Integrative View of the Diversity and Evolution of SWEET and SemiSWEET Sugar Transporters

Correlating sugar transporter expression and activities to identify transporters for an orphan sugar substrate

Filamentous fungi like Neurospora crassa are able to take up and metabolize important sugars present, for example, in agricultural and human food wastes. However, only a fraction of all putative sugar transporters in filamentous fungi has been characterized to date, and for many sugar substrates, the corresponding transporters are unknown. In N. crassa, only 14 out of the 42 putative major facilitator superfamily (MFS)-type sugar transporters have been characterized so far. To uncover this hidden potential for biotechnology, it is therefore necessary to find new strategies. By correlation of the uptake profile of sugars of interest after different induction conditions with the expression profiles of all 44 genes encoding predicted sugar transporters in N. crassa, together with an exhaustive phylogenetic analysis using sequences of characterized fungal sugar transporters, we aimed to identify transporter candidates for the tested sugars. Following this approach, we found a high correlation of uptake rates and expression strengths for many sugars with dedicated transporters, like galacturonic acid and arabinose, while the correlation is loose for sugars that are transported by several transporters due to functional redundancy. Nevertheless, this combinatorial approach allowed us to elucidate the uptake system for the disaccharide lactose, a by-product of the dairy industry, which consists of the two main cellodextrin transporters CDT-1 and CDT-2 with a minor contribution of the related transporter NCU00809. Moreover, a non-MFS transporter involved in glycerol transport was also identified. Deorphanization of sugar transporters or identification of transporters for orphan sugar substrates by correlation of uptake kinetics with transporter expression and phylogenetic information can thus provide a way to optimize the reuse of food industry by-products and agricultural wastes by filamentous fungi in order to create economic value and reduce their environmental impact. KEY POINTS: • The Neurospora crassa genome contains 30 uncharacterized putative sugar transporter genes. • Correlation of transporter expression and sugar uptake profiles can help to identify transporters for orphan sugar substrates. • CDT-1, CDT-2, and NCU00809 are key players in the transport of the dairy by-product lactose in N. crassa.

Transcriptional responses to salinity-induced changes in cell wall morphology of the euryhaline diatom Pleurosira laevis

Diatoms are unicellular algae with morphologically diverse silica cell walls, which are called frustules. The mechanism of frustule morphogenesis has attracted attention in biology and nanomaterials engineering. However, the genetic regulation of the morphology remains unclear. We therefore used transcriptome sequencing to search for genes involved in frustule morphology in the centric diatom Pleurosira laevis, which exhibits morphological plasticity between flat and domed valve faces in salinity 2 and 7, respectively. We observed differential expression of transposable elements (TEs) and transporters, likely due to osmotic response. Up-regulation of mechanosensitive ion channels and down-regulation of Ca2+-ATPases in cells with flat valves suggested that cytosolic Ca2+ levels were changed between the morphologies. Calcium signaling could be a mechanism for detecting osmotic pressure changes and triggering morphological shifts. We also observed an up-regulation of ARPC1 and annexin, involved in the regulation of actin filament dynamics known to affect frustule morphology, as well as the up-regulation of genes encoding frustule-related proteins such as BacSETs and frustulin. Taken together, we propose a model in which salinity-induced morphogenetic changes are driven by upstream responses, such as the regulation of cytosolic Ca2+ levels, and downstream responses, such as Ca2+-dependent regulation of actin dynamics and frustule-related proteins. This study highlights the sensitivity of euryhaline diatoms to environmental salinity and the role of active cellular processes in controlling gross valve morphology under different osmotic pressures.

Structure, evolution, and roles of SWEET proteins in growth and stress responses in plants

Carbohydrates are exported by the SWEET family of transporters, which is a novel class of carriers that can transport sugars across cell membranes and facilitate sugar's long-distance transport from source to sink organs in plants. SWEETs play crucial roles in a wide range of physiologically important processes by regulating apoplastic and symplastic sugar concentrations. These processes include host-pathogen interactions, abiotic stress responses, and plant growth and development. In the present review, we (i) describe the structure and organization of SWEETs in the cell membrane, (ii) discuss the roles of SWEETs in sugar loading and unloading processes, (iii) identify the distinct functions of SWEETs in regulating plant growth and development including flower, fruit, and seed development, (iv) shed light on the importance of SWEETs in modulating abiotic stress resistance, and (v) describe the role of SWEET genes during plant-pathogen interaction. Finally, several perspectives regarding future investigations for improving the understanding of sugar-mediated plant defenses are proposed.

Genome-wide identification of SWEET genes reveals their roles during seed development in peanuts

Sugar Will Eventually be Exported Transporter (SWEET) proteins are highly conserved in various organisms and play crucial roles in sugar transport processes. However, SWEET proteins in peanuts, an essential leguminous crop worldwide, remain lacking in systematic characterization. Here, we identified 94 SWEET genes encoding the conservative MtN3/saliva domains in three peanut species, including 47 in Arachis hypogea, 23 in Arachis duranensis, and 24 in Arachis ipaensis. We observed significant variations in the exon-intron structure of these genes, while the motifs and domain structures remained highly conserved. Phylogenetic analysis enabled us to categorize the predicted 286 SWEET proteins from eleven species into seven distinct groups. Whole genome duplication/segment duplication and tandem duplication were the primary mechanisms contributing to the expansion of the total number of SWEET genes. In addition, an investigation of cis-elements in the potential promoter regions and expression profiles across 22 samples uncovered the diverse expression patterns of AhSWEET genes in peanuts. AhSWEET24, with the highest expression level in seeds from A. hypogaea Tifrunner, was observed to be localized on both the plasma membrane and endoplasmic reticulum membrane. Moreover, qRT-PCR results suggested that twelve seed-expressed AhSWEET genes were important in the regulation of seed development across four different peanut varieties. Together, our results provide a foundational basis for future investigations into the functions of SWEET genes in peanuts, especially in the process of seed development.

Identification of HuSWEET Family in Pitaya (Hylocereus undatus) and Key Roles of HuSWEET12a and HuSWEET13d in Sugar Accumulation

The sugar composition and content of fruit have a significant impact on their flavor and taste. In pitaya, or dragon fruit, sweetness is a crucial determinant of fruit taste and consumer preference. The sugars will eventually be exported transporters (SWEETs), a novel group of sugar transporters that have various physiological functions, including phloem loading, seed filling, nectar secretion, and fruit development. However, the role of SWEETs in sugar accumulation in pitaya fruit is not yet clear. Here, we identified 19 potential members (HuSWEET genes) of the SWEET family in pitaya and analyzed their conserved motifs, physiochemical characteristics, chromosomal distribution, gene structure, and phylogenetic relationship. Seven highly conserved α-helical transmembrane domains (7-TMs) were found, and the HuSWEET proteins can be divided into three clades based on the phylogenetic analysis. Interestingly, we found two HuSWEET genes, HuSWEET12a and HuSWEET13d, that showed strong preferential expressions in fruits and an upward trend during fruit maturation, suggesting they have key roles in sugar accumulation in pitaya. This can be further roughly demonstrated by the fact that transgenic tomato plants overexpressing HuSWEET12a/13d accumulated high levels of sugar in the mature fruit. Together, our result provides new insights into the regulation of sugar accumulation by SWEET family genes in pitaya fruit, which also set a crucial basis for the further functional study of the HuSWEETs.

Camellia oleifera CoSWEET10 Is Crucial for Seed Development and Drought Resistance by Mediating Sugar Transport in Transgenic Arabidopsis

Sugar transport from the source leaf to the sink organ is critical for seed development and crop yield, as well as for responding to abiotic stress. SWEETs (sugar will eventually be exported transporters) mediate sugar efflux into the reproductive sink and are therefore considered key candidate proteins for sugar unloading during seed development. However, the specific mechanism underlying the sugar unloading to seeds in Camellia oleifera remains elusive. Here, we identified a SWEET gene named CoSWEET10, which belongs to Clade III and has high expression levels in the seeds of C. oleifera. CoSWEET10 is a plasma membrane-localized protein. The complementation assay of CoSWEET10 in SUSY7/ura3 and EBY.VW4000 yeast strains showed that CoSWEET10 has the ability to transport sucrose, glucose, and fructose. Through the C. oleifera seeds in vitro culture, we found that the expression of CoSWEET10 can be induced by hexose and sucrose, and especially glucose. By generating the restoration lines of CoSWEET10 in Arabidopsis atsweet10, we found that CoSWEET10 restored the seed defect phenotype of the mutant by regulating soluble sugar accumulation and increased plant drought tolerance. Collectively, our study demonstrates that CoSWEET10 plays a dual role in promoting seed development and enhancing plant drought resistance as a sucrose and hexose transporter.

Abscisic acid and regulation of the sugar transporter gene MdSWEET9b promote apple sugar accumulation

Enhancing fruit sugar contents, especially for high-flavonoid apples with a sour taste, is one of the main goals of horticultural crop breeders. This study analyzed sugar accumulation and the underlying mechanisms in the F2 progenies of a hybridization between the high-sugar apple (Malus × domestica) variety "Gala" and high-flavonoid apple germplasm "CSR6R6". We revealed that MdSWEET9b (sugars will eventually be exported transporter) helps mediate sugar accumulation in fruits. Functional characterization of MdSWEET9b in yeast mutants lacking sugar transport as well as in overexpressing and CRISPR/Cas9 knockdown apple calli revealed MdSWEET9b could transport sucrose specifically, ultimately promoting normal yeast growth and accumulation of total sugar contents. Moreover, MdWRKY9 bound to the MdSWEET9b promoter and regulated its activity, which responded to abscisic acid (ABA) signaling. Furthermore, MdWRKY9 interacted with MdbZIP23 (basic leucine zipper) and MdbZIP46, key ABA signal transducers, at the protein and DNA levels to enhance its regulatory effect on MdSWEET9b expression, thereby influencing sugar accumulation. Based on the contents of ABA in lines with differing sugar contents and the effects of ABA treatments on fruits and calli, we revealed ABA as one of the main factors responsible for the diversity in apple fruit sugar content. The results of this study have clarified how MdSWEET9b influences fruit sugar accumulation, while also further elucidating the regulatory effects of the ABA-signaling network on fruit sugar accumulation. This work provides a basis for future explorations of the crosstalk between hormone and sugar metabolism pathways.

Physiological implications of SWEETs in plants and their potential applications in improving source-sink relationships for enhanced yield

The sugars will eventually be exported transporters (SWEET) family of transporters in plants is identified as a novel class of sugar carriers capable of transporting sugars, sugar alcohols and hormones. Functioning in intercellular sugar transport, SWEETs influence a wide range of physiologically important processes. SWEETs regulate the development of sink organs by providing nutritional support from source leaves, responses to abiotic stresses by maintaining intracellular sugar concentrations, and host-pathogen interactions through the modulation of apoplastic sugar levels. Many bacterial and fungal pathogens activate the expression of SWEET genes in species such as rice and Arabidopsis to gain access to the nutrients that support virulence. The genetic manipulation of SWEETs has led to the generation of bacterial blight (BB)-resistant rice varieties. Similarly, while the overexpression of the SWEETs involved in sucrose export from leaves and pathogenesis led to growth retardation and yield penalties, plants overexpressing SWEETs show improved disease resistance. Such findings demonstrate the complex functions of SWEETs in growth and stress tolerance. Here, we review the importance of SWEETs in plant-pathogen and source-sink interactions and abiotic stress resistance. We highlight the possible applications of SWEETs in crop improvement programmes aimed at improving sink and source strengths important for enhancing the sustainability of yield. We discuss how the adverse effects of the overexpression of SWEETs on plant growth may be overcome.

Solid-state NMR 13C and 15 N resonance assignments of Vibrio sp. SemiSWEET transporter in lipid bilayers

The Sugar Will Eventually be Exported Transporter (SWEET) family is a new class of transporters that plays crucial roles in the cellular sugar transport process. Their bacterial homologs, known as SemiSWEETs, are among the smallest transporters and can be used as a prototype for studying the biological properties of sugar transporters. Here, a set of dipolar-based multidimensional solid-state NMR spectra were employed to investigate the structure of Vibrio sp. SemiSWEET (Vs-SemiSWEET) reconstituted in the native-like lipid bilayers. A nearly complete (88% of the amino acid residues) backbone and side-chain ¹³C and ¹⁵ N chemical shift assignments of Vs-SemiSWEET were obtained. The overall secondary structure of Vs-SemiSWEET predicted from the obtained ¹³C and ¹⁵ N chemical shifts is similar to that from X-ray crystallography, with some differences, reflecting the influence of the membrane environments to the structure of membrane proteins.

Phylogenetic analysis and structural prediction reveal the potential functional diversity between green algae SWEET transporters

Sugar-Will-Eventually-be-Exported-Transporters (SWEETs) are an important family of sugar transporters that appear to be ubiquitous in all organisms. Recent research has determined the structure of SWEETs in higher plants, identified specific residues required for monosaccharide or disaccharide transport, and begun to understand the specific functions of individual plant SWEET proteins. However, in green algae (Chlorophyta) these transporters are poorly characterised. This study identified SWEET proteins from across representative Chlorophyta with the aim to characterise their phylogenetic relationships and perform protein structure modelling in order to inform functional prediction. The algal genomes analysed encoded between one and six SWEET proteins, which is much less than a typical higher plant. Phylogenetic analysis identified distinct clusters of over 70 SWEET protein sequences, taken from almost 30 algal genomes. These clusters remain separate from representative higher or non-vascular plant SWEETs, but are close to fungi SWEETs. Subcellular localisation predictions and analysis of conserved amino acid residues revealed variation between SWEET proteins of different clusters, suggesting different functionality. These findings also showed conservation of key residues at the substrate-binding site, indicating a similar mechanism of substrate selectivity and transport to previously characterised higher plant monosaccharide-transporting SWEET proteins. Future work is now required to confirm the predicted sugar transport specificity and determine the functional role of these algal SWEET proteins.

Characterization of the SWEET Gene Family in Longan (Dimocarpus longan) and the Role of DlSWEET1 in Cold Tolerance

Sugars will eventually be exported transporters (SWEET), a group of relatively novel sugar transporters, that play important roles in phloem loading, seed and fruit development, pollen development, and stress response in plants. Longan (Dimocarpus longan), a subtropic fruit tree with high economic value, is sensitive to cold. However, whether the SWEET gene family plays a role in conferring cold tolerance upon longan remains unknown. Here, a total of 20 longan SWEET (DlSWEET) genes were identified, and their phylogenetic relationships, gene structures, cis-acting elements, and tissue-specific expression patterns were systematically analyzed. This family is divided into four clades. Gene structures and motifs analyses indicated that the majority of DlSWEETs in each clade shared similar exon–intron organization and conserved motifs. Tissue-specific gene expression suggested diverse possible functions for DlSWEET genes. Cis-elements analysis and quantitative real-time PCR (qRT-PCR) analysis revealed that DlSWEET1 responded to cold stress. Notably, the overexpression of DlSWEET1 improved cold tolerance in transgenic Arabidopsis, suggesting that DlSWEET1 might play a positive role in D. longan’s responses to cold stress. Together, these results contribute to a better understanding of SWEET genes, which could serve as a foundation for the further functional identification of these genes.

When SWEETs Turn Tweens: Updates and Perspectives

Sugar translocation between cells and between subcellular compartments in plants requires either plasmodesmata or a diverse array of sugar transporters. Interactions between plants and associated microorganisms also depend on sugar transporters. The sugars will eventually be exported transporter (SWEET) family is made up of conserved and essential transporters involved in many critical biological processes. The functional significance and small size of these proteins have motivated crystallographers to successfully capture several structures of SWEETs and their bacterial homologs in different conformations. These studies together with molecular dynamics simulations have provided unprecedented insights into sugar transport mechanisms in general and into substrate recognition of glucose and sucrose in particular. This review summarizes our current understanding of the SWEET family, from the atomic to the whole-plant level. We cover methods used for their characterization, theories about their evolutionary origins, biochemical properties, physiological functions, and regulation. We also include perspectives on the future work needed to translate basic research into higher crop yields.

Sugar Transporters in Plasmodiophora brassicae: Genome-Wide Identification and Functional Verification

Plasmodiophora brassicae, an obligate intracellular pathogen, can hijack the host’s carbohydrates for survival. When the host plant is infected by P. brassicae, a large amount of soluble sugar accumulates in the roots, especially glucose, which probably facilitates the development of this pathogen. Although a complete glycolytic and tricarboxylic acid cycle (TCA) cycle existed in P. brassicae, very little information about the hexose transport system has been reported. In this study, we screened 17 putative sugar transporters based on information about their typical domains. The structure of these transporters showed a lot of variation compared with that of other organisms, especially the number of transmembrane helices (TMHs). Phylogenetic analysis indicated that these sugar transporters were far from the evolutionary relationship of other organisms and were unique in P. brassicae. The hexose transport activity assay indicated that eight transporters transported glucose or fructose and could restore the growth of yeast strain EBY.VW4000, which was deficient in hexose transport. The expression level of these glucose transporters was significantly upregulated at the late inoculation time when resting spores and galls were developing and a large amount of energy was needed. Our study provides new insights into the mechanism of P. brassicae survival in host cells by hijacking and utilizing the carbohydrates of the host.

From Functional Characterization to the Application of SWEET Sugar Transporters in Plant Resistance Breeding

The occurrence of plant diseases severely affects the quality and quantity of plant production. Plants adapt to the constant invasion of pathogens and gradually form a series of defense mechanisms, such as pathogen-associated molecular pattern-triggered immunity and microbial effector-triggered immunity. Moreover, many pathogens have evolved to inhibit the immune defense system and acquire plant nutrients as a result of their coevolution with plants. The sugars will eventually be exported transporters (SWEETs) are a novel family of sugar transporters that function as uniporters. They provide a channel for pathogens, including bacteria, fungi, and viruses, to hijack sugar from the host. In this review, we summarize the functions of SWEETs in nectar secretion, grain loading, senescence, and long-distance transport. We also focus on the interaction between the SWEET genes and pathogens. In addition, we provide insight into the potential application of SWEET genes to enhance disease resistance through the use of genome editing tools. The summary and perspective of this review will deepen our understanding of the role of SWEETs during the process of pathogen infection and provide insights into resistance breeding.

Genome-wide identification of the SWEET gene family mediating the cold stress response in Prunus mume

The Sugars Will Eventually be Exported Transporter (SWEET) gene family encodes a family of sugar transporters that play essential roles in plant growth, reproduction, and biotic and abiotic stresses. Prunus mume is a considerable ornamental wood plant with high edible and medicinal values; however, its lack of tolerance to low temperature has severely limited its geographical distribution. To investigate whether this gene family mediates the response of P. mume to cold stress, we identified that the P. mume gene family consists of 17 members and divided the family members into four groups. Sixteen of these genes were anchored on six chromosomes, and one gene was anchored on the scaffold with four pairs of segmental gene duplications and two pairs of tandem gene duplications. Cis-acting regulatory element analysis indicated that the PmSWEET genes are potentially involved in P. mume development, including potentially regulating roles in procedure, such as circadian control, abscisic acid-response and light-response, and responses to numerous stresses, such as low-temperature and drought. We performed low-temperature treatment in the cold-tolerant cultivar 'Songchun' and cold-sensitive cultivar 'Zaolve' and found that the expression of four of 17 PmSWEETs was either upregulated or downregulated with prolonged treatment times. This finding indicates that these family members may potentially play a role in cold stress responses in P. mume. Our study provides a basis for further investigation of the role of SWEET proteins in the development of P. mume and its responses to cold stress.

Insights into the gene and protein structures of the CaSWEET family members in chickpea (Cicer arietinum), and their gene expression patterns in different organs under various stress and abscisic acid treatments

'Sugars Will Eventually be Exported Transporters' (SWEETs) are a group of sugar transporters that play crucial roles in various biological processes, particularly plant stress responses. However, no information is available yet for the CaSWEET family in chickpea. Here, we identified all putative CaSWEET members in chickpea, and obtained their major characteristics, including physicochemical patterns, chromosomal distribution, subcellular localization, gene organization, conserved motifs and three-dimensional protein structures. Subsequently, we explored available transcriptome data to compare spatiotemporal transcript abundance of CaSWEET genes in various major organs. Finally, we studied the changes in their transcript levels in leaves and/or roots following dehydration and exogenous abscisic acid treatments using RT-qPCR to obtain valuable information underlying their potential roles in chickpea responses to water-stress conditions. Our results provide the first insights into the characteristics of the CaSWEET family members and a foundation for further functional characterizations of selected candidate genes for genetic engineering of chickpea.

Expression Patterns of Sugar Transporter Genes in the Allocation of Assimilates and Abiotic Stress in Lily

During the growth cycle of lilies, assimilates undergo a process of accumulation, consumption and reaccumulation in bulbs and are transported and allocated between aboveground and underground organs and tissues. The sink-source relationship changes with the allocation of assimilates, affecting the vegetative growth and morphological establishment of lilies. In this study, the carbohydrate contents in different tissues of five critical stages during lily development were measured to observe the assimilates allocation. The results showed bulbs acted as the main source to provide energy before the budding stage (S3); after the flowering stage (S4), bulbs began to accumulate assimilates as a sink organ again. During the period when the plant height was 30cm with leaf-spread (S2), leaves mainly accumulated assimilates from bulbs through the symplastic pathway, while when leaves were fully expanded, it transformed to export carbohydrates. At the S4 stage, flowers became a new active sink with assimilates influx. To further understand the allocation of assimilates, 16 genes related to sugar transport and metabolism (ST genes) were identified and categorized into different subfamilies based on the phylogenetic analysis, and their protein physicochemical properties were also predicted. Tissue-specific analysis showed that most of the genes were highly expressed in stems and petals, and it was mainly the MST (monosaccharide transporter) genes that were obviously expressed in petals during the S4 stage, suggesting that they may be associated with the accumulation of carbohydrates in flowers and thus affect flower development process. LoSWEET14 (the Sugar will eventually be exported transporters) was significantly correlated with starch in scales and with soluble sugar in leaves. Sugar transporters LoHXT6 and LoSUT1 were significantly correlated with soluble sugar and sucrose in leaves, suggesting that these genes may play key roles in the accumulation and transportation of assimilates in lilies. In addition, we analyzed the expression patterns of ST genes under different abiotic stresses, and the results showed that all genes were significantly upregulated. This study lays a solid foundation for further research on molecular mechanism of sink-source change and response to abiotic stresses in lilies.

Emerging Roles of SWEET Sugar Transporters in Plant Development and Abiotic Stress Responses

Sugars are the major source of energy in living organisms and play important roles in osmotic regulation, cell signaling and energy storage. SWEETs (Sugars Will Eventually be Exported Transporters) are the most recent family of sugar transporters that function as uniporters, facilitating the diffusion of sugar molecules across cell membranes. In plants, SWEETs play roles in multiple physiological processes including phloem loading, senescence, pollen nutrition, grain filling, nectar secretion, abiotic (drought, heat, cold, and salinity) and biotic stress regulation. In this review, we summarized the role of SWEET transporters in plant development and abiotic stress. The gene expression dynamics of various SWEET transporters under various abiotic stresses in different plant species are also discussed. Finally, we discuss the utilization of genome editing tools (TALENs and CRISPR/Cas9) to engineer SWEET genes that can facilitate trait improvement. Overall, recent advancements on SWEETs are highlighted, which could be used for crop trait improvement and abiotic stress tolerance.

Sugar transporters and their molecular tradeoffs during abiotic stress responses in plants

Sugars as photosynthates are well known as energy providers and as building blocks of various structural components of plant cells, tissues and organs. Additionally, as a part of various sugar signaling pathways, they interact with other cellular machinery and influence many important cellular decisions in plants. Sugar signaling is further reliant on the differential distribution of sugars throughout the plant system. The distribution of sugars from source to sink tissues or within organelles of plant cells is a highly regulated process facilitated by various sugar transporters located in plasma membranes and organelle membranes, respectively. Sugar distribution, as well as signaling, is impacted during unfavorable environments such as extreme temperatures, salt, nutrient scarcity, or drought. Here, we have discussed the mechanism of sugar transport via various types of sugar transporters as well as their differential response during environmental stress exposure. The functional involvement of sugar transporters in plant's abiotic stress tolerance is also discussed. Besides, we have also highlighted the challenges in engineering sugar transporter proteins as well as the undeciphered modules associated with sugar transporters in plants. Thus, this review provides a comprehensive discussion on the role and regulation of sugar transporters during abiotic stresses and enables us to target the candidate sugar transporter(s) for crop improvement to develop climate-resilient crops.

Understanding the role of SWEET genes in fruit development and abiotic stress in pomegranate (Punica granatum L.)

BACKGROUND

The Sugar Will Eventually Be Exported Transporters (SWEET), consisting of the MtN3 and salvia domain, are sugar transporters having an active role in diverse activities in plants such as pollen nutrition, phloem loading, nectar secretion, reproductive tissue development, and plant-pathogen interaction. The SWEET genes have been characterized only in a few fruit crop species.

METHODS AND RESULTS

In this study, a total of 15 SWEET genes were identified in the pomegranate (Punica granatum) genome. The gene structure, transmembrane (TM) helices, domain architecture, and phylogenetic relationships of these genes were evaluated using computational approaches. Genes were further classified as Semi-SWEETs or SWEETs based on the TM domains. Similarly, pomegranate, Arabidopsis, rice, and soybean SWEETs were studied together to classify into major groups. In addition, analysis of RNAseq transcriptome data was performed to study SWEEET gene expression dynamics in different tissue. The expression suggests that SWEETs are mostly expressed in pomegranate peel. In addition, PgSWEET13 was found to be differentially expressed under high salinity stress in pomegranate. Further, quantitative PCR analysis confirmed the expression of four candidate genes in leaf and stem tissues.

CONCLUSION

The information provided here will help to understand the role of SWEET genes in fruit development and under abiotic stress conditions in pomegranate.

Sugar transporters of the SWEET family and their role in arbuscular mycorrhiza

Plant sugar transporters play an essential role in the organism’s productivity by carrying out carbohydrate transportation from source cells in the leaves to sink cells in the cortex. In addition, they aid in the regulation of a substantial part of the exchange of nutrients with microorganisms in the rhizosphere (bacteria and fungi), an ty essential to the formation of symbiotic relationships. This review pays special attention to carbohydrate nutrition during the development of arbuscular mycorrhiza (AM), a symbiosis of plants with fungi from the Glomeromycotina subdivision. This relationship results in the host plant receiving micronutrients from the mycosymbiont, mainly phosphorus, and the fungus receiving carbon assimilation products in return. While the eff icient nutrient transport pathways in AM symbiosis are yet to be discovered, SWEET sugar transporters are one of the three key families of plant carbohydrate transporters. Specif ic AM symbiosis transporters can be identif ied among the SWEET proteins. The survey provides data on the study history, structure and localization, phylogeny and functions of the SWEET proteins. A high variability of both the SWEET proteins themselves and their functions is noted along with the fact that the same proteins may perform different functions in different plants. A special role is given to the SWEET transporters in AM development. SWEET transporters can also play a key role in abiotic stress tolerance, thus allowing plants to adapt to adverse environmental conditions. The development of knowledge about symbiotic systems will contribute to the creation of microbial preparations for use in agriculture in the Russian Federation.

Glycosylation of SARS-CoV-2: structural and functional insights

The COVID-19 pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Similar to other coronaviruses, its particles are composed of four structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins. S, E, and M proteins are glycosylated, and the N protein is phosphorylated. The S protein is involved in the interaction with the host receptor human angiotensin-converting enzyme 2 (hACE2), which is also heavily glycosylated. Recent studies have revealed several other potential host receptors or factors that can increase or modulate the SARS-CoV-2 infection. Interestingly, most of these molecules bear carbohydrate residues. While glycans acquired by the viruses through the hijacking of the host machinery help the viruses in their infectivity, they also play roles in immune evasion or modulation. Glycans play complex roles in viral pathobiology, both on their own and in association with carrier biomolecules, such as proteins or glycosaminoglycans (GAGs). Understanding these roles in detail can help in developing suitable strategies for prevention and therapy of COVID-19. In this review, we sought to emphasize the interplay of SARS-CoV-2 glycosylated proteins and their host receptors in viral attachment, entry, replication, and infection. Moreover, the implications for future therapeutic interventions targeting these glycosylated biomolecules are also discussed in detail.

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

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

Identification of novel membrane proteins for improved lignocellulose conversion

Lignocellulose processing yields a heterogeneous mixture of substances, which are poorly utilized by current industrial strains. For efficient valorization of recalcitrant biomass, it is critical to identify and engineer new membrane proteins that enable the broad uptake of hydrolyzed substrates. Whereas glucose consumption rarely presents a bottleneck for cell factories, there is also a lack of transporters that allow co-consumption of glucose with other abundant biomass sugars such as xylose. This review discusses recent efforts to bioinformatically identify membrane proteins of high biotech potential for lignocellulose conversion and metabolic engineering in both model and nonconventional organisms. Of particular interest are transporters sourced from anaerobic gut fungi resident to large herbivores, which produce Sugars Will Eventually be Exported Transporters (SWEETs) that enhance xylose transport in the yeast Saccharomyces cerevisiae and enable glucose and xylose co-utilization. Additionally, recently identified fungal cellodextrin transporters are valuable alternatives to mitigate glucose repression and transporter inhibition.

Role of Placental Glucose Transporters in Determining Fetal Growth

Maternal nutrient availability and its transport through the placenta are crucial for fetal development. Nutrients are transported to the fetus via specific transporters present on the microvillous (MVM) and basal membrane (BM) of the placenta. Glucose is the most abundant nutrient transferred to the fetus and plays a key role in the fetal growth and development. The transfer of glucose across the human placenta is directly proportional to maternal glucose concentrations, and is mediated by glucose transporter family proteins (GLUTs). Maternal glucose concentration influences expression and activity of GLUTs in the MVM (glucose uptake) and BM (glucose delivery). Alteration in the number and function of these transporters may affect the growth and body composition of the fetus. The thin-fat phenotype of the Indian baby (low ponderal index, high adiposity) is proposed as a harbinger of future metabolic risk. We propose that placental function mediated through nutrient transporters contributes to the phenotype of the baby, specifically that glucose transporters will influence neonatal fat. This review discusses the role of various glucose transporters in the placenta in determining fetal growth and body composition, in light of the above hypothesis.

Plant SWEETs: from sugar transport to plant-pathogen interaction and more unexpected physiological roles

Sugars Will Eventually be Exported Transporters (SWEETs) have important roles in numerous physiological mechanisms where sugar efflux is critical, including phloem loading, nectar secretion, seed nutrient filling, among other less expected functions. They mediate low affinity and high capacity transport, and in angiosperms this family is composed by 20 paralogs on average. As SWEETs facilitate the efflux of sugars, they are highly susceptible to hijacking by pathogens, making them central players in plant-pathogen interaction. For instance, several species from the Xanthomonas genus are able to upregulate the transcription of SWEET transporters in rice (Oryza sativa), upon the secretion of transcription-activator-like effectors. Other pathogens, such as Botrytis cinerea or Erysiphe necator, are also capable of increasing SWEET expression. However, the opposite behavior has been observed in some cases, as overexpression of the tonoplast AtSWEET2 during Pythium irregulare infection restricted sugar availability to the pathogen, rendering plants more resistant. Therefore, a clear-cut role for SWEET transporters during plant-pathogen interactions has so far been difficult to define, as the metabolic signatures and their regulatory nodes, which decide the susceptibility or resistance responses, remain poorly understood. This fuels the still ongoing scientific question: what roles can SWEETs play during plant-pathogen interaction? Likewise, the roles of SWEET transporters in response to abiotic stresses are little understood. Here, in addition to their relevance in biotic stress, we also provide a small glimpse of SWEETs importance during plant abiotic stress, and briefly debate their importance in the particular case of grapevine (Vitis vinifera) due to its socioeconomic impact.

A SWEET surprise: Anaerobic fungal sugar transporters and chimeras enhance sugar uptake in yeast

In the yeast Saccharomyces cerevisiae, microbial fuels and chemicals production on lignocellulosic hydrolysates is constrained by poor sugar transport. For biotechnological applications, it is desirable to source transporters with novel or enhanced function from nonconventional organisms in complement to engineering known transporters. Here, we identified and functionally screened genes from three strains of early-branching anaerobic fungi (Neocallimastigomycota) that encode sugar transporters from the recently discovered Sugars Will Eventually be Exported Transporter (SWEET) superfamily in Saccharomyces cerevisiae. A novel fungal SWEET, NcSWEET1, was identified that localized to the plasma membrane and complemented growth in a hexose transporter-deficient yeast strain. Single cross-over chimeras were constructed from a leading NcSWEET1 expression-enabling domain paired with all other candidate SWEETs to broadly scan the sequence and functional space for enhanced variants. This led to the identification of a chimera, NcSW1/PfSW2:TM5-7, that enhanced the growth rate significantly on glucose, fructose, and mannose. Additional chimeras with varied cross-over junctions identified residues in TM1 that affect substrate selectivity. Furthermore, we demonstrate that NcSWEET1 and the enhanced NcSW1/PfSW2:TM5-7 variant facilitated novel co-consumption of glucose and xylose in S. cerevisiae. NcSWEET1 utilized 40.1% of both sugars, exceeding the 17.3% utilization demonstrated by the control HXT7(F79S) strain. Our results suggest that SWEETs from anaerobic fungi are beneficial tools for enhancing glucose and xylose co-utilization and offers a promising step towards biotechnological application of SWEETs in S. cerevisiae.

Role of transporters in plant disease resistance

Plants being sessile have evolved numerous mechanisms to meet the changing environmental and growth conditions. Plant pathogens are responsible for devastating disease epidemics in many species. Transporter proteins are an integral part of plant growth and development, and several studies have documented their role in pathogen disease resistance. In this review, we analyze the studies on genome-wide identifications of plant transporters like sugars will eventually be exported transporters (SWEET), multidrug and toxic compound extrusion (MATE) transporters, ATP-binding cassette (ABC) transporters, natural resistance-associated macrophage proteins (NRAMP), and sugar transport proteins (STPs), all having a significant role in plant disease resistance. The mechanism of action of these transporters, their solute specificity, and the potential application of recent molecular biology approaches deploying these transporters for the development of disease-resistant plants are also discussed. The applications of genome editing tools, such as CRIPSR/Cas9, are also presented. Altogether the information included in this article gives a better understanding of the role of transporter proteins during plant-pathogen interaction.

SWEET Transporters and the Potential Functions of These Sequences in Tea (Camellia sinensis)

Tea (Camellia sinensis) is an important economic beverage crop. Its flowers and leaves could be used as healthcare tea for its medicinal value. SWEET proteins were recently identified in plants as sugar transporters, which participate in diverse physiological processes, including pathogen nutrition, seed filling, nectar secretion, and phloem loading. Although SWEET genes have been characterized and identified in model plants, such as Arabidopsis thaliana and Oryza sativa, there is very little knowledge of these genes in C. sinensis. In this study, 28 CsSWEETs were identified in C. sinensis and further phylogenetically divided into four subfamilies with A. thaliana. These identified CsSWEETs contained seven transmembrane helixes (TMHs) which were generated by an ancestral three-TMH unit with an internal duplication experience. Microsynteny analysis revealed that the large-scale duplication events were the main driving forces for members from CsSWEET family expansion in C. sinensis. The expression profiles of the 28 CsSWEETs revealed that some genes were highly expressed in reproductive tissues. Among them, CsSWEET1a might play crucial roles in the efflux of sucrose, and CsSWEET17b could control fructose content as a hexose transporter in C. sinensis. Remarkably, CsSWEET12 and CsSWEET17c were specifically expressed in flowers, indicating that these two genes might be involved in sugar transport during flower development. The expression patterns of all CsSWEETs were differentially regulated under cold and drought treatments. This work provided a systematic understanding of the members from the CsSWEET gene family, which would be helpful for further functional studies of CsSWEETs in C. sinensis.

Genome-wide association study reveals new genes involved in leaf trichome formation in polyploid oilseed rape (Brassica napus L.)

Leaf trichomes protect against various biotic and abiotic stresses in plants. However, there is little knowledge about this trait in oilseed rape (Brassica napus). Here, we demonstrated that hairy leaves were less attractive to Plutella xylostella larvae than glabrous leaves. We established a core germplasm collection with 290 accessions for a genome-wide association study (GWAS) of the leaf trichome trait in oilseed rape. We compared the transcriptomes of the shoot apical meristem (SAM) between hairy- and glabrous-leaf genotypes to narrow down the candidate genes identified by GWAS. The single nucleotide polymorphisms and the different transcript levels of BnaA.GL1.a, BnaC.SWEET4.a, BnaC.WAT1.a and BnaC.WAT1.b corresponded to the divergence of the hairy- and glabrous-leaf phenotypes, indicating the role of sugar and/or auxin signalling in leaf trichome initiation. The hairy-leaf SAMs had lower glucose and sucrose contents but higher expression of putative auxin responsive factors than the glabrous-leaf SAMs. Spraying of exogenous auxin (8 μm) increased leaf trichome number in certain genotypes, whereas spraying of sucrose (1%) plus glucose (6%) slightly repressed leaf trichome initiation. These data contribute to the existing knowledge about the genetic control of leaf trichomes and would assist breeding towards the desired leaf surface type in oilseed rape.

Genome-Wide Profiling and Phylogenetic Analysis of the SWEET Sugar Transporter Gene Family in Walnut and Their Lack of Responsiveness to Xanthomonas arboricola pv. juglandis Infection

Following photosynthesis, sucrose is translocated to sink organs, where it provides the primary source of carbon and energy to sustain plant growth and development. Sugar transporters from the SWEET (sugar will eventually be exported transporter) family are rate-limiting factors that mediate sucrose transport across concentration gradients, sustain yields, and participate in reproductive development, plant senescence, stress responses, as well as support plant-pathogen interaction, the focus of this study. We identified 25 SWEET genes in the walnut genome and distinguished each by its individual gene structure and pattern of expression in different walnut tissues. Their chromosomal locations, cis-acting motifs within their 5' regulatory elements, and phylogenetic relationship patterns provided the first comprehensive analysis of the SWEET gene family of sugar transporters in walnut. This family is divided into four clades, the analysis of which suggests duplication and expansion of the SWEET gene family in Juglans regia. In addition, tissue-specific gene expression signatures suggest diverse possible functions for JrSWEET genes. Although these are commonly used by pathogens to harness sugar products from their plant hosts, little was known about their role during Xanthomonas arboricola pv. juglandis (Xaj) infection. We monitored the expression profiles of the JrSWEET genes in different tissues of "Chandler" walnuts when challenged with pathogen Xaj417 and concluded that SWEET-mediated sugar translocation from the host is not a trigger for walnut blight disease development. This may be directly related to the absence of type III secretion system-dependent transcription activator-like effectors (TALEs) in Xaj417, which suggests different strategies are employed by this pathogen to promote susceptibility to this major aboveground disease of walnuts.

Genome-wide identification and expression analysis of SWEET gene family in Litchi chinensis reveal the involvement of LcSWEET2a/3b in early seed development

BACKGROUND

SWEETs (Sugar Will Eventually be Exported transporters) function as sugar efflux transporters that perform diverse physiological functions, including phloem loading, nectar secretion, seed filling, and pathogen nutrition. The SWEET gene family has been identified and characterized in a number of plant species, but little is known about in Litchi chinensis, which is an important evergreen fruit crop.

RESULTS

In this study, 16 LcSWEET genes were identified and nominated according to its homologous genes in Arabidopsis and grapevine. Multiple sequence alignment showed that the 7 alpha-helical transmembrane domains (7-TMs) were basically conserved in LcSWEETs. The LcSWEETs were divided into four clades (Clade I to Clade IV) by phylogenetic tree analysis. A total of 8 predicted motifs were detected in the litchi LcSWEET genes. The 16 LcSWEET genes were unevenly distributed in 9 chromosomes and there was one pairs of segmental duplicated events by synteny analysis. The expression patterns of the 16 LcSWEET genes showed higher expression levels in reproductive organs. The temporal and spatial expression patterns of LcSWEET2a and LcSWEET3b indicated they play central roles during early seed development.

CONCLUSIONS

The litchi genome contained 16 SWEET genes, and most of the genes were expressed in different tissues. Gene expression suggested that LcSWEETs played important roles in the growth and development of litchi fruits. Genes that regulate early seed development were preliminarily identified. This work provides a comprehensive understanding of the SWEET gene family in litchi, laying a strong foundation for further functional studies of LcSWEET genes and improvement of litchi fruits.

SWEET Gene Family in Medicago truncatula: Genome-Wide Identification, Expression and Substrate Specificity Analysis

SWEET (Sugars Will Eventually be Exported Transporter) proteins mediate the translocation of sugars across cell membranes and play crucial roles in plant growth and development as well as stress responses. In this study, a total of 25 SWEET genes were identified from the Medicago truncatula genome and were divided into four clades based on the phylogenetic analysis. The MtSWEET genes are distributed unevenly on the M. truncatula chromosomes, and eight and 12 MtSWEET genes are segmentally and tandemly duplicated, respectively. Most MtSWEET genes contain five introns and encode proteins with seven transmembrane helices (TMHs). Besides, nearly all MtSWEET proteins have relatively conserved membrane domains, and contain conserved active sites. Analysis of microarray data showed that some MtSWEET genes are specifically expressed in disparate developmental stages or tissues, such as flowers, developing seeds and nodules. RNA-seq and qRT-PCR expression analysis indicated that many MtSWEET genes are responsive to various abiotic stresses such as cold, drought, and salt treatments. Functional analysis of six selected MtSWEETs in yeast revealed that they possess diverse transport activities for sucrose, fructose, glucose, galactose, and mannose. These results provide new insights into the characteristics of the MtSWEET genes, which lay a solid foundation for further investigating their functional roles in the developmental processes and stress responses of M. truncatula.

Cloning and Functional Assessments of Floral-Expressed SWEET Transporter Genes from Jasminum sambac

Sugar transporters of the SWEET family mediate cross membrane movement of mono- and disaccharides and play vital roles in diverse physiological and pathophysiological processes, including sink-source relationship, pathogen responses, reproductive growth, and development. However, it remains to be determined how these transporters function in non-module plants of agricultural significance, given the evolutionarily diverse traits. In this study, we combined transcriptome analysis, rapid amplification of cDNA ends-cloning (RACE-cloning), expression profiling, and heterologous functional assay to identify SWEET genes that may have potential roles during flower opening and sexual reproduction in Jasminum sambac . During the anthesis, the floral organs of J. sambac express seven SWEET homologous genes from all four clades of the family. JsSWEET9 and 2 are significantly upregulated when flowers are fully opened, up to 6- and 3-fold compared to unopened buds, respectively. The other transporters, JsSWEET1, 5, 10, and 17 are also accumulated slightly at stage associated with fragrance release, whereas only the vacuole transporter JsSWEET16 showed small decrease in transcript level after anthesis. The JsSWEET5, a clade II member, is capable to complement yeast cell uptake on most tested sugar substrates with a preference for hexoses, while the clade I transporter JsSWEET1 mediates merely galactose import when expressed in yeast. Our results provide first evidence for further investigation on sugar transport and allocation during flowering and reproductive processes in J. sambac.

Structure, evolution and diverse physiological roles of SWEET sugar transporters in plants

Present review describes the structure, evolution, transport mechanism and physiological functions of SWEETs. Their application using TALENs and CRISPR/CAS9 based genomic editing approach is discussed. Sugars Will Eventually be Exported Transporters (SWEET) proteins were first identified in plants as the novel family of sugar transporters which mediates the translocation of sugars across cell membranes. The SWEET family of sugar transporters is unique in terms of their structure which contains seven predicted transmembrane domains with two internal triple-helix bundles which possibly originate due to prokaryotic gene duplication. SWEETs perform diverse physiological functions such as pollen nutrition, nectar secretion, seed filling, phloem loading, and pathogen nutrition which we have discussed in the present review. We also discuss how transcriptional activator-like effector nucleases (TALENs) and CRISPR/CAS9 genome editing tools are used to engineer SWEET mutants which modulate pathogen resistance in plants and its applications in the field of agriculture. The expression of SWEETs promises to implement insights into many other cellular transport mechanisms. To conclude, the present review highlights the recent aspects which will further develop better understanding of molecular evolution, structure, and function of SWEET transporters in plants.

Molecular Basis of the Glucose Transport Mechanism in Plants

The SWEET family belongs to a class of transporters in plants that undergoes large conformational changes to facilitate transport of sugar molecules across the cell membrane (SWEET, Sugars Will Eventually Be Exported Transporter). However, the structures of their functionally relevant conformational states in the transport cycle have not been reported. In this study, we have characterized the conformational dynamics and complete transport cycle of glucose in the OsSWEET2b transporter using extensive molecular dynamics simulations. Using Markov state models, we estimated the free energy barrier associated with different states as well as for the glucose transport mechanism. SWEETs undergo a structural transition to outward-facing (OF), occluded (OC), and inward-facing (IF) and strongly support an alternate access transport mechanism. The glucose diffuses freely from outside to inside the cell without causing major conformational changes which means that the conformations of glucose unbound and bound snapshots are exactly the same for OF, OC, and IF states. We identified a network of hydrophobic core residues at the center of the transporter that restricts the glucose entry to the cytoplasmic side and acts as an intracellular hydrophobic gate. The mechanistic predictions from molecular dynamics simulations are validated using site-directed mutagenesis experiments. Our simulation also revealed hourglass-like intermediate states making the pore radius narrower at the center. This work provides new fundamental insights into how substrate-transporter interactions actively change the free energy landscape of the transport cycle to facilitate enhanced transport activity.

Further studies on sugar transporter (SWEET) genes in wheat (Triticum aestivum L.)

SWEET proteins represent one of the largest sugar transporter family in the plant kingdom and play crucial roles in plant development and stress responses. In the present study, a total of 108 TaSWEET genes distributed on all the 21 wheat chromosomes were identified using the latest whole genome sequence (as against 59 genes reported in an earlier report). These 108 genes included 14 of the 17 types reported in Arabidopsis and also included three novel types. Tandem duplications (22) and segmental duplications (5) played a significant role in the expansion of TaSWEET family. A number of cis-elements were also identified in the promoter regions of TaSWEET genes, indicating response of TaSWEET genes during development and also during biotic/abiotic stresses. The TaSWEET proteins carried 4-7 trans-membrane helices (TMHs) showing diversity in structure. Phylogenetic analysis using SWEET proteins of wheat and 8 other species gave four well-known clusters. Expression analysis involving both in silico and in planta indicated relatively higher expression of TaSWEET genes in water/heat sensitive and leaf rust resistant genotypes. The results provided insights into the functional role of TaSWEETs in biotic and abiotic stresses, which may further help in planning strategies to develop high yielding wheat varieties tolerant to environmental stresses.

Catalytic, Computational, and Evolutionary Analysis of the d-Lactate Dehydrogenases Responsible for d-Lactic Acid Production in Lactic Acid Bacteria

d-Lactate dehydrogenase (d-LDH) catalyzes the reversible reaction pyruvate + NADH + H⁺ ↔ lactate + NAD⁺, which is a principal step in the production of d-lactate in lactic acid bacteria. In this study, we identified and characterized the major d-LDH (d-LDH1) from three d-LDHs in Leuconostoc mesenteroides, which has been extensively used in food processing. A molecular simulation study of d-LDH1 showed that the conformation changes during substrate binding. During catalysis, Tyr101 and Arg235 bind the substrates by hydrogen bonds and His296 acts as a general acid/base for proton transfer. These residues are also highly conserved and have coevolved. Point mutations proved that the substrate binding sites and catalytic site are crucial for enzyme activity. Network and phylogenetic analyses indicated that d-LDH1 and the homologues are widely distributed but are most abundant in bacteria and fungi. This study expands the understanding of the functions, catalytic mechanism, and evolution of d-LDH.

New Insight Into the Diversity of SemiSWEET Sugar Transporters and the Homologs in Prokaryotes

Sugars will eventually be exported transporters (SWEETs) and SemiSWEETs represent a family of sugar transporters in eukaryotes and prokaryotes, respectively. SWEETs contain seven transmembrane helices (TMHs), while SemiSWEETs contain three. The functions of SemiSWEETs are less studied. In this perspective article, we analyzed the diversity and conservation of SemiSWEETs and further proposed the possible functions. 1,922 SemiSWEET homologs were retrieved from the UniProt database, which is not proportional to the sequenced prokaryotic genomes. However, these proteins are very diverse in sequences and can be classified into 19 clusters when >50% sequence identity is required. Moreover, a gene context analysis indicated that several SemiSWEETs are located in the operons that are related to diverse carbohydrate metabolism. Several proteins with seven TMHs can be found in bacteria, and sequence alignment suggested that these proteins in bacteria may be formed by the duplication and fusion. Multiple sequence alignments showed that the amino acids for sugar translocation are still conserved and coevolved, although the sequences show diversity. Among them, the functions of a few amino acids are still not clear. These findings highlight the challenges that exist in SemiSWEETs and provide future researchers the foundation to explore these uncharted areas.

dbSWEET: An Integrated Resource for SWEET Superfamily to Understand, Analyze and Predict the Function of Sugar Transporters in Prokaryotes and Eukaryotes

SWEET (Sweet Will Eventually be Exported Transporter) proteins have been recently discovered and form one of the three major families of sugar transporters. Homologs of SWEET are found in both prokaryotes and eukaryotes. Bacterial SWEET homologs have three transmembrane segments forming a triple-helical bundle and the functional form is dimers. Eukaryotic SWEETs have seven transmembrane helical segments forming two triple-helical bundles with a linker helix. Members of SWEET homologs have been shown to be involved in several important physiological processes in plants. However, not much is known regarding the biological significance of SWEET homologs in prokaryotes and in mammals. We have collected more than 2000 SWEET homologs from both prokaryotes and eukaryotes. For each homolog, we have modeled three different conformational states representing outward open, inward open and occluded states. We have provided details regarding substrate-interacting residues and residues forming the selectivity filter for each SWEET homolog. Several search and analysis options are available. The users can generate a phylogenetic tree and structure-based sequence alignment for selected set of sequences. With no metazoan SWEETs functionally characterized, the features observed in the selectivity filter residues can be used to predict the potential substrates that are likely to be transported across the metazoan SWEETs. We believe that this database will help the researchers to design mutational experiments and simulation studies that will aid to advance our understanding of the physiological role of SWEET homologs. This database is freely available to the scientific community at http://bioinfo.iitk.ac.in/bioinfo/dbSWEET/Home.