Sucrose Transporter Research Articles

Integrative analysis of metabolome and transcriptome reveals the improvements of seed quality in vegetable soybean (Glycine max (L.) Merr.)

Vegetable soybean is derived from grain soybean. Seeds of vegetable soybean are bigger, sweeter, and have smoother texture and better flavor than those of grain soybean. To better understand the improvements of seed quality in vegetable soybean, comparative metabolome and transcriptome analyses were performed in the developing seeds between grain (Williams 82) and vegetable (Jiaoda 133) soybeans. A total of 299 differential metabolites were identified between two genotypes, with an increase in free amino acids, carbohydrates, sterols, and flavonoids and a decrease in fatty acid in vegetable soybean. Thousands of differentially expressed genes (DEGs) were identified by transcriptome analysis. DEGs were used for weighted gene co-expression network analysis (WGCNA), yielding 16 co-expression modules. The expression patterns of DEGs within these modules were distinct between two genotypes. Functional enrichment analysis revealed that metabolic pathways, including alanine, aspartate and glutamate metabolism, fatty acid degradation, starch and sucrose metabolism, sucrose transport, and flavonoid biosynthesis, were up-regulated, whereas photosynthesis, arginine biosynthesis, arginine and proline metabolism, glycolysis/gluconeogenesis, and fatty acid biosynthesis were down-regulated in vegetable soybean. Reasonably, the alterations of metabolic pathways corresponding to DEGs partly explained the formation of differential metabolites. These findings provide a better understanding of seed development and breeding improvements of vegetable soybean.

Changes in sucrose metabolic enzymes to water stress in contrasting rice genotypes

Photosynthesis is the key process that controls metabolic movements in plants. Water stress reduces the rate of photosynthesis, leading to energy depletion and yield loss. Sucrose is the fundamental product of photosynthesis in plants, which acts as an important energy substrate and regulator of plant growth and developmental signals. In this study, two rice varieties, drought tolerant PTB7 (Parambuvattan) and drought susceptible PTB23 (Cheriya Aryan) were subjected to water deficit stress treatments to understand the changes in various physiological, biochemical mechanisms and sucrose metabolizing enzymes. Under water stress conditions, root biomass increased as compared to shoot biomass, which led to an increase in the root to shoot (R/S) ratio. Water stress enhanced the sucrose content in both leaves and roots of the rice plant, which may be due to the increased sucrose metabolism and transport under water stress conditions. Since then, there has been a notable increase in the activity of enzymes (α-amylase and β-amylase) and expression of genes such as sucrose transporter (SUC2) and sucrose synthase (OsSuSy) genes. In conclusion, the results showed that changes in sucrose distribution, metabolism, and transport in plants under water stress conditions contribute to tolerance in rice to water stress conditions.

Molecular Mechanisms of Early Flowering in Tomatoes Induced by Manganese Ferrite (MnFe2O4) Nanomaterials.

Nanomaterials (NMs) have demonstrated enormous potential to improve agricultural production. Ten mg L-1 of customized manganese ferrite (MnFe2O4) NMs was selected as the optimal dose based on its outstanding effects on promoting tomato flowering and production. After the foliar application before flowering, MnFe2O4 NMs increased the leaf chlorophyll content by 20 percent, and significantly upregulated the expressions of ferredoxin, PsaA, and PsbA in leaves, likely by serving as an electron donor, leading to a significant increase in photosynthesis efficiency by 13.3%. Long distance transport of sucrose was then confirmed by the upregulation of sucrose transporter SUT1 and SUT2 in NM-treated leaves and meristems. The genes associated with gibberellin biosynthesis, including GA20ox2, GA20ox3, and SIGAST, and a flowering induction gene SFT, were also significantly upregulated. Importantly, the flowering time was 13 days earlier by MnFe2O4 NMs over the control. At the reproductive stage, MnFe2O4 NMs increased pollen activity and ovule size, leading to the significant increase in fruit number per plant, single fruit weight, and fruit weight per plant by 50%, 30%, and 75%, respectively. Metabolically, a significant increase of glucose-6-phosphate, phenylalanine, rutin, and ascorbic acid (vitamin C), as well as a significant decrease of tomatine and methionine, demonstrates an increased nutritional value of the tomato fruits. A verified companion field experiment showed an increase of 84.1% in total tomato production with the MnFe2O4 NM amendment. These findings provide support for the early flowering and yield improvement in nano-enabled agricultural systems.

The sucrose transport regulator OsDOF11 mediates cytokinin degradation during rice development.

Photosynthetic tissues are dynamic structures whose homeostasis depends on the coordination of two antagonistic processes: self-maintenance and supporting sink tissues. The balance of these processes determines plant development, which might be mediated by cytokinin. However, little is known about the link between sucrose transport signaling and cytokinin. Rice (Oryza sativa) DNA BINDING WITH ONE FINGER11 (OsDOF11) is a transcription factor that mediates sucrose transport by inducing the expression of sucrose transporter genes. Here, we found that OsDOF11 loss-of-function mutants showed a semi-dwarf phenotype with a smaller cell length due to increased cytokinin content in source tissues. RNA sequencing and reverse transcription quantitative PCR analyses revealed that genes involved in cytokinin signaling and metabolism were affected in osdof11 mutants. Yeast one-hybrid, dual-luciferase reporter, and chromatin immunoprecipitation experiments showed that OsDOF11 directly binds to the promoter regions of O. sativa CYTOKININ OXIDASE/DEHYDROGENASE4 (OsCKX4). Moreover, mutation of osckx4 in the osdof11 osckx4 double mutant rescued the semi-dwarf phenotype of the osdof11 mutant. Interestingly, exogenous application of kinetin promoted OsDOF11 expression earlier than OsCKX4, and overexpression of O. sativa VIN3-LIKE 2 caused an increase in active cytokinin levels and induced OsDOF11 transcript levels. Taken together, our results suggest a model in which both a sucrose transport regulator (OsDOF11) and cytokinin via OsCKX4 establish a feedback loop to maintain dynamic tissue homeostasis.

Transcriptional analysis of the early ripening of ‘Summer Black’ grape in response to abscisic acid treatment

Berry quality and harvesting time affect grape (Vitis vinifera L.) market values. Abscisic acid (ABA) applications improve fruit quality and hasten ripening in grapes. However, the molecular mechanisms behind these promotive effects, especially ripening acceleration, remain unclear. Here, treatments of different ABA concentrations were applied to ‘Summer Black’ grape from véraison onwards. The ABA applications accelerated berry ripening and improved quality attributes, such as increased sugar and anthocyanin accumulations, as well as a deepening of color. RNA sequencing was performed to characterize the gene expression involved in ripening-related processes. A total of 1,880 non-redundant differentially expressed genes were identified by paired-comparisons between the ABA-treated and control samples at three sampling time points. The transcripts associated with starch and sucrose metabolism, flavonoid biosynthesis, and flavone and flavonol biosynthesis were significantly overrepresented. The change in the anthocyanin content following ABA treatments coincided with the expression patterns of key structural genes and an R2R2-MYB transcription factor associated with anthocyanin synthesis. Additionally, genes involved in both sucrose transportation and metabolism, such as HEXOSE TRANSPORTER and INVERTASE, had increased expression levels after ABA treatments, which was consistent with the responses of sucrose-relate enzymes. Thus, we hypothesize that ABA applications regulate many genes involved in flavonoid biosynthesis, sucrose metabolism and sugar transport that accelerate anthocyanin and hexose accumulation, thereby improving grape quality and allowing earlier harvests. These results provide extensive candidate genes involved in the ripening process under ABA treatment in grapes.

OsSWEET11b, a potential sixth leaf blight susceptibility gene involved in sugar transport-dependent male fertility.

SWEETs play important roles in intercellular sugar transport. Induction of SWEET sugar transporters by Transcription Activator-Like effectors (TALe) of Xanthomonas ssp. is key for virulence in rice, cassava and cotton. We identified OsSWEET11b with roles in male fertility and potential bacterial blight (BB) susceptibility in rice. While single ossweet11a or 11b mutants were fertile, double mutants were sterile. As clade III SWEETs can transport gibberellin (GA), a key hormone for spikelet fertility, sterility and BB susceptibility might be explained by GA transport deficiencies. However, in contrast with the Arabidopsis homologues, OsSWEET11b did not mediate detectable GA transport. Fertility and susceptibility therefore are likely to depend on sucrose transport activity. Ectopic induction of OsSWEET11b by designer TALe enabled TALe-free Xanthomonas oryzae pv. oryzae (Xoo) to cause disease, identifying OsSWEET11b as a potential BB susceptibility gene and demonstrating that the induction of host sucrose uniporter activity is key to virulence of Xoo. Notably, only three of six clade III SWEETs are targeted by known Xoo strains from Asia and Africa. The identification of OsSWEET11b is relevant for fertility and for protecting rice against emerging Xoo strains that target OsSWEET11b.

The Tiny Companion Matters: The Important Role of Protons in Active Transports in Plants.

In plants, the translocation of molecules, such as ions, metabolites, and hormones, between different subcellular compartments or different cells is achieved by transmembrane transporters, which play important roles in growth, development, and adaptation to the environment. To facilitate transport in a specific direction, active transporters that can translocate their substrates against the concentration gradient are needed. Examples of major active transporters in plants include ATP-binding cassette (ABC) transporters, multidrug and toxic compound extrusion (MATE) transporters, monosaccharide transporters (MSTs), sucrose transporters (SUTs), and amino acid transporters. Transport via ABC transporters is driven by ATP. The electrochemical gradient across the membrane energizes these secondary transporters. The pH in each cell and subcellular compartment is tightly regulated and yet highly dynamic, especially when under stress. Here, the effects of cellular and subcellular pH on the activities of ABC transporters, MATE transporters, MSTs, SUTs, and amino acid transporters will be discussed to enhance our understanding of their mechanics. The relation of the altered transporter activities to various biological processes of plants will also be addressed. Although most molecular transport research has focused on the substrate, the role of protons, the tiny counterparts of the substrate, should also not be ignored.

A transcriptional landscape underlying sugar import for grain set in maize.

Developing seed depends on sugar supply for its growth and yield formation. Maize (Zea mays L.) produces the largest grains among cereals. However, there is a lack of holistic understanding of the transcriptional landscape of genes controlling sucrose transport to, and utilization within, maize grains. By performing in-depth data mining of spatio-temporal transcriptomes coupled with histological and heterologous functional analyses, we identified transporter genes specifically expressed in the maternal-filial interface, including (i) ZmSWEET11/13b in the placento-chalazal zone, where sucrose is exported into the apoplasmic space, and (ii) ZmSTP3, ZmSWEET3a/4c (monosaccharide transporters), ZmSUT1, and ZmSWEET11/13a (sucrose transporters) in the basal endosperm transfer cells for retrieval of apoplasmic sucrose or hexoses after hydrolysis by extracellular invertase. In the embryo and its surrounding regions, an embryo-localized ZmSUT4 and a cohort of ZmSWEETs were specifically expressed. Interestingly, drought repressed those ZmSWEETs likely exporting sucrose but enhanced the expression of most transporter genes for uptake of apoplasmic sugars. Importantly, this drought-induced fluctuation in gene expression was largely attenuated by an increased C supply via controlled pollination, indicating that the altered gene expression is conditioned by C availability. Based on the analyses above, we proposed a holistic model on the spatio-temporal expression of genes that likely govern sugar transport and utilization across maize maternal and endosperm and embryo tissues during the critical stage of grain set. Collectively, the findings represent an advancement towards a holistic understanding of the transcriptional landscape underlying post-phloem sugar transport in maize grain and indicate that the drought-induced changes in gene expression are attributable to low C status.

Characterization and functional divergence of genes encoding sucrose transporters in oilseeds castor bean

Castor bean (Ricinus communis L.) is an economically important non-edible oilseed crop. Its seed oils are rich in hydroxy fatty acid, which are highly valuable with a wide range of industrial applications. Sucrose transportation is critical in regulating the growth, development and oilseed yield in castor bean. The transporters or carriers (SUTs or SUCs) play a central role in orchestrating sucrose allocation and aiding in plant adaptation to diverse stresses. In this study, based on castor bean genome, three RcSUCs (RcSUC2, RcSUC3 and RcSUC4) were identified and characterized. The expressional profiles of RcSUCs in different tissues such as leaf, stem, root, phloem and seed tissues exhibited a distinct divergence of gene expression, suggesting that the functions of RcSUC2, RcSUC3 and RcSUC4 are differentiated into long or short-distance transportation among tissues. Additionally, under abiotic stresses including hot temperature, low temperature, drought and salt stresses, the sugar allocation among leaf, stem and roots was tested. The expressional changes of RcSUCs in leaf, stem and root tissues were associated with sugar transportation and allocation. Taken together, the differential expression of RcSUCs among tissues responsing to abiotic stress suggested functional differences in sucrose transport and redistribution in different tissues. This study is helpful to understand the physiological and molecular mechanisms of sucrose transportation and allocation among tissues in heterotrophic oilseeds, and could provide clues for genetic improvement and optimization of cultivation practices.

Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought.

Maintaining phloem transport under water stress is expected to be crucial to whole-plant drought tolerance, but the traits that benefit phloem function under drought are poorly understood. Nearly half of surveyed angiosperm species, including important crops, use sucrose transporter proteins to actively load sugar into the phloem. Plants can alter transporter abundance in response to stress, providing a potential mechanism for active-loading species to closely regulate phloem loading rates to avoid drought-induced reductions or failures in phloem transport. We developed an integrated xylem-phloem-stomatal model to test this hypothesis by quantifying the joint impacts of transporter kinetics, phloem anatomy, and plant water status on sucrose export to sinks. We parameterized the model with phloem hydraulic resistances and sucrose transporter kinetic parameters compiled from the literature, and simulated loading regulation by allowing loading rates to decline exponentially with phloem pressure to prevent excessive sucrose concentrations from inducing viscosity limitations. In the absence of loading regulation, where loading rates were independent of phloem pressure, most resistance values produced unrealistic phloem pressures owing to viscosity effects, even under well-watered conditions. Conversely, pressure-regulated loading helped to control viscosity buildup and improved export to sinks for both lower and higher resistant phloem pathways, while maintaining realistic phloem pressures. Regulation also allowed for rapid loading and export in wet conditions while maintaining export and viable phloem pressures during drought. Therefore, we expect feedbacks between phloem pressure and loading to be critical to carbon transport in active-loading species, especially under drought, and for transporter kinetics to be strongly coordinated with phloem architecture and plant water status. This work provides an important and underexplored physiological framework to understand the ecophysiology of phloem transport under drought and to enhance the genetic engineering of crop plants.

Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink.

Sucrose is transported from sources (mature leaves) to sinks (importing tissues such as roots, stems, fruits, and seeds) through the phloem tissues in veins. In many herbaceous crop species, sucrose must first be effluxed to the cell wall by a sugar transporter of the SWEET family prior to being taken up into phloem companion cells or sieve elements by a different sugar transporter, called SUT or SUC. The import of sucrose into these cells is termed apoplasmic phloem loading. In sinks, sucrose can similarly exit the phloem apoplasmically or, alternatively, symplasmically through plasmodesmata into connecting parenchyma storage cells. Recent advances describing the regulation and manipulation of sugar transporter expression and activities provide stimulating new insights into sucrose phloem loading in sources and unloading processes in sink tissues. Additionally, new breakthroughs have revealed distinct subpopulations of cells in leaves with different functions pertaining to phloem loading. These and other discoveries in sucrose transport are discussed.

Mechanical properties of stem and physiological–biochemical responses of cucumber under different N and K conditions

In order to investigate the effects of N and K on the mechanical properties of cucumber stems, the physiological and biochemical indexes under different N and K conditions were compared. The analysis showed that the pho-tosynthetic rate was affected by both stress and the self-senescence of cucumber plants, and that the crops under N and K stress were stimulated to activate a highly effective antioxidant system to cope with the damage. Sugar, as the basis for the synthesis and structure formation of the stems, was highly related to stem strength, and polyphenol oxidase in the study of stem strength was limited but indispensable. Compared with CK, the epidermal penetration strength under LN, LNLK, and LNHK treatments increased by at least 10%, while the strength under HN, LK, HNLK, and HNHK treatments decreased by at least 7%. To sum up, it was the photosynthesis rate, the sucrose transportation, as well as the futile cycling of K+ that were the keys of stem strength.

Phytoplasma Infection Blocks Starch Breakdown and Triggers Chloroplast Degradation, Leading to Premature Leaf Senescence, Sucrose Reallocation, and Spatiotemporal Redistribution of Phytohormones.

Witches’-broom (WB, excessive initiation, and outgrowth of axillary buds) is one of the remarkable symptoms in plants caused by phytoplasmas, minute wall-less intracellular bacteria. In healthy plants, axillary bud initiation and outgrowth are regulated by an intricate interplay of nutrients (such as sugars), hormones, and environmental factors. However, how these factors are involved in the induction of WB by phytoplasma is poorly understood. We postulated that the WB symptom is a manifestation of the pathologically induced redistribution of sugar and phytohormones. Employing potato purple top phytoplasma and its alternative host tomato (Solanum lycopersicum), sugar metabolism and transportation, and the spatiotemporal distribution of phytohormones were investigated. A transmission electron microscopy (TEM) analysis revealed that starch breakdown was inhibited, resulting in the degradation of damaged chloroplasts, and in turn, premature leaf senescence. In the infected source leaves, two marker genes encoding asparagine synthetase (Sl-ASN) and trehalose-6-phosphate synthase (Sl-TPS) that induce early leaf senescence were significantly up-regulated. However, the key gibberellin biosynthesis gene that encodes ent-kaurene synthase (Sl-KS) was suppressed. The assessment of sugar content in various infected tissues (mature leaves, stems, roots, and leaf axils) indicated that sucrose transportation through phloem was impeded, leading to sucrose reallocation into the leaf axils. Excessive callose deposition and the resulting reduction in sieve pore size revealed by aniline blue staining and TEM provided additional evidence to support impaired sugar transport. In addition, a spatiotemporal distribution study of cytokinin and auxin using reporter lines detected a cytokinin signal in leaf axils where the axillary buds initiated. However, the auxin responsive signal was rarely present in such leaf axils, but at the tips of the newly elongated buds. These results suggested that redistributed sucrose as well as cytokinin in leaf axils triggered the axillary bud initiation, and auxin played a role in the bud elongation. The expression profiles of genes encoding squamosa promoter-binding proteins (Sl-SBP1), and BRANCHED1 (Sl-BRC1a and Sl-BRC1b) that control axillary bud release, as determined by quantitative reverse transcription (qRT)-PCR, indicated their roles in WB induction. However, their interactions with sugars and cytokinins require further study. Our findings provide a comprehensive insight into the mechanisms by which phytoplasmas induce WB along with leaf chlorosis, little leaf, and stunted growth.

Sugar loading is not required for phloem sap flow in maize plants.

Phloem transport of photoassimilates from leaves to non-photosynthetic organs, such as the root and shoot apices and reproductive organs, is crucial to plant growth and yield. For nearly 90 years, evidence has been generally consistent with the theory of a pressure-flow mechanism of phloem transport. Central to this hypothesis is the loading of osmolytes, principally sugars, into the phloem to generate the osmotic pressure that propels bulk flow. Here we used genetic and light manipulations to test whether sugar import into the phloem is required as the driving force for phloem sap flow. Using carbon-11 radiotracer, we show that a maize sucrose transporter1 (sut1) loss-of-function mutant has severely reduced export of carbon from photosynthetic leaves (only ~4% of the wild type level). Yet, the mutant remarkably maintains phloem pressure at ~100% and sap flow speeds at ~50-75% of those of wild type. Potassium (K+) abundance in the phloem was elevated in sut1 mutant leaves. Fluid dynamic modelling supports the conclusion that increased K+ loading compensated for decreased sucrose loading to maintain phloem pressure, and thereby maintained phloem transport via the pressure-flow mechanism. Furthermore, these results suggest that sap flow and transport of other phloem-mobile nutrients and signalling molecules could be regulated independently of sugar loading into the phloem, potentially influencing carbon-nutrient homoeostasis and the distribution of signalling molecules in plants encountering different environmental conditions.

Identification of microRNAs involved in sucrose accumulation in sugarcane (Saccharum species hybrid)

The genetic improvement efforts in sugarcane are largely centered around increasing its sucrose accumulation potential. Sugarcane genotypes show variable regulation of genes involved in synthesis, accumulation, transport, storage and subsequent retention of sucrose. MicroRNAs (miRNAs), which are a class of small non-coding RNAs play a very important role in post-transcriptional regulation of genes. Very little has been reported on the miRNA-mediated regulation of sucrose accumulation in sugarcane. In this study, two sugarcane genotypes contrasting for sucrose content along with two extreme bulks from their F1 population were used to identify differentially expressed miRNAs which might play a regulatory role in sucrose accumulation. A total of 161 miRNAs were identified, of them, 23 were significantly differential for contrasting sucrose content. The target genes of the identified miRNAs were predicted and some of them were validated using transcriptome library-based gene expression levels. Nineteen differential miRNAs and six selected target genes were validated using quantitative PCR. The study revealed that miRNA might be involved in sucrose accumulation in sugarcane, not by directly targeting the sucrose metabolism genes but by targeted regulation of transcription factors (PCF8, GRFs, ARF, etc.), and genes involved in carbohydrate metabolism, vegetative to mature phase transition, stress response, ion transport, growth and developmental pathways in sugarcane like RAV6, calcium-dependent protein kinase, HSP20, diacylglycerol kinase, squamosa promoter binding protein, etc. This study identifies miRNA candidates involved in sucrose accumulation and enriches the small RNA inventory of sugarcane.

Sugar Metabolism and Transcriptome Analysis Reveal Key Sugar Transporters during Camellia oleifera Fruit Development.

Camellia oleifera is a widely planted woody oil crop with economic significance because it does not occupy cultivated land. The sugar-derived acetyl-CoA is the basic building block in fatty acid synthesis and oil synthesis in C. oleifera fruit; however, sugar metabolism in this species is uncharacterized. Herein, the changes in sugar content and metabolic enzyme activity and the transcriptomic changes during C. oleifera fruit development were determined in four developmental stages (CR6: young fruit formation; CR7: expansion; CR9: oil transformation; CR10: ripening). CR7 was the key period of sugar metabolism since it had the highest amount of soluble sugar, sucrose, and glucose with a high expression of genes related to sugar transport (four sucrose transporters (SUTs) or and one SWEET-like gene, also known as a sugar, will eventually be exported transporters) and metabolism. The significant positive correlation between their expression and sucrose content suggests that they may be the key genes responsible for sucrose transport and content maintenance. Significantly differentially expressed genes enriched in the starch and sucrose metabolism pathway were observed in the CR6 versus CR10 stages according to KEGG annotation. The 26 enriched candidate genes related to sucrose metabolism provide a molecular basis for further sugar metabolism studies in C. oleifera fruit.

Carbon Metabolism and the ROS Scavenging System Participate in Nostoc flagelliforme's Adaptive Response to Dehydration Conditions through Protein Acetylation.

Acetylation represents an extensively occurring protein post-translational modification (PTM) that plays a key role in many cellular physiological and biochemical processes. However, studies on PTMs such as acetylation of lysine (LysAc) in cyanobacteria are still rare. In this study, a quantitative LysAc approach (acetylome) on the strains of Nostoc flagelliforme subjected to different dehydration treatments was conducted. We observed that starch contents were significantly accumulated due to dehydration treatments, and we identified 2474 acetylpeptides and 1060 acetylproteins based on acetylome analysis. Furthermore, an integrative analysis was performed on acetylome and nontargeted metabolism, and the results showed that many KEGG terms were overlapped for both omics analyses, including starch and sucrose metabolism, transporter activity, and carbon metabolism. In addition, time series clustering was analyzed, and some proteins related to carbon metabolism and the ROS scavenging system were significantly enriched in the list of differentially abundant acetylproteins (DAAPs). These protein expression levels were further tested by qPCR. A working model was finally proposed to show the biological roles of protein acetylation from carbon metabolism and the ROS scavenging system in response to dehydration in N. flagelliforme. We highlighted that LysAc was essential for the regulation of key metabolic enzymes in the dehydration stress response.

Xylella fastidiosa and Drought Stress in Olive Trees: A Complex Relationship Mediated by Soluble Sugars.

Simple SummaryCarbohydrates play important roles in tolerance to both biotic and abiotic stressors. Xylella fastidiosa, the causal agent of “Olive Quick Decline Syndrome”, is a quarantine pathogen that induces drought stress in the host, aggravated by eventual water shortage, which is a frequent environmental condition in Mediterranean olive groves. At present, the resistance mechanisms shown by few resistant olive cultivars (e.g., cv Leccino) are not completely known; therefore, the aim of this research is to understand whether sugar metabolism is involved in the cross-talk mechanisms of biotic and abiotic responses. The results show that drought stress response induces effects beneficial to resistance of Xylella fastidiosa in cv Leccino. In the current context of global climate change, this study supports the importance of investigating the complex drought–disease interaction to detect resistance traits and thus find ways to counter the threat of this pathogen in the future.Xylella fastidiosa (Xf) subsp. pauca “De Donno” is the etiological agent of “Olive Quick Decline Syndrome” (OQDS) on olive trees (Olea europaea L.); the presence of the bacterium causes xylem vessel occlusions inducing a drought stress and the development of leaf scorch symptoms, which may be worsened by water shortage in summer. In order to evaluate how the two stress factors overlap each other, the carbohydrate content and the expression patterns of genes related to carbohydrate metabolism have been evaluated in two olive cvs trees (Cellina di Nardò, susceptible to Xf, and Leccino, resistant to Xf) reporting transcriptional dynamics elicited by Xf infection, drought, or combined stress (drought/Xf). In the Xf-susceptible Cellina di Nardò plants, Xf and its combination with drought significantly decrease total sugars compared to control (−27.0% and −25.7%, respectively). In contrast, the Xf-resistant Leccino plants show a more limited reduction in sugar content in Xf-positive conditions (−20.1%) and combined stresses (−11.1%). Furthermore, while the amount of glucose decreases significantly in stressed Cellina di Nardò plants (≈18%), an increase was observed in Leccino plants under drought/Xf combined stresses (+11.2%). An opposite behavior among cvs was also observed for sucrose, as an accumulation of the disaccharide was recorded in stressed Leccino plants (≈37%). The different response to combined stress by Xf-resistant plants was confirmed considering genes coding for the sucrose or monosaccharide transporter (OeSUT1, OeMST2), the cell wall or vacuolar invertase (OeINV-CW, OeINV-V), the granule-bound starch synthase I (OeGBSSI) and sucrose synthase (OeSUSY), with a higher expression than at least one single stress (e.g., ≈1-fold higher or more than Xf for OeMST2, OeINV-CW, OeINV-V, OeGBSSI). It is probable that the pathways involved in drought stress response induce positive effects useful for pathogen resistance in cv Leccino, confirming the importance of investigating the mechanisms of cross-talk of biotic and abiotic responses.

Metabolic regulation of subcellular sucrose cleavage inferred from quantitative analysis of metabolic functions

Quantitative analysis of experimental metabolic data is frequently challenged by non-intuitive, complex patterns which emerge from regulatory networks. The complex output of metabolic regulation can be summarised by metabolic functions which comprise information about dynamics of metabolite concentrations. In a system of ordinary differential equations, metabolic functions reflect the sum of biochemical reactions which affect a metabolite concentration, and their integration over time reveals metabolite concentrations. Further, derivatives of metabolic functions provide essential information about system dynamics and elasticities. Here, invertase-driven sucrose hydrolysis was simulated in kinetic models on a cellular and subcellular level. Both Jacobian and Hessian matrices of metabolic functions were derived for quantitative analysis of kinetic regulation of sucrose metabolism. Model simulations suggest that transport of sucrose into the vacuole represents a central regulatory element in plant metabolism during cold acclimation which preserves control of metabolic functions and limits feedback-inhibition of cytosolic invertases by elevated hexose concentrations.

Genome-wide identification, expression and functional analysis of sugar transporters in sorghum (Sorghum bicolor L.)

Sugar transporters are essential for osmotic process regulation, various signaling pathways and plant growth and development. Currently, few studies are available on the function of sugar transporters in sorghum (Sorghum bicolor L.). In this study, we performed a genome-wide survey of sugar transporters in sorghum. In total, 98 sorghum sugar transporters (SSTs) were identified via BLASTP. These SSTs were classified into three families based on the phylogenetic and conserved domain analysis, including six sucrose transporters (SUTs), 23 sugars will eventually be exported transporters (SWEETs), and 69 monosaccharide transporters (MSTs). The sorghum MSTs were further divided into seven subfamilies, including 24 STPs, 23 PLTs, two VGTs, four INTs, three pGlcT/SBG1s, five TMTs, and eight ERDs. Chromosomal localization of the SST genes showed that they were randomly distributed on 10 chromosomes, and substantial clustering was evident on the specific chromosomes. Twenty-seven SST genes from the families of SWEET, ERD, STP, and PLT were found to cluster in eight tandem repeat event regions. In total, 22 SSTs comprising 11 paralogous pairs and accounting for 22.4% of all the genes were located on the duplicated blocks. The different subfamilies of SST proteins possessed the same conserved domain, but there were some differences in features of the motif and transmembrane helices (TMH). The publicly-accessible RNA-sequencing data and real-time PCR revealed that the SST genes exhibited distinctive tissue specific patterns. Functional studies showed that seven SSTs were mainly located on the cell membrane and membrane organelles, and 14 of the SSTs could transport different types of monosaccharides in yeast. These findings will help us to further elucidate their roles in the sorghum sugar transport and sugar signaling pathways.