Genome-Wide Discovery and Expression Profiling of the SWEET Sugar Transporter Gene Family in Woodland Strawberry (Fragaria vesca) under Developmental and Stress Conditions: Structural and Evolutionary Analysis

The SWEET(sugar will eventually be exported transporter)family proteins are a recently identified class of sugar transporters that are essential for various physiological processes.Although the functions of the SWEET proteins have been identified in a number of species,to date,there have been no reports of the functions of the SWEET genes in woodland strawberries(Fragaria vesca).In this study,we identified 15 genes that were highly homolo-gous to the A.thaliana AtSWEET genes and designated them as FvSWEET1–FvSWEET15.We then conducted a structural and evolutionary analysis of these 15 FvSWEET genes.The phylogenetic analysis enabled us to categor-ize the predicted 15 SWEET proteins into four distinct groups.We observed slight variations in the exon‒intron structures of these genes,while the motifs and domain structures remained highly conserved.Additionally,the developmental and biological stress expression profiles of the 15 FvSWEET genes were extracted and analyzed.Finally,WGCNA coexpression network analysis was run to search for possible interacting genes of FvSWEET genes.The results showed that the FvSWEET10 genes interacted with 20 other genes,playing roles in response to bacterial and fungal infections.The outcomes of this study provide insights into the further study of FvSWEET genes and may also aid in the functional characterization of the FvSWEET genes in woodland strawberries.

Identification and expression analysis of sugar transporter family genes reveal the role of ZmSTP2 and ZmSTP20 in maize disease resistance

Sugar is an indispensable source of energy for plant growth and development, and it requires the participation of sugar transporter proteins(STPs) for crossing the hydrophobic barrier in plants. Here, we systematically identified the genes encoding sugar transporters in the genome of maize(Zea mays L.), analyzed their expression patterns under different conditions, and determined their functions in disease resistance. The results showed that the mazie sugar transporter family contained 24 members, all of which were predicted to be distributed on the cell membrane and had a highly conserved transmembrane transport domain. The tissue-specific expression of the maize sugar transporter genes was analyzed, and the expression level of these genes was found to be significantly different in different tissues. The analysis of biotic and abiotic stress data showed that the expression levels of the sugar transporter genes changed significantly under different stress factors. The expression levels of Zm STP2 and Zm STP20 continued to increase following Fusarium graminearum infection. By performing disease resistance analysis of zmstp2 and zmstp20 mutants, we found that after inoculation with Cochliobolus carbonum, Setosphaeria turcica, Cochliobolus heterostrophus, and F. graminearum, the lesion area of the mutants was significantly higher than that of the wild-type B73 plant. In this study, the genes encoding sugar transporters in maize were systematically identified and analyzed at the whole genome level. The expression patterns of the sugar transporter-encoding genes in different tissues of maize and under biotic and abiotic stresses were revealed, which laid an important theoretical foundation for further elucidation of their functions.

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

Sugar transporters are essential for osmotic process regulation,various signaling pathways and plant growth and development.Currently,few studies are available on the function of sugar transporters in sorghum(Sorghum bicolor L.).In this study,we performed a genome-wide survey of sugar transporters in sorghum.In total,98 sorghum sugar transporters(SSTs)were identified via BLASTP.These SSTs were classified into three families based on the phylogenetic and conserved domain analysis,including six sucrose transporters(SUTs),23 sugars will eventually be exported transporters(SWEETs),and 69 monosaccharide transporters(MSTs).The sorghum MSTs were further divided into seven subfamilies,including 24 STPs,23 PLTs,two VGTs,four INTs,three p Glc T/SBG1 s,five TMTs,and eight ERDs.Chromosomal localization of the SST genes showed that they were randomly distributed on 10 chromosomes,and substantial clustering was evident on the specific chromosomes.Twenty-seven SST genes from the families of SWEET,ERD,STP,and PLT were found to cluster in eight tandem repeat event regions.In total,22 SSTs comprising 11 paralogous pairs and accounting for 22.4%of all the genes were located on the duplicated blocks.The different subfamilies of SST proteins possessed the same conserved domain,but there were some differences in features of the motif and transmembrane helices(TMH).The publicly-accessible RNA-sequencing data and real-time PCR revealed that the SST genes exhibited distinctive tissue specific patterns.Functional studies showed that seven SSTs were mainly located on the cell membrane and membrane organelles,and 14 of the SSTs could transport different types of monosaccharides in yeast.These findings will help us to further elucidate their roles in the sorghum sugar transport and sugar signaling pathways.

Identification and Characterization of Tonoplast Sugar Transporter (TST) Gene Family in Cucumber

Tonoplast sugar transporters(TSTs)play essential roles in regulating plant growth,development,and response to various biotic and abiotic stresses.In this study,a total of three TST genes were identified by a genome-wide analysis in cucumber.Phylogenetic analysis showed that TST proteins from cucumber and other plant species can be classified into five groups,and nearly all TST members in the same groups displayed similarmotif distributions,transmembrane(TM)domains,and gene structures.All of the three CsTST genes possess a number of development-,stress-,and hormone-related cis-elements in the promoter sequences.Meanwhile,qRT-PCR assays revealed that the CsTST1 was expressed in fruits,flowers,leaves,and other tissues,and its expression varied significantly under various abiotic stresses such as cold,salt,drought(PEG),and abscisic acid(ABA).Finally,functional analysis of CsTST1 in yeast revealed that it was able to complement the deficiency in galactose,mannose and sucrose transport.These results revealed that CsTST1 can act as a functional sugar transporter to play important roles in cucumber growth and response to abiotic stress probably through affecting carbohydrate distribution.

Evidence of the predominance of passive symplastic phloem loading and sugar transport with leaf ageing in Camellia oleifera

Phloem loading and transport of sugar from leaves to sink tissues such as fruits are crucial for yield formation.Camellia oleifera is an evergreen horticultural crop with high value;however,its low production limits the development of the C.oleifera industry.In this study,using a combination of ultrastructural observation,fluorescence loss in photobleaching(FLIP)and inhibitor treatment,we revealed that C.oleifera leaves mainly adopt a symplastic loading route from mesophyll cells to the surrounding vascular bundle cells in minor veins.HPLC assays showed that sucrose is the main sugar transported and only a small amount of raffinose or stachyose was detected in petioles,supporting a passive symplastic loading route in C.oleifera leaves.Compared to leaves grown this year(LT),the carbohydrate synthesis capacity in leaves grown last year(LL)was decreased while LL retained more soluble sugar,suggesting a decrease in transport capacity with leaf ageing.TEM and tissue staining showed that a reduction in plasmodesmata density leads to a decline in the degree of cellular coupling and is responsible for the weakening transport capacity in older leaves.RNA-seq revealed several differentially expressed genes(DEGs)including CoPDCB1-1,CoSUT1 and CoSWEET12,which are likely involved in the regulation of phloem loading and sugar transport.An expression correlation network is constructed between PD-callose binding protein genes,sugar transporter genes and senescence-associated genes.Collectively,this study provides the evidence of the passive symplastic phloem loading pathway in C.oleifera leaves and constructs the correlation between sugar transport and leaf ageing.

Impaired SWEET-mediated sugar transportation impacts starch metabolism in developing rice seeds

Sugar transportation and sugar-to-starch metabolism are considered important processes in seed development and embryo viability.A few plant SWEET proteins acting as sugar transporters have been reported to function in inflorescence and/or seed development.Here,we identified seven members of the 21 Os SWEET genes in rice that play essential roles in sugar transportation and sugar-to-starch conversion in seed development.Nineteen Os SWEET genes exhibiting different expression patterns during inflorescence and seed development were knocked out individually by CRISPR/Cas9.One third of the mutants showed decreased fertile pollen viability and shriveled mature caryopses,resulting in weakened seed traits.Grain fill-related genes but not representative grain shape-regulating genes showed attenuated expression in the mutants.Seed of each of these mutants accumulated more sucrose,glucose or fructose but less starch.Among all Os SWEET genes,Os SWEET4 and Os SWEET11 had major effects on caryopsis development.The sugar-to-starch metabolic pathway was significantly altered in ossweet11 mutants based on differential expression analysis in RNA sequencing assays,confirming that Os SWEET11 functions as a sugar transporter with a key role in seed development.These results help to decipher the multiple functions of Os SWEET genes and to show how they might be used in genetic improvement of rice.

The Physiological Mechanism of Postphloem Sugar Transport in Citrus Fruit

The dynamics of translocation and partitioning of 14C-phothsynthates, the concentration of sucrose in fruit tissues and the effects of the membrane carrier- and ATPase-specific inhibitors on 14C-sucrose uptake by juice sacs of the satsuma mandarin(Citrus unshiu Marc. cv. Miyagawa wase)fruit were examined at the stage of fruit enlargement and fruit full ripe. Kinetic data of 14C-photosynthate translocation indicated that the rate of photosynthate transport into juice sacs decreased with fruit maturation and sugar accumulation. Along the photosynthate translocation path, i. e. from vascular bundles to segment epidermis then to juice sacs, a descending sugar gradient was observed. With fruit maturation and sugar accumulation in juice sacs, the 14C photosynthate gradient increased, whereas the static sucrose concentration gradient decreased with fruit maturation and sugar accumulation. The higher gradient of specific 14C radioactivity was considered to favor diffusion and sugar transport into juice sacs at the later stage of fruit development. The rate of uptake 14C-sucrose by juice sacs of satsuma mandarin fruit was markedly reduced by PCMBS, EB, DNP and NO3-treatment. The above results suggested the participation of a carrier-mediated, energy-dependent sugar active transport process in juice sacs of satsuma mandarin fruit.

Genome-wide identification, characterization, and expression analysis of the SWEET gene family in cucumber

SWEETs （sugars will eventually be exported transporters） are a novel class of recently identified sugar transporters that play important roles in diverse physiological processes. However, only a few species of the plant SWEETgene family have been functionally identified. Up till now, there has been no systematic analysis of the SWEETgene family in Cucurbitaceae crops. Here, a genome-wide characterization of this family was conducted in cucumber（Cucumis sativus L.）. A total of 17 CsSWEETgenes were identified, which are not evenly distributed over the seven cucumber chromosomes. Cucumber SWEET protein sequences possess seven conserved domains and two putative serine phosphorylation sites. The phylo- genetic tree of the SWEET genes in cucumber, Arabidopsis thaliana, and Oryza sativa was constructed, and all the SWEET genes were divided into four clades. In addition, a number of putative cis-elements were identified in the promoter regions of these CsSWEET genes： nine types involved in phytohormone responses and eight types involved in stress responses. Moreover, the transcript levels of CsSWEETgenes were analyzed in various tissues using quantitative real-time polymerase chain reaction. A majority （70.58%） of the CsSWEET genes were confined to reproductive tissue development. Finally, 18 putative watermelon ClaSWEETgenes and 18 melon CmSWEETgenes were identified that showed a high degree of similarity with CsSWEETgenes. The results from this study provided a basic understanding of the CsSWEETgenes and may also facilitate future research to elucidate the function of SWEET genes in cucumber and other Cucurbitaceae crops.

Genome-wide identification and transcriptome profiling reveal great expansion of SWEET gene family and their wide-spread responses to abiotic stress in wheat (Triticum aestivum L.)

The Sugars Will Eventually be Exported Jransporter(SWEET)gene family,identified as sugar transporters,has been demonstrated to play key roles in phloem loading,grain filling,pollen nutrition,and plant-pathogen interactions.To date,the study of SWEET genes in response to abiotic stress is very limited.In this study,we performed a genome-wide identification of the SWEET gene family in wheat and examined their expression profiles under mutiple abiotic stresses.We identified a total of 105 wheat SWEET genes,and phylogenic analysis revealed that they fall into five clades,with clade V specific to wheat and its closely related species.Of the 105 wheat SWEET genes,59%exhibited significant expression changes after stress treatments,including drought,heat,heat combined with drought,and salt stresses,and more up-regulated genes were found in response to drought and salt stresses.Further hierarchical clustering analysis revealed that SWEET genes exhibited differential expression patterns in response to different stress treatments or in different wheat cultivars.Moreover,different phylogenetic clades also showed distinct response to abiotic stress treatments.Finally,we found that homoeologous SWEET genes from different wheat subgenomes exhibited differential expression patterns in response to different abiotic stress treatments.The genome-wide analysis revealed the great expansion of SWEET gene family in wheat and their wide participation in abiotic stress response.The expression partitioning of SWEET homoeologs under abiotic stress conditions may confer greater flexibility for hexaploid wheat to adapt to ever changing environments.

A genome-wide analysis of SWEET gene family in cotton and their expressions under different stresses

Background： The SWEET （Sugars will eventually be exported transporters） gene family plays multiple roles in plant physiological activities and development process. It participates in reproductive development and in the process of sugar transport and absorption, plant senescence and stress responses and plant-pathogen interaction. However, thecomprehensive analysis of SWEET genes has not been reported in cotton. Results： In this study, we identified 22, 31, 55 and 60 SWEETgenes from the sequenced genomes of Gossypium orboreum, G. rairnondii, G. hirsutum and G. borbadense, respectively. Phylogenetic tree analysis showed that the SWEET genes could be divided into four groups, which were further classified into 14 sub-clades. Further analysis of chromosomal location, synteny analysis and gene duplication suggested that the orthologs showed a good collinearity and segmental duplication events played a crucial role in the expansion of the family in cotton. Specific MtN3_slv domains were highly conserved between Arabidopsis and cotton by exon-intron organization and motif analysis. In addition, the expression pattern in different tissues indicated that the duplicated genes in cotton might have acquired new functions as a result of sub-functionalization or neo-functionalization. The expression pattern of SWEET genes showed that the different genes were induced by diverse stresses. The identification and functional analysis of SWEET genes in cotton may provide more candidate genes for genetic modification. Conclusion： SWEET genes were classified into four clades in cotton. The expression patterns suggested that the duplicated genes might have experienced a functional divergence. This work provides insights into the evolution of SWEETgenes and more candidates for specific genetic modification, which will be useful in future research.

Arabidopsis thaliana AtUTr7 Encodes a Golgi-Localized UDP-Glucose/UDP-Galactose Transporter that Affects Lateral Root Emergence

Nucleotide sugar transporters （NSTs） are antiporters comprising a gene family that plays a fundamental role in the biosynthesis of complex cell wall polysaccharides and glycoproteins in plants. However, due to the limited number of related mutants that have observable phenotypes, the biological function（s） of most NSTs in cell wall biosynthesis and assembly have remained elusive. Here, we report the characterization of AtUTr7 from Arabidopsis （Arabidopsis thaliana （L.） Heynh.）, which is homologous to multi-specific UDP-sugar transporters from Drosophila melanogaster, humans, and Caenorhabditis elegans. We show that AtUTr7 possesses the common structural characteristics conserved among NSTs. Using a green fluorescent protein （GFP） tagged version, we demonstrate that AtUTr7 is localized in the Golgi apparatus. We also show that AtUTr7 is widely expressed, especially in the roots and in specific floral organs. Additionally, the results of an in vitro nucleotide sugar transport assay carried out with a tobacco and a yeast expression system suggest that AtUTr7 is capable of transferring UDP-Gal and UDP-GIc, but not a range of other UDP- and GDP-sugars, into the Golgi lumen. Mutants lacking expression of AtUTr7 exhibited an early proliferation of lateral roots as well as distorted root hairs when cultivated at high sucrose concentrations. Furthermore, the distribution of homogalacturonan with a low degree of methyl esterification differed in lateral root tips of the mutant compared to wild-type plants, although additional analytical procedures revealed no further differences in the composition of the root cell walls. This evidence suggests that the transport of UDP-Gal and UDP-GIc into the Golgi under conditions of high root biomass production plays a role in lateral root and root hair development.

Functional characterization of a novel, highly expressed ion-driven sugar antiporter in the thoracic muscles of Helicoverpa armigera

Sugar transporters(STs),which mainly mediate cellular sugar exchanges,play critical physiological roles in living organisms,and they may be responsible for sugar exchanges among various insect tissues.However,the molecular and physiological functions of insect STs are largely unknown.Here,16 STs of Helicoverpa armigera were identified.A phylogenetic analysis classified the putative HaSTs into 12 sub-families,and those identified in this study were distributed into 6 sub-families.Real-time polymerase chain reaction indicated that the 16 HaSTs had diverse tissue-specific expression levels.One transporter,HaST10,was highly expressed in thoracic muscles.A functional study using a Xenopus oocyte expression system revealed that HaST10 mediated both H+-driven trehalose and Na+-driven glucose antiport activities with high transport efficiency and low affinity levels.A HaST10 knockout clearly impaired the performance of H.armigera.Thus,HaST10 may participate in sugar-supply regulation and have essential physiological roles in H.armigera.

Diverse Functional Roles of Monosaccharide Transporters and their Homologs in Vascular Plants： A Physiological Perspective

Vascular plants contain two gene families that encode monosaccharide transporter proteins. The classical monosaccharide transporter（-like） gene superfamily is large and functionally diverse, while the recently identified SWEET transporter family is smaller and, thus far, only found to transport glucose. These transporters play essential roles at many levels, ranging from organelles to the whole plant. Many family members are essential for cellular homeostasis and reproductive success. Although most transporters do not directly participate in long-distance transport, their indirect roles greatly impact carbon allocation and transport flux to the heterotrophic tissues of the plant. Functional characterization of some members from both gene families has revealed their diverse roles in carbohydrate partitioning, phloem function, resource allocation, plant defense, and sugar signaling. This review highlights the broad impacts and implications of monosaccharide transport by describing some of the functional roles of the monosaccharide transporter（-like） superfamily and the SWEET transporter family.

The Medicago truncatula Sucrose Transporter Family： Characterization and Implication of Key Members in Carbon Partitioning towards Arbuscular Mycorrhizal Fungi

We identified de novo sucrose transporter （SUT） genes involved in long-distance transport of sucrose from photosynthetic source leaves towards sink organs in the model leguminous species Medicago truncatula. The iden- tification and functional analysis of sugar transporters provide key information on mechanisms that underlie carbon partitioning in plant-microorganism interactions. In that way, full-length sequences of the M. truncatula SUT （MtSUT） family were retrieved and biochemical characterization of MtSUT members was performed by heterologous expression in yeast. The MtSUT family now comprises six genes which distribute among Dicotyledonous clades. MtSUTI-1 and MtSUT4-1 are key members in regard to their expression profiles in source leaves and sink roots and were characterized as functional H~/sucrose transporters. Physiological and molecular responses to phosphorus supply and inoculation by the arbuscular mycorrhizal fungus （AMF） Glomus intraradices was studied by gene expression and sugar quantification analyses. Sucrose represents the main sugar transport form in M. truncatula and the expression profiles of MtSUTI-1, MtSUT2, and MtSUT4-1 highlight a fine-tuning regulation for beneficial sugar fluxes towards the fungal symbiont. Taken together, these results suggest distinct functions for proteins from the SUT1, SUT2, and SUT4 clades in plant and in bio- trophic interactions.