When SWEETs Turn Tweens: Updates and Perspectives

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

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

Sugar transporters PpSWEET9a and PpSWEET14 synergistically mediate peach sucrose allocation from source leaves to fruit

Sugar content is a critical indicator of fruit quality and is mainly controlled by sugar transporters. Sugars will eventually be exported transporters (SWEET) proteins play an indispensable role in sugar allocation between and within plant organs. Sucrose is the major sugar in many fruits and the predominant form of sugar translocated in peach (Prunus persica). However, the role of the multiple peach SWEET genes in sucrose allocation to fruit remains elusive. In this study, a total of 19 SWEET candidates have been identified in the peach genome, and two Clade III SWEET genes, PpSWEET9a and PpSWEET14, are found to be highly expressed in mature source leaves and branches. Complementation assays, transgene manipulations, and protein interaction studies reveal that PpSWEET9a and PpSWEET14 serve as sucrose efflux proteins and form a heterooligomer that synergistically directs sucrose allocation from source leaves to fruits. Our findings provide insights into the effect of SWEETs on sugar accumulation in peach fruit and identify genetic candidates for improving fruit quality.

Brown planthoppers manipulate rice sugar transporters to benefit their own feeding

The feeding of piercing-sucking insect herbivores often elicits changes in their host plants that benefit the insect.1 In addition to thwarting a host's defense responses, these phloem-feeding insects may manipulate source-sink signaling so as to increase resources consumed.2,3 To date, the molecular mechanisms underlying herbivore-induced resource reallocation remain less investigated. Brown planthopper (BPH), an important rice pest, feeds on the phloem and oviposits into leaf sheaths. BPH herbivory increases sugar accumulations 5-fold in the phloem sap of leaf sheaths and concurrently induces the expression of two clade III SWEET genes, SWEET13 and SWEET14, in leaf tissues, but not in leaf sheaths of attacked rice plants. Mutations of both genes by genome editing attenuate resistance to BPH without alterations of known chemical and physical defense responses. Moreover, BPH-elicited sugar levels in the phloem sap were significantly reduced in sweet13/14 mutants, which is likely to attenuate BPH feeding behavior on sweet13/14 mutants. In one of the two field seasons tested, the sweet13/14 mutants showed comparable yield to wild types, and in the other season, the mutants demonstrated stronger BPH resistance. These preliminary results suggested that the mutations in these SWEET transporters could enhance BPH resistance without yield penalties. Given that sweet13/14 mutants also exhibit resistance to bacterial blight pathogen, Xanthomonas oryzae pv. oryzae, these SWEET genes could serve as excellent molecular targets for the breeding of resistant rice cultivars.

Characterization and Expression Analysis of Sugar Transporters through Partial Least Square Structural Equation Model (PLS-SEM) Revealed Their Role in Pepper (Capsicum annuum L.)

Pepper (Capsicum annuum L.) is one of the most important economic crops in the world. By controlling the transport and distribution of photosynthetic products between cells and organs, sugar transporters are widely involved in growth and development, environmental adaptation, and microbial interactions. The present study was carried out at the genome-wide level to systematically characterize sugar transporters. As a result, 50 MST, 3 SUT, and 29 SWEET genes were identified and classified. The expression pattern of sugar transporters in pepper was analyzed by transcriptomic data. The expression properties of sugar transporters were further explored in pepper varieties with significant differences in weight, shape, and pungency. It was shown that the pepper sugar transporter genes had obvious spatiotemporal specific expression characteristics and exhibited variety-specific expression preferences. We focus on analyzing candidate genes that may be involved in fruit development and expansion. We further explore the response of pepper sugar transporters to adversity stress using a structural equation model. Finally, we found that the MST, SUT, and SWEET families are collectively involved in balancing pepper resistance to abiotic stress by coordinating the expression strengths of different family members. Our study may contribute to the functional study of pepper sugar transporter genes and create the prospect of utilizing sugar transporter gene resources to improve pepper variety.

Deep mutational scanning reveals sequence to function constraints for SWEET family transporters

Protein science is entering a transformative phase enabled by deep mutational scans that provide an unbiased view of the residue level interactions that mediate function. However, it has yet to be extensively used to characterize the mutational and evolutionary landscapes of plant proteins. Here, we apply the method to explore sequence-function relationships within the sugar transporter AtSWEET13. DMS results describe how mutational interrogation throughout different regions of the protein affects AtSWEET13 abundance and transport function. Our results identify novel transport-enhancing mutations that are validated using the FRET sensor assays. Extending DMS results to phylogenetic analyses reveal the role of transmembrane helix 4 (TM4) which makes the SWEET family transporters distinct from prokaryotic SemiSWEETs. We show that transmembrane helix 4 is intolerant to motif swapping with other clade-specific SWEET TM4 compositions, despite accommodating single point-mutations towards aromatic and charged polar amino acids. We further show that the transfer learning approaches based on physics and ML based In silico variant prediction tools have limited utility for engineering plant proteins as they were unable to reproduce our experimental results. We conclude that DMS can produce datasets which, when combined with the right predictive computational frameworks, can direct plant engineering efforts through derivative phenotype selection and evolutionary insights.

Deletion of the sugar importer gene OsSWEET1b accelerates sugar starvation-promoted leaf senescence in rice

Leaf senescence is a combined response of plant cells stimulated by internal and external signals. Sugars acting as signaling molecules or energy metabolites can influence the progression of leaf senescence. Both sugar starvation and accumulation can promote leaf senescence with diverse mechanisms that are reported in different species. Sugars Will Eventually be Exported Transporters (SWEETs) are proposed to play essential roles in sugar transport, but whether they have roles in senescence and the corresponding mechanism are unclear. Here, we functionally characterized a sugar transporter, OsSWEET1b, which transports sugar and promotes senescence in rice (Oryza sativa L.). OsSWEET1b could import glucose and galactose when heterologously expressed in Xenopus oocytes and translocate glucose and galactose from the extracellular apoplast into the intracellular cytosol in rice. Loss of function of OsSWEET1b decreased glucose and galactose accumulation in leaves. ossweet1b mutants showed accelerated leaf senescence under natural and dark-induced conditions. Exogenous application of glucose and galactose complemented the defect of OsSWEET1b deletion-promoted senescence. Moreover, the senescence-activated transcription factor OsWRKY53, acting as a transcriptional repressor, genetically functions upstream of OsSWEET1b to suppress its expression. OsWRKY53-overexpressing plants had attenuated sugar accumulation, exhibiting a similar phenotype as the ossweet1b mutants. Our findings demonstrate that OsWRKY53 downregulates OsSWEET1b to impair its influx transport activity, leading to compromised sugar accumulation in the cytosol of rice leaves where sugar starvation promotes leaf senescence.

SWEET family transporters act as water conducting carrier proteins in plants

Dedicated water channels are involved in the facilitated diffusion of water molecules across the cell membrane in plants. Transporter proteins are also known to transport water molecules along with substrates, however the molecular mechanism of water permeation is not well understood in plant transporters. Here, we show plant sugar transporters from the SWEET (Sugar Will Eventually be Exported Transporter) family act as water-conducting carrier proteins via a variety of passive and active mechanisms that allow diffusion of water molecules from one side of the membrane to the other. This study provides a molecular perspective on how plant membrane transporters act as water carrier proteins, a topic that has not been extensively explored in literature. Water permeation in membrane transporters could occur via four distinct mechanisms which form our hypothesis for water transport in SWEETs. These hypothesis are tested using molecular dynamics simulations of the outward-facing, occluded, and inward-facing state of AtSWEET1 to identify the water permeation pathways and the flux associated with them. The hydrophobic gates at the center of the transport tunnel act as a barrier that restricts water permeation. We have performed in silico single and double mutations of the hydrophobic gate residues to examine the changes in the water conductivity. Surprisingly, the double mutant allows the water permeation to the intracellular half of the membrane and forms a continuous water channel. These computational results are validated by experimentally examining the transport of hydrogen peroxide molecules by the AtSWEET family of transporters. We have also shown that the transport of hydrogen peroxide follows the similar mechanism as water transport in AtSWEET1. Finally, we conclude that similar water-conduction states are also present in other SWEET transporters due to the high sequence and structure conservation exhibited by this transporter family.

The Arabidopsis AtSWEET13 transporter discriminates sugars by selective facial and positional substrate recognition

Transporters are targeted by endogenous metabolites and exogenous molecules to reach cellular destinations, but it is generally not understood how different substrate classes exploit the same transporter's mechanism. Any disclosure of plasticity in transporter mechanism when treated with different substrates becomes critical for developing general selectivity principles in membrane transport catalysis. Using extensive molecular dynamics simulations with an enhanced sampling approach, we select the Arabidopsis sugar transporter AtSWEET13 as a model system to identify the basis for glucose versus sucrose molecular recognition and transport. Here we find that AtSWEET13 chemical selectivity originates from a conserved substrate facial selectivity demonstrated when committing alternate access, despite mono-/di-saccharides experiencing differing degrees of conformational and positional freedom throughout other stages of transport. However, substrate interactions with structural hallmarks associated with known functional annotations can help reinforce selective preferences in molecular transport.

From acidity to sweetness: a comprehensive review of carbon accumulation in grape berries

Most of the carbon found in fruits at harvest is imported by the phloem. Imported carbon provide the material needed for the accumulation of sugars, organic acids, secondary compounds, in addition to the material needed for the synthesis of cell walls. The accumulation of sugars during fruit development influences not only sweetness but also various parameters controlling fruit composition (fruit "quality"). The accumulation of organic acids and sugar in grape berry flesh cells is a key process for berry development and ripening. The present review presents an update of the research on grape berry development, anatomical structure, sugar and acid metabolism, sugar transporters, and regulatory factors.

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.

How plants sense and respond to osmotic stress

Drought is one of the most serious abiotic stresses to land plants. Plants sense and respond to drought stress to survive under water deficiency. Scientists have studied how plants sense drought stress, or osmotic stress caused by drought, ever since Charles Darwin, and gradually obtained clues about osmotic stress sensing and signaling in plants. Osmotic stress is a physical stimulus that triggers many physiological changes at the cellular level, including changes in turgor, cell wall stiffness and integrity, membrane tension, and cell fluid volume, and plants may sense some of these stimuli and trigger downstream responses. In this review, we emphasized water potential and movements in organisms, compared putative signal inputs in cell wall-containing and cell wall-free organisms, prospected how plants sense changes in turgor, membrane tension, and cell fluid volume under osmotic stress according to advances in plants, animals, yeasts, and bacteria, summarized multilevel biochemical and physiological signal outputs, such as plasma membrane nanodomain formation, membrane water permeability, root hydrotropism, root halotropism, Casparian strip and suberin lamellae, and finally proposed a hypothesis that osmotic stress responses are likely to be a cocktail of signaling mediated by multiple osmosensors. We also discussed the core scientific questions, provided perspective about the future directions in this field, and highlighted the importance of robust and smart root systems and efficient source-sink allocations for generating future high-yield stress-resistant crops and plants.

Functional Characterization of CsSWEET5a, a Cucumber Hexose Transporter That Mediates the Hexose Supply for Pollen Development and Rescues Male Fertility in Arabidopsis

Pollen cells require large amounts of sugars from the anther to support their development, which is critical for plant sexual reproduction and crop yield. Sugars Will Eventually be Exported Transporters (SWEETs) have been shown to play an important role in the apoplasmic unloading of sugars from anther tissues into symplasmically isolated developing pollen cells and thereby affect the sugar supply for pollen development. However, among the 17 CsSWEET genes identified in the cucumber (Cucumis sativus L.) genome, the CsSWEET gene involved in this process has not been identified. Here, a member of the SWEET gene family, CsSWEET5a, was identified and characterized. The quantitative real-time PCR and β-glucuronidase expression analysis revealed that CsSWEET5a is highly expressed in the anthers and pollen cells of male cucumber flowers from the microsporocyte stage (stage 9) to the mature pollen stage (stage 12). Its subcellular localization indicated that the CsSWEET5a protein is localized to the plasma membrane. The heterologous expression assays in yeast demonstrated that CsSWEET5a encodes a hexose transporter that can complement both glucose and fructose transport deficiencies. CsSWEET5a can significantly rescue the pollen viability and fertility of atsweet8 mutant Arabidopsis plants. The possible role of CsSWEET5a in supplying hexose to developing pollen cells via the apoplast is also discussed.

Substrate selectivity and unique sequence signatures in SWEET/semiSWEET homologs of four taxonomic groups: Sequence analysis and phylogenetic studies

The recently discovered SWEET (Sugar Will Eventually be Exported Transporter) proteins are involved in the selective transport of monosaccharides and disaccharides. The prokaryotic counterparts, semiSWEETs, form dimers with each monomer forming a triple-helix transmembrane bundle (THB). The longer eukaryotic SWEETs have seven transmembrane helices with two THBs and a linker helix. Structures of semiSWEETs/SWEETs have been determined experimentally. Experimental studies revealed the role of plant SWEETs in vital physiological processes and identified residues responsible for substrate selectivity. However, SWEETs/semiSWEETs from metazoans and bacteria are not characterized. In this study, we used structure-based sequence alignment and compared more than 2000 SWEET/semiSWEETs from four different taxonomic groups. Conservation of residue/chemical property was examined at all positions. Properties of clades/subclades of phylogenetic trees from each taxonomic group were analyzed. Conservation pattern of known residues in the selectivity-filter was used to predict the substrate preference of plant SWEETs and some clusters of metazoans and bacteria. Some residues at the gating and substrate-binding regions, pore-facing positions and at the helix-helix interface are conserved across all taxonomic groups. Conservation of polar/charged residues at specific pore-facing positions, helix-helix interface and in loops seems to be unique for plant SWEETs. Overall, the number of conserved residues is less in metazoan SWEETs. Plant and metazoan SWEETs exhibit high conservation of four and three proline residues respectively in "proline tetrad." Further experimental studies can validate the predicted substrate selectivity and significance of conserved polar/charged/aromatic residues at structurally and functionally important positions of SWEETs/semiSWEETs in plants, metazoans and bacteria.

Plasma membrane-localized hexose transporter OsSWEET1b, affects sugar metabolism and leaf senescence

KEY MESSAGE

OsSWEET1b is a hexose transporter protein, which localized in cell membranes and interacting with itself to form homodimer and knockout of OsSWEET1b resulted in reduced leaves sugar content and accelerating leaf senescence. In the rice genome, the SWEET gene family contains 21 homologous members, but the role of some of them in rice growth and development is still unknown. The function of the sugar transporter OsSWEET1b protein in rice was identified in this research. Expression analysis showed that the expression levels of OsSWEET1b in leaves were higher than that in other tissues. The hexose transport experiment confirmed that OsSWEET1b has glucose and galactose transporter activity in yeast. Subcellular localization indicates that OsSWEET1b protein was targeted to the plasma membrane and BiFC analysis showed that OsSWEET1b interacts with itself to form homodimers. Functional analysis demonstrated that the ossweet1b mutant plants were have reduced the sucrose, glucose, fructose, starch and galactose contents, and induced carbon starvation-related gene expression, which might lead to carbon starvation in leaves at filling stage. The ossweet1b knockout plants showed decreased chlorophyll content and antioxidant enzyme activity, and increased ROS accumulation in leaves, leading to leaf cell death and premature senescence phenotype at filling stage. In ossweet1b mutants, the leaf senescence-related gene expression levels were increased and the abundance of photosynthesis-related proteins was decreased. Loss of OsSWEET1b were affected the starch, sucrose metabolism and carbon fixation in photosynthetic organelles pathway by RNA-seq analysis. The destruction of OsSWEET1b function will cause sugar starvation, decreased photosynthesis and leaf senescence, which leading to reduced rice yield. Collectively, our results suggest that the OsSWEET1b plays a key role in rice leaves carbohydrate metabolism and leaf senescence.

The Arabidopsis SWEET1 and SWEET2 uniporters recognize similar substrates while differing in subcellular localization

Sugars Will Eventually be Exported Transporters (SWEETs) are central for sugar allocation in plants. The SWEET family has approximately 20 homologs in most plant genomes, and despite extensive research on their structures and molecular functions, it is still unclear how diverse SWEETs recognize different substrates. Previous work using SweetTrac1, a biosensor constructed by the intramolecular fusion of a conformation-sensitive fluorescent protein in the plasma membrane transporter SWEET1 from Arabidopsis thaliana, identified common features in the transporter's substrates. Here, we report SweetTrac2, a new biosensor based on the Arabidopsis vacuole membrane transporter SWEET2, and use it to explore the substrate specificity of this second protein. Our results show that SWEET1 and SWEET2 recognize similar substrates but some with different affinities. Sequence comparison and mutagenesis analysis support the conclusion that the differences in affinity depend on nonspecific interactions involving previously uncharacterized residues in the substrate-binding pocket. Furthermore, SweetTrac2 can be an effective tool for monitoring sugar transport at vacuolar membranes that would be otherwise challenging to study.

Pineapple SWEET10 is a glucose transporter

SWEET transporters are a unique class of sugar transporters that play vital roles in various developmental and physiological processes in plants. While the functions of SWEETs have been well established in model plants such as Arabidopsis, their functions in economically important fruit crops like pineapple have not been well studied. Here we aimed to investigate the substrate specificity of pineapple SWEETs by comparing the protein sequences of known glucose and sucrose transporters in Arabidopsis with those in pineapple. Our genome-wide approach and 3D structure comparison showed that the Arabidopsis SWEET8 homolog in pineapple, AcSWEET10, shares similar sequences and protein properties responsible for glucose transport. To determine the functional conservation of AcSWEET10, we tested its ability to complement glucose transport mutants in yeast and analyzed its expression in stamens and impact on the microspore phenotype and seed set in transgenic Arabidopsis. The results showed that AcSWEET10 is functionally equivalent to AtSWEET8 and plays a critical role in regulating microspore formation through the regulation of the Callose synthase5 (CalS5), which highlights the importance of SWEET transporters in pineapple. This information could have important implications for improving fruit crop yield and quality by manipulating SWEET transporter activity.

An update on sugar allocation and accumulation in fruits

Fruit sweetness is determined by the amount and composition of sugars in the edible flesh. The accumulation of sugar is a highly orchestrated process that requires coordination of numerous metabolic enzymes and sugar transporters. This coordination enables partitioning and long-distance translocation of photoassimilates from source tissues to sink organs. In fruit crops, sugars ultimately accumulate in the sink fruit. Whereas tremendous progress has been achieved in understanding the function of individual genes associated with sugar metabolism and sugar transport in non-fruit crops, there is less known about the sugar transporters and metabolic enzymes responsible for sugar accumulation in fruit crop species. This review identifies knowledge gaps and can serve as a foundation for future studies, with comprehensive updates focusing on (1) the physiological roles of the metabolic enzymes and sugar transporters responsible for sugar allocation and partitioning and that contribute to sugar accumulation in fruit crops; and (2) the molecular mechanisms underlying the transcriptional and posttranslational regulation of sugar transport and metabolism. We also provide insights into the challenges and future directions of studies on sugar transporters and metabolic enzymes and name several promising genes that should be targeted with gene editing in the pursuit of optimized sugar allocation and partitioning to enhance sugar accumulation in fruits.

Sugar accumulation enhancement in sorghum stem is associated with reduced reproductive sink strength and increased phloem unloading activity

Sweet sorghum has emerged as a promising source of bioenergy mainly due to its high biomass and high soluble sugar yield in stems. Studies have shown that loss-of-function Dry locus alleles have been selected during sweet sorghum domestication, and decapitation can further boost sugar accumulation in sweet sorghum, indicating that the potential for improving sugar yields is yet to be fully realized. To maximize sugar accumulation, it is essential to gain a better understanding of the mechanism underlying the massive accumulation of soluble sugars in sweet sorghum stems in addition to the Dry locus. We performed a transcriptomic analysis upon decapitation of near-isogenic lines for mutant (d, juicy stems, and green leaf midrib) and functional (D, dry stems and white leaf midrib) alleles at the Dry locus. Our analysis revealed that decapitation suppressed photosynthesis in leaves, but accelerated starch metabolic processes in stems. SbbHLH093 negatively correlates with sugar levels supported by genotypes (DD vs. dd), treatments (control vs. decapitation), and developmental stages post anthesis (3d vs.10d). D locus gene SbNAC074A and other programmed cell death-related genes were downregulated by decapitation, while sugar transporter-encoding gene SbSWEET1A was induced. Both SbSWEET1A and Invertase 5 were detected in phloem companion cells by RNA in situ assay. Loss of the SbbHLH093 homolog, AtbHLH093, in Arabidopsis led to a sugar accumulation increase. This study provides new insights into sugar accumulation enhancement in bioenergy crops, which can be potentially achieved by reducing reproductive sink strength and enhancing phloem unloading.

Coordination of carbon assimilation, allocation, and utilization for systemic improvement of cereal yield

The growth of yield outputs is dwindling after the first green revolution, which cannot meet the demand for the projected population increase by the mid-century, especially with the constant threat from extreme climates. Cereal yield requires carbon (C) assimilation in the source for subsequent allocation and utilization in the sink. However, whether the source or sink limits yield improvement, a crucial question for strategic orientation in future breeding and cultivation, is still under debate. To narrow the knowledge gap and capture the progress, we focus on maize, rice, and wheat by briefly reviewing recent advances in yield improvement by modulation of i) leaf photosynthesis; ii) primary C allocation, phloem loading, and unloading; iii) C utilization and grain storage; and iv) systemic sugar signals (e.g., trehalose 6-phosphate). We highlight strategies for optimizing C allocation and utilization to coordinate the source-sink relationships and promote yields. Finally, based on the understanding of these physiological mechanisms, we envisage a future scenery of "smart crop" consisting of flexible coordination of plant C economy, with the goal of yield improvement and resilience in the field population of cereals crops.

AtSWEET1 negatively regulates plant susceptibility to root-knot nematode disease

The root-knot nematode Meloidogyne incognita is a pathogenic pest that causes severe economic loss to agricultural production by forming a parasitic relationship with its hosts. During the development of M. incognita in the host plant roots, giant cells are formed as a nutrient sink. However, the roles of sugar transporters during the giant cells gain sugar from the plant cells are needed to improve. Meanwhile, the eventual function of sugars will eventually be exported transporters (SWEETs) in nematode-plant interactions remains unclear. In this study, the expression patterns of Arabidopsis thaliana SWEETs were examined by inoculation with M. incognita at 3 days post inoculation (dpi) (penetration stage) and 18 dpi (developing stage). We found that few AtSWEETs responded sensitively to M. incognita inoculation, with the highest induction of AtSWEET1 (AT1G21460), a glucose transporter gene. Histological analyses indicated that the β-glucuronidase (GUS) and green fluorescent protein (GFP) signals were observed specifically in the galls of AtSWEET1-GUS and AtSWEET1-GFP transgenic plant roots, suggesting that AtSWEET1 was induced specifically in the galls. Genetic studies have shown that parasitism of M. incognita was significantly affected in atsweet1 compared to wild-type and complementation plants. In addition, parasitism of M. incognita was significantly affected in atsweet10 but not in atsweet13 and atsweet14, expression of which was induced by inoculation with M. incognita. Taken together, these data prove that SWEETs play important roles in plant and nematode interactions.

Comparative transcriptome profiling reveals the importance of GmSWEET15 in soybean susceptibility to Sclerotinia sclerotiorum

Soybean sclerotinia stem rot (SSR) is a disease caused by Sclerotinia sclerotiorum that causes incalculable losses in soybean yield each year. Considering the lack of effective resistance resources and the elusive resistance mechanisms, we are urged to develop resistance genes and explore their molecular mechanisms. Here, we found that loss of GmSWEET15 enhanced the resistance to S. sclerotiorum, and we explored the molecular mechanisms by which gmsweet15 mutant exhibit enhanced resistance to S. sclerotiorum by comparing transcriptome. At the early stage of inoculation, the wild type (WT) showed moderate defense response, whereas gmsweet15 mutant exhibited more extensive and intense transcription reprogramming. The gmsweet15 mutant enriched more biological processes, including the secretory pathway and tetrapyrrole metabolism, and it showed stronger changes in defense response, protein ubiquitination, MAPK signaling pathway-plant, plant-pathogen interaction, phenylpropanoid biosynthesis, and photosynthesis. The more intense and abundant transcriptional reprogramming of gmsweet15 mutant may explain how it effectively delayed colonization by S. sclerotiorum. In addition, we identified common and specific differentially expressed genes between WT and gmsweet15 mutant after inoculation with S. sclerotiorum, and gene sets and genes related to gmsweet15_24 h were identified through Gene Set Enrichment Analysis. Moreover, we constructed the protein-protein interaction network and gene co-expression networks and identified several groups of regulatory networks of gmsweet15 mutant in response to S. sclerotiorum, which will be helpful for the discovery of candidate functional genes. Taken together, our results elucidate molecular mechanisms of delayed colonization by S. sclerotiorum after loss of GmSWEET15 in soybean, and we propose novel resources for improving resistance to SSR.

Plasma Membrane-Localized Transporter MdSWEET12 Is Involved in Sucrose Unloading in Apple Fruit

Sugar content is an important factor determining the flavor in apple fruit. Sugar unloading is a prerequisite step for sugar accumulation. However, little is known about sugar unloading mechanisms in apple. Transcriptomic sequencing of two apple varieties, "Envy" and "Pacific Rose," with significantly different sugar content was performed. MdSWEET12a from the SWEET transporter family was differentially expressed. Further study of the MdSWEET12a showed that this plasma membrane-localized transporter protein-encoding gene was mainly expressed in sieve element-companion cells (SE-CC) in the fruit, which was positively correlated with the sucrose accumulation during the development of "Envy" apple. Consistently manipulating the gene expression through either transient overexpression or silencing significantly increased or decreased the sugar content in apple fruit, respectively. Complementary growth experiments in mutant yeast cells indicated that MdSWEET12a transported sucrose. Heterologous expression of MdSWEET12a in tomato increased the expression of genes related to sugar metabolism and transport, leading to increased sugar content. These findings underpin the involvement of MdSWEET12a in sugar unloading in apple fruit.

Impairment of sugar transport in the vascular system acts on nitrogen remobilization and nitrogen use efficiency in Arabidopsis

Carbon (C) and nitrogen (N) metabolisms have long been known to be coupled, and this is required for adjusting nitrogen use efficiency (NUE). Despite this intricate relationship, it is still unclear how deregulation of sugar transport impacts N allocation. Here, we investigated in Arabidopsis the consequences of the simultaneous downregulation of the genes coding for the sugar transporters SWEET11, SWEET12, SWEET16, and SWEET17 on various anatomical and physiological traits ranging from the stem's vascular system development to plant biomass production, seed yield, and N remobilization and use efficiency. Our results show that intracellular sugar exchanges mediated by SWEET16 and SWEET17 proteins specifically impact vascular development but do not play a significant role in the distribution of N. Most importantly, we showed that the double mutant swt11 swt12, which has an impacted vascular development, displays an improved NUE and nitrogen remobilization to the seeds. In addition, a significant negative correlation between sugar and amino acids contents and the inflorescence stem radial growth exists, highlighting the complex interaction between the maintenance of C/N homeostasis and the inflorescence stem development. Our results thus deepen the link between sugar transport, C/N allocation, and vascular system development.

Identification and expression analysis of pineapple sugar transporters reveal their role in the development and environmental response

In plants, sugars are required for several essential functions, including growth, storage, signaling, defense and reproduction. Sugar transporters carry out the controlled movement of sugars from source (leaves) to sink (fruits and roots) tissues and determine the overall development of the plant. Various types of sugar transporter families have been described in plants, including sucrose transporters (SUC/SUT), monosaccharide transporter (MST) and SWEET (from "Sugar Will Eventually be Exported Transporters"). However, the information about pineapple sugar transporters is minimal. This study systematically identified and classified 45 MST and 4 SUC/SUT genes in the pineapple genome. We found that the expression patterns of sugar transporter genes have a spatiotemporal expression in reproductive and vegetative tissues indicating their pivotal role in reproductive growth and development. Besides, different families of sugar transporters have a diel expression pattern in photosynthetic and non-photosynthetic tissues displaying circadian rhythm associated participation of sugar transporters in the CAM pathway. Moreover, regulation of the stress-related sugar transporters during cold stress indicates their contribution to cold tolerance in pineapple. Heterologous expression (yeast complementation assays) of sugar transporters in a mutant yeast strain suggested that SUT1/2 have the ability to transport sucrose, and STP13, STP26, pGlcT-L2 and TMT4 are able to transport glucose, whereas SWEET11/13 transport both sucrose and fructose. The information provided here would help researchers further explore the underlying molecular mechanism involved in the sugar metabolism of pineapple.

Sucrose rather than GA transported by AtSWEET13 and AtSWEET14 supports pollen fitness at late anther development stages

Both sugar and the hormone gibberellin (GA) are essential for anther-enclosed pollen development and thus for plant productivity in flowering plants. Arabidopsis (Arabidopsis thaliana) AtSWEET13 and AtSWEET14, which are expressed in anthers and associated with seed yield, transport both sucrose and GA. However, it is still unclear which substrate transported by them directly affects anther development and seed yield. Histochemical staining, cross-sectioning and microscopy imaging techniques were used to investigate and interpret the phenotypes of the atsweet13;14 double mutant during anther development. Genetic complementation of atsweet13;14 using AtSWEET9, which transports sucrose but not GA, and the GA transporter AtNPF3.1, respectively, was conducted to test the substrate preference relevant to the biological process. The loss of both AtSWEET13 and AtSWEET14 resulted in reduced pollen viability and therefore decreased pollen germination. AtSWEET9 fully rescued the defects in pollen viability and germination of atsweet13;14, whereas AtNPF3.1 failed to do so, indicating that AtSWEET13/14-mediated sucrose rather than GA is essential for pollen fertility. AtSWEET13 and AtSWEET14 function mainly at the anther wall during late anther development stages, and they probably are responsible for sucrose efflux into locules to support pollen development to maturation, which is vital for subsequent pollen viability and germination.

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.

Improved Understanding of Sugar Transport in Various Plants

A few recent reviews have addressed progress and perspectives in the field of sugar transport in plants rather comprehensively [...]

Multiagent Reinforcement Learning-Based Adaptive Sampling for Conformational Dynamics of Proteins

Machine learning is increasingly applied to improve the efficiency and accuracy of molecular dynamics (MD) simulations. Although the growth of distributed computer clusters has allowed researchers to obtain higher amounts of data, unbiased MD simulations have difficulty sampling rare states, even under massively parallel adaptive sampling schemes. To address this issue, several algorithms inspired by reinforcement learning (RL) have arisen to promote exploration of the slow collective variables (CVs) of complex systems. Nonetheless, most of these algorithms are not well-suited to leverage the information gained by simultaneously sampling a system from different initial states (e.g., a protein in different conformations associated with distinct functional states). To fill this gap, we propose two algorithms inspired by multiagent RL that extend the functionality of closely related techniques (REAP and TSLC) to situations where the sampling can be accelerated by learning from different regions of the energy landscape through coordinated agents. Essentially, the algorithms work by remembering which agent discovered each conformation and sharing this information with others at the action-space discretization step. A stakes function is introduced to modulate how different agents sense rewards from discovered states of the system. The consequences are three-fold: (i) agents learn to prioritize CVs using only relevant data, (ii) redundant exploration is reduced, and (iii) agents that obtain higher stakes are assigned more actions. We compare our algorithm with other adaptive sampling techniques (least counts, REAP, TSLC, and AdaptiveBandit) to show and rationalize the gain in performance.

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.