analysis of sugar beet in response to salt, cold and biotic stresses were conducted by RNA-Seq or SSH methods. At the translational level, more than 800 differentially expressed proteins in response to salt, K⁺/Na⁺ ratio, iron deficiency and resupply and heavy metal (zinc) stress were identified by quantitative proteomics techniques. Understanding how sugar beet respond and tolerate biotic and abiotic stresses is important for boosting sugar beet productivity under these challenging conditions. In order to minimize the negative impact of these stresses, studying how the sugar beet has evolved stress coping mechanisms will provide new insights and lead to novel strategies for improving the breeding of stress-resistant sugar beet and other crops.

Transcriptome analysis of acerola fruit ripening: insights into ascorbate, ethylene, respiration, and softening metabolisms

The first transcriptome coupled to metabolite analyses reveals major trends during acerola fruit ripening and shed lights on ascorbate, ethylene signalling, cellular respiration, sugar accumulation, and softening key regulatory genes. Acerola is a fast growing and ripening fruit that exhibits high amounts of ascorbate. During ripening, the fruit experience high respiratory rates leading to ascorbate depletion and a quickly fragile and perishable state. Despite its growing economic importance, understanding of its developmental metabolism remains obscure due to the absence of genomic and transcriptomic data. We performed an acerola transcriptome sequencing that generated over 600 million reads, 40,830 contigs, and provided the annotation of 25,298 unique transcripts. Overall, this study revealed the main metabolic changes that occur in the acerola ripening. This transcriptional profile linked to metabolite measurements, allowed us to focus on ascorbate, ethylene, respiration, sugar, and firmness, the major metabolism indicators for acerola quality. Our results suggest a cooperative role of several genes involved in AsA biosynthesis (PMM, GMP1 and 3, GME1 and 2, GGP1 and 2), translocation (NAT3, 4, 6 and 6-like) and recycling (MDHAR2 and DHAR1) pathways for AsA accumulation in unripe fruits. Moreover, the association of metabolites with transcript profiles provided a comprehensive understanding of ethylene signalling, respiration, sugar accumulation and softening of acerola, shedding light on promising key regulatory genes. Overall, this study provides a foundation for further examination of the functional significance of these genes to improve fruit quality traits.

More Transporters, More Substrates: The Arabidopsis Major Facilitator Superfamily Revisited

The Major Facilitator Superfamily (MFS) is ubiquitous in living organisms and represents the largest group of secondary active membrane transporters. In plants, significant research efforts have focused on the role of specific families within the MFS, particularly those transporting macronutrients (C, N, and P) that constitute the vast majority of the members of this superfamily. Other MFS families remain less explored, although a plethora of additional substrates and physiological functions have been uncovered. Nevertheless, the lack of a systematic approach to analyzing the MFS as a whole has obscured the high diversity and versatility of these transporters. Here, we present a phylogenetic analysis of all annotated MFS domain-containing proteins encoded in the Arabidopsis thaliana genome and propose that this superfamily of transporters consists of 218 members, clustered in 22 families. In reviewing the available information regarding the diversity in biological functions and substrates of Arabidopsis MFS members, we provide arguments for intensified research on these membrane transporters to unveil the breadth of their physiological relevance, disclose the molecular mechanisms underlying their mode of action, and explore their biotechnological potential.

Quantitative Proteome Analysis Reveals Changes in the Protein Landscape During Grape Berry Development With a Focus on Vacuolar Transport Proteins

The vacuole plays a central role in fruit growth and quality formation, yet its proteomic landscape is largely unknown. In the present study, a protocol for isolating intact vacuoles from grape flesh tissue was successfully established. Quantitative proteome analysis identified 2533 proteins from five sampling dates along Cabernet Sauvignon berry development from stage I to III; among them, 1443 proteins were identified on all five sampling dates in at least two biological replicates per sample and were designated core proteome, and 1820 were recruited as differentially abundant proteins (DAPs) by sequential pairwise comparisons using arbitrary fold change of >1.5 and P < 0.05. Metabolism consistently constituted the largest category of identified proteins for both core proteome and DAPs, together with a consistently high proportion of protein-fate category proteins, indicating that the classic lytic functions of vegetative cell vacuoles are maintained throughout berry development; accumulation of metabolites involved in high sugar and other berry qualities in the late developmental stage added to the conventional lytic role of the flesh cell vacuoles. Overall increases in abundance of the DAPs were seen in the transporter proteins, membrane fusion/vesicle trafficking, and protein-fate categories, and decreased abundance was seen for DAPs in the stress, energy and cytoskeleton categories as berry development progressed. A very pronounced proteomic change was revealed between late stage I and mid stage II, with 915 increased and 114 decreased DAPs, demonstrating a significant surge of the vacuolar proteome underlying the rather static phenotypical and physiological phase. We identified 161 transport proteins with differential abundance, including proton pumps, aquaporins, sugar transporters, ATP-binding cassette transporters and ion transport proteins, together with organic compound transport proteins, the highest number and variety of berry tonoplast transporters found in grape proteome efforts to date. We further found a pre-positive increment of 96 transport proteins from the middle of stage II, before the berry undergoes its dramatic physiological changes at and following véraison. Our results are the first to describe the proteome of a vacuole-enriched preparation, toward understanding the functions of the largest compartment in berry cells during grape growth and ripening.

High V-PPase activity is beneficial under high salt loads, but detrimental without salinity

The membrane-bound proton-pumping pyrophosphatase (V-PPase), together with the V-type H+ -ATPase, generates the proton motive force that drives vacuolar membrane solute transport. Transgenic plants constitutively overexpressing V-PPases were shown to have improved salinity tolerance, but the relative impact of increasing PPi hydrolysis and proton-pumping functions has yet to be dissected. For a better understanding of the molecular processes underlying V-PPase-dependent salt tolerance, we transiently overexpressed the pyrophosphate-driven proton pump (NbVHP) in Nicotiana benthamiana leaves and studied its functional properties in relation to salt treatment by primarily using patch-clamp, impalement electrodes and pH imaging. NbVHP overexpression led to higher vacuolar proton currents and vacuolar acidification. After 3 d in salt-untreated conditions, V-PPase-overexpressing leaves showed a drop in photosynthetic capacity, plasma membrane depolarization and eventual leaf necrosis. Salt, however, rescued NbVHP-hyperactive cells from cell death. Furthermore, a salt-induced rise in V-PPase but not of V-ATPase pump currents was detected in nontransformed plants. The results indicate that under normal growth conditions, plants need to regulate the V-PPase pump activity to avoid hyperactivity and its negative feedback on cell viability. Nonetheless, V-PPase proton pump function becomes increasingly important under salt stress for generating the pH gradient necessary for vacuolar proton-coupled Na+ sequestration.

In Concert: Orchestrated Changes in Carbohydrate Homeostasis Are Critical for Plant Abiotic Stress Tolerance

The sessile lifestyle of higher plants is accompanied by their remarkable ability to tolerate unfavorable environmental conditions. This is because, during evolution, plants developed a sophisticated repertoire of molecular and metabolic reactions to cope with changing biotic and abiotic challenges. In particular, the abiotic factors light intensity and ambient temperature are characterized by altering their amplitude within comparably short periods of time and are causative for onset of dynamic plant responses. These rapid responses in plants are also classified as 'acclimation reactions' which differ, due to their reversibility and duration, from non-reversible 'adaptation reactions'. In this review, we demonstrate the remarkable importance of stress-induced changes in carbohydrate homeostasis of plants exposed to high light or low temperatures. These changes represent a co-ordinated process comprising modifications of (i) the concentrations of selected sugars; (ii) starch turnover; (iii) intracellular sugar compartmentation; and (iv) corresponding gene expression patterns. The critical importance of these individual processes has been underlined in the recent past by the analyses of a large number of mutant plants. The outcome of these analyses raised our understanding of acclimation processes in plants per se but might even become instrumental to develop new concepts for directed breeding approaches with the aim to increase abiotic stress tolerance of crop species, which in most cases have high stress sensitivity. The latter direction of plant research is of special importance since abiotic stress stimuli strongly impact on crop productivity and are expected to become even more pronounced because of human activities which alter environmental conditions rapidly.

Overexpression of the tonoplast sugar transporter CmTST2 in melon fruit increases sugar accumulation

Fruits are an important part of the human diet and sugar content is a major criterion used to evaluate fruit quality. Melon fruit accumulate high sugar concentrations during their development; however, the mechanism through which these sugars are transported into the vacuoles of fruit cells for storage remains unclear. In this study, three tonoplast sugar transporters (TSTs), CmTST1, CmTST2, and CmTST3, were isolated from melon plants. Analysis of subcellular localization revealed that all these proteins were targeted to the tonoplast, and evaluation of spatial expression and promoter-GUS activity indicated that they had different expression patterns in the plant. RT-PCR and qRT-PCR results indicated that CmTST2 exhibited the highest expression level among the TST isoforms during melon fruit development. Histochemical and immunohistochemistry localization experiments were performed to identify the tissue- and cell-type localization of CmTST2 in the fruit, and the results indicated that both its transcription and translation were in the mesocarp and vascular cells. Overexpressing the CmTST2 gene in strawberry fruit and cucumber plants by transient expression and stable transformation, respectively, both increased sucrose, fructose, and glucose accumulation in the fruit. The results indicate that CmTST2 plays an important role in sugar accumulation in melon fruit.

Sugar compartmentation as an environmental stress adaptation strategy in plants

The sessile nature of plants has driven their evolution to cope flexibly with ever-changing surrounding environments. The development of stress tolerance traits is complex, and a broad range of cellular processes are involved. Recent studies have revealed that sugar transporters contribute to environmental stress tolerance in plants, suggesting that sugar flow is dynamically fluctuated towards optimization of cellular conditions in adverse environments. Here, we highlight sugar compartmentation mediated by sugar transporters as an adaptation strategy against biotic and abiotic stresses. Competition for sugars between host plants and pathogens shapes their evolutionary arms race. Pathogens, which rely on host-derived carbon, manipulate plant sugar transporters to access sugars easily, while plants sequester sugars from pathogens by enhancing sugar uptake activity. Furthermore, we discuss pathogen tactics to circumvent sugar competition with host plants. Sugar transporters also play a role in abiotic stress tolerance. Exposure to abiotic stresses such as cold or drought stress induces sugar accumulation in various plants. We also discuss how plants allocate sugars under such conditions. Collectively, these findings are relevant to basic plant biology as well as potential applications in agriculture, and provide opportunities to improve crop yield for a growing population.

Main and epistatic loci studies in soybean for Sclerotinia sclerotiorum resistance reveal multiple modes of resistance in multi-environments

Genome-wide association (GWAS) and epistatic (GWES) studies along with expression studies in soybean [Glycine max (L.) Merr.] were leveraged to dissect the genetics of Sclerotinia stem rot (SSR) [caused by Sclerotinia sclerotiorum (Lib.) de Bary], a significant fungal disease causing yield and quality losses. A large association panel of 466 diverse plant introduction accessions were phenotyped in multiple field and controlled environments to: (1) discover sources of resistance, (2) identify SNPs associated with resistance, and (3) determine putative candidate genes to elucidate the mode of resistance. We report 58 significant main effect loci and 24 significant epistatic interactions associated with SSR resistance, with candidate genes involved in a wide range of processes including cell wall structure, hormone signaling, and sugar allocation related to plant immunity, revealing the complex nature of SSR resistance. Putative candidate genes [for example, PHYTOALEXIN DEFFICIENT 4 (PAD4), ETHYLENE-INSENSITIVE 3-LIKE 1 (EIL3), and ETHYLENE RESPONSE FACTOR 1 (ERF1)] clustered into salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) pathways suggest the involvement of a complex hormonal network typically activated by both necrotrophic (ET/JA) and biotrophic (SA) pathogens supporting that S. sclerotiorum is a hemibiotrophic plant pathogen.

Proteomic analysis reveals dynamic regulation of fruit development and sugar and acid accumulation in apple

Understanding the fruit developmental process is critical for fruit quality improvement. Here, we report a comprehensive proteomic analysis of apple fruit development over five growth stages, from young fruit to maturity, coupled with metabolomic profiling. A tandem mass tag (TMT)-based comparative proteomics approach led to the identification and quantification of 7098 and 6247 proteins, respectively. This large-scale proteomic dataset presents a global view of the critical pathways involved in fruit development and metabolism. When linked with metabolomics data, these results provide new insights into the modulation of fruit development, the metabolism and storage of sugars and organic acids (mainly malate), and events within the energy-related pathways for respiration and glycolysis. We suggest that the key steps identified here (e.g. those involving the FK2, TST, EDR6, SPS, mtME and mtMDH switches), can be further targeted to confirm their roles in accumulation and balance of fructose, sucrose and malate. Moreover, our findings imply that the primary reason for decreases in amino acid concentrations during fruit development is related to a reduction in substrate flux via glycolysis, which is mainly regulated by fructose-bisphosphate aldolase and bisphosphoglycerate mutase.

OEP40, a Regulated Glucose-permeable β-Barrel Solute Channel in the Chloroplast Outer Envelope Membrane

Chloroplasts and mitochondria are unique endosymbiotic cellular organelles surrounded by two membranes. Essential metabolic networking between these compartments and their hosting cells requires the exchange of a large number of biochemical pathway intermediates in a directed and coordinated fashion across their inner and outer envelope membranes. Here, we describe the identification and functional characterization of a highly specific, regulated solute channel in the outer envelope of chloroplasts, named OEP40. Loss of OEP40 function in Arabidopsis thaliana results in early flowering under cold temperature. The reconstituted recombinant OEP40 protein forms a high conductance β-barrel ion channel with subconductant states in planar lipid bilayers. The OEP40 channel is slightly cation-selective PK+/PCl- ≈ 4:1 and rectifying (i⃗/i⃖ ≅ 2) with a slope conductance of Ḡmax ≅ 690 picosiemens. The OEP40 channel has a restriction zone diameter of ≅1.4 nm and is permeable for glucose, glucose 1-phosphate and glucose 6-phosphate, but not for maltose. Moreover, channel properties are regulated by trehalose 6-phosphate, which cannot permeate. Altogether, our results indicate that OEP40 is a "glucose-gate" in the outer envelope membrane of chloroplasts, facilitating selective metabolite exchange between chloroplasts and the surrounding cell.

Combined Cytological and Transcriptomic Analysis Reveals a Nitric Oxide Signaling Pathway Involved in Cold-Inhibited Camellia sinensis Pollen Tube Growth

Nitric oxide (NO) as a signaling molecule plays crucial roles in many abiotic stresses in plant development processes, including pollen tube growth. Here, the signaling networks dominated by NO during cold stress that inhibited Camellia sinensis pollen tube growth are investigated in vitro. Cytological analysis show that cold-induced NO is involved in the inhibition of pollen tube growth along with disruption of the cytoplasmic Ca(2+) gradient, increase in ROS content, acidification of cytoplasmic pH and abnormalities in organelle ultrastructure and cell wall component distribution in the pollen tube tip. Furthermore, differentially expressed genes (DEGs)-related to signaling pathway, such as NO synthesis, cGMP, Ca(2+), ROS, pH, actin, cell wall, and MAPK cascade signal pathways, are identified and quantified using transcriptomic analyses and qRT-PCR, which indicate a potential molecular mechanism for the above cytological results. Taken together, these findings suggest that a complex signaling network dominated by NO, including Ca(2+), ROS, pH, RACs signaling and the crosstalk among them, is stimulated in the C. sinensis pollen tube in response to cold stress, which further causes secondary and tertiary alterations, such as ultrastructural abnormalities in organelles and cell wall construction, ultimately resulting in perturbed pollen tube extension.

A Benchtop Fractionation Procedure for Subcellular Analysis of the Plant Metabolome

Although compartmentation is a key feature of eukaryotic cells, biological research is frequently limited by methods allowing for the comprehensive subcellular resolution of the metabolome. It has been widely accepted that such a resolution would be necessary in order to approximate cellular biochemistry and metabolic regulation, yet technical challenges still limit both the reproducible subcellular fractionation and the sample throughput being necessary for a statistically robust analysis. Here, we present a method and a detailed protocol which is based on the non-aqueous fractionation technique enabling the assignment of metabolites to their subcellular localization. The presented benchtop method aims at unraveling subcellular metabolome dynamics in a precise and statistically robust manner using a relatively small amount of tissue material. The method is based on the separation of cellular fractions via density gradients consisting of organic, non-aqueous solvents. By determining the relative distribution of compartment-specific marker enzymes together with metabolite profiles over the density gradient it is possible to estimate compartment-specific metabolite concentrations by correlation. To support this correlation analysis, a spreadsheet is provided executing a calculation algorithm to determine the distribution of metabolites over subcellular compartments. The calculation algorithm performs correlation of marker enzyme activity and metabolite abundance accounting for technical errors, reproducibility and the resulting error propagation. The method was developed, tested and validated in three natural accessions of Arabidopsis thaliana showing different ability to acclimate to low temperature. Particularly, amino acids were strongly shuffled between subcellular compartments in a cold-sensitive accession while a cold-tolerant accession was characterized by a stable subcellular metabolic homeostasis. Finally, we conclude that subcellular metabolome analysis is essential to unambiguously unravel regulatory strategies being involved in plant-environment interactions.

Effects of cold acclimation on sugar metabolism and sugar-related gene expression in tea plant during the winter season

Sugar plays an essential role in plant cold acclimation (CA), but the interaction between CA and sugar remains unclear in tea plants. In this study, during the whole winter season, we investigated the variations of sugar contents and the expression of a large number of sugar-related genes in tea leaves. Results indicated that cold tolerance of tea plant was improved with the development of CA during early winter season. At this stage, starch was dramatically degraded, whereas the content of total sugars and several specific sugars including sucrose, glucose and fructose were constantly elevated. Beyond the CA stage, the content of starch was maintained at a low level during winter hardiness (WH) period and then was elevated during de-acclimation (DC) period. Conversely, the content of sugar reached a peak at WH stage followed by a decrease during DC stage. Moreover, gene expression results showed that, during CA period, sugar metabolism-related genes exhibited different expression pattern, in which beta-amylase gene (CsBAM), invertase gene (CsINV5) and raffinose synthase gene (CsRS2) engaged in starch, sucrose and raffinose metabolism respectively were solidly up-regulated; the expressions of sugar transporters were stimulated in general except the down-regulations of CsSWEET2, 3, 16, CsERD6.7 and CsINT2; interestingly, the sugar-signaling related CsHXK3 and CsHXK2 had opposite expression patterns at the early stage of CA. These provided comprehensive insight into the effects of CA on carbohydrates indicating that sugar accumulation contributes to tea plant cold tolerance during winter season, and a simply model of sugar regulation in response to cold stimuli is proposed.

Intracellular and cell-to-apoplast compartmentation of carbohydrate metabolism

In most plants, carbohydrates represent the major energy store as well as providing the building blocks for essential structural polymers. Although the major pathways for carbohydrate biosynthesis, degradation, and transport are well characterized, several key steps have only recently been discovered. In addition, several novel minor metabolic routes have been uncovered in the past few years. Here we review current studies of plant carbohydrate metabolism detailing the expanding compendium of functionally characterized transport proteins as well as our deeper comprehension of more minor and conditionally activated metabolic pathways. We additionally explore the pertinent questions that will allow us to enhance our understanding of the response of both major and minor carbohydrate fluxes to changing cellular circumstances.