The sugar transporter inventory of tomato: genome-wide identification and expression analysis

A transcriptome-wide identification of ATP-binding cassette (ABC) transporters revealed participation of ABCB subfamily in abiotic stress management of Glycyrrhiza glabra L

Transcriptome-wide survey divulged a total of 181 ABC transporters in G. glabra which were phylogenetically classified into six subfamilies. Protein-Protein interactions revealed nine putative GgABCBs (-B6, -B14, -B15, -B25, -B26, -B31, -B40, -B42 &-B44) corresponding to five AtABCs orthologs (-B1, -B4, -B11, -B19, &-B21). Significant transcript accumulation of ABCB6 (31.8 folds), -B14 (147.5 folds), -B15 (17 folds), -B25 (19.7 folds), -B26 (18.31 folds), -B31 (61.89 folds), -B40 (1273 folds) and -B42 (51 folds) was observed under the influence of auxin. Auxin transport-specific inhibitor, N-1-naphthylphthalamic acid, showed its effectiveness only at higher (10 µM) concentration where it down regulated the expression of ABCBs, PINs (PIN FORMED) and TWD1 (TWISTED DWARF 1) genes in shoot tissues, while their expression was seen to enhance in the root tissues. Further, qRT-PCR analysis under various growth conditions (in-vitro, field and growth chamber), and subjected to abiotic stresses revealed differential expression implicating role of ABCBs in stress management. Seven of the nine genes were shown to be involved in the stress physiology of the plant. GgABCB6, 15, 25 and ABCB31 were induced in multiple stresses, while GgABCB26, 40 & 42 were exclusively triggered under drought stress. No study pertaining to the ABC transporters from G. glabra is available till date. The present investigation will give an insight to auxin transportation which has been found to be associated with plant growth architecture; the knowledge will help to understand the association between auxin transportation and plant responses under the influence of various conditions.

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

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

Fruit Quality and Metabolomic Analyses of Fresh Food Accessions Provide Insights into the Key Carbohydrate Metabolism in Blueberry

Blueberry is a nutrient-rich berry, and its taste and flavor directly determine the consumer preference. Until now, few studies have focused on the comparison of fresh food quality and the key metabolites in superior fresh-eating blueberry cultivars. Herein, fruit quality indicators of 10 highbush blueberry cultivars were evaluated using 'Bluerain' as the control. Appearance quality analysis of fruits showed that 'Brigitta' had a larger fruit size and 'Anna' was the smallest. 'Anna' fruits, followed by 'O'Neal', had the highest ratio of soluble solids to acidity because of their lowest titratable acidity content. Despite the high soluble sugar content, the antioxidants in 'Anna' fruits such as total flavonoids, anthocyanins and vitamin C were lowest among all cultivars, while 'Duke' seemed to have opposite patterns. Furthermore, a total of 553 and 557 metabolites were identified by non-targeted metabolomics liquid chromatography-tandem mass spectrometry (LC-MS/MS) in positive and negative ion mode, respectively. Particularly, the numbers of differentially accumulated metabolites (DAMs) were the most between the 'O'Neal' vs. 'Bluerain' group. The DAMs involved in the metabolic pathways, sesquiterpenoid and triterpenoid biosynthesis, monoterpenoid biosynthesis, galactose metabolism, starch and sucrose metabolism, may be mainly related to the synthesis of flavor and carbohydrate substances. Moreover, the expression patterns of genes involved in sugar metabolism were verified by quantitative real-time PCR (qRT-PCR) analysis in different cultivars. Therefore, the systematical comparison of the quality characteristics, metabolites and expression profiles of related genes in highbush blueberries with good flavor could provide some basis for further research on fresh fruit breeding of blueberries.

Interaction between sugar transport and plant development

Endogenous programs and constant interaction with the environment regulate the development of the plant organism and its individual organs. Sugars are necessary building blocks for plant and organ growth and at the same time act as critical integrators of the metabolic state into the developmental program. There is a growing recognition that the specific type of sugar and its subcellular or tissue distribution is sensed and translated to developmental responses. Therefore, the transport of sugars across membranes is a key process in adapting plant organ properties and overall development to the nutritional state of the plant. In this review, we discuss how plants exploit various sugar transporters to signal growth responses, for example, to control the development of sink organs such as roots or fruits. We highlight which sugar transporters are involved in root and shoot growth and branching, how intracellular sugar allocation can regulate senescence, and, for example, control fruit development. We link the important transport processes to downstream signaling cascades and elucidate the factors responsible for the integration of sugar signaling and plant hormone responses.

GhSTP18, a member of sugar transport proteins family, negatively regulates salt stress in cotton

The sugar transporter protein (STP) family has been shown to play important roles in plant growth, development, and stress response. However, it has not been studied in cotton compared to other major crops. In this study, we identified 90 STP genes from four cotton species, performed bioinformatic analysis, and focused on the role of GhSTP18 in salt stress. According to our results, cotton STP proteins were divided into four subgroups according to the phylogenetic tree. A synteny analysis suggested that whole-genome duplication (WGD) and segmental duplication were key drivers in the expansion of the STP gene family. The transcriptomic data analysis showed that 29 GhSTP genes exhibited sink-specific expression. Quantitative real time-polymerase chain reaction (qRT-PCR) analyses revealed that expression of GhSTP18 was induced by salt treatment, heat treatment, cold treatment, and drought treatment, and continuously increased during a salt stress time course. Notably, GhSTP18 encodes a plasma membrane-localized galactose transporter. Suppression of GhSTP18 transcription by a virus-induced gene silencing (VIGS) assay reduced sensitivity to salt stress in cotton, indicating that GhSTP18 negatively regulates plant salt tolerance. These results provide an important reference and resource for further studying and deploying STP genes for cotton improvement.

Sugars and sucrose transporters in pollinia of Phalaenopsis aphrodite (Orchidaceae)

The pollen grains of Phalaenopsis orchids are clumped tightly together, packed in pollen dispersal units called pollinia. In this study, the morphology, cytology, biochemistry, and sucrose transporters in pollinia of Phalaenopsis orchids were investigated. Histochemical detection was used to characterize the distribution of sugars and callose at the different development stages of pollinia. Ultra-performance liquid chromatography-high resolution-tandem mass spectrometry data indicated that P. aphrodite accumulated abundant saccharides such as sucrose, galactinol, myo-inositol, and glucose, and trace amounts of raffinose and trehalose in mature pollinia. We found that galactinol synthase (PAXXG304680) and trehalose-6-phosphate phosphatase (PAXXG016120) genes were preferentially expressed in mature pollinia. The P. aphrodite genome was identified as having 11 sucrose transporters (SUTs). Our qRT-PCR confirmed that two SUTs (PAXXG030250 and PAXXG195390) were preferentially expressed in the pollinia. Pollinia germinated in pollen germination media (PGM) supplemented with 10% sucrose showed increased callose production and enhanced pollinia germination, but there was no callose or germination in PGM without sucrose. We show that P. aphrodite accumulates high levels of sugars in mature pollinia, providing nutrients and enhanced SUT gene expression for pollinia germination and tube growth.

Identification and molecular characterization of novel sucrose transporters in the hexaploid wheat (Triticum aestivum L.)

Common wheat (Triticum aestivum) is a major cereal crop grown and consumed globally. Recent advances in sequencing technology have facilitated the exploration of large and repetitive genomes. Plant sucrose transporter (SUT) genes are vital components of energy transport systems that play prominent roles in various plant functions, such as signaling and stress regulation. In this study, we identified and analyzed five novel sucrose transporter genes in wheat. The wheat sucrose transporter genes were divided into five clades based on their phylogenetic relationships. Synteny analysis revealed that synteny in the genome is highly conserved between wheat and rye, barley, and Brachypodium. Furthermore, the cis-element analysis indicated that sucrose transporter genes might be regulated by light and some phytohormone-related transcriptional factors. Overall, plant tissue-specific gene expression revealed enhanced expression of the transporter genes in the root and stem, whereas they were differentially expressed under abiotic stress treatments (cold, heat, NaCl, PEG-6000, and sucrose). These results indicate that each TaSUT gene may play a crucial role in stabilizing plants under stress by actively regulating the energy demands of cells. The findings of this study may provide a basis for further research on sucrose transporters and their significant roles in plant energy metabolism as well as in abiotic stress response, signaling, and regulation.

Widely targeted metabolomic, transcriptomic, and metagenomic profiling reveal microbe-plant-metabolic reprogramming patterns mediated by Streptomyces pactum Act12 enhance the fruit quality of Capsicum annuum L

Plant growth-promoting rhizobacteria, such as Streptomyces pactum Act12, promote crop growth and stress resistance, but their contribution to fruit quality is still poorly understood. Herein we conducted a field experiment to ascertain the effects of S. pactum Act12-mediated metabolic reprogramming and underlying mechanisms in pepper (Capsicum annuum L.) fruit based on widely targeted metabolomic and transcriptomic profiling. We additionally performed metagenomic analysis to elucidate the potential relationship between S. pactum Act12-mediated reshaping of rhizosphere microbial communities and pepper fruit quality. Soil inoculation with S. pactum Act12 considerably increased the accumulation of capsaicinoids, carbohydrates, organic acids, flavonoids, anthraquinones, unsaturated fatty acids, vitamins, and phenolic acids in pepper fruit samples. Consequently, fruit flavor, taste, and color were modified, accompanied by elevated contents of nutrients and bioactive compounds. Increased microbial diversity and recruitment of potentially beneficial taxa were observed in inoculated soil samples, with crosstalk between microbial gene functions and pepper fruit metabolism. The reformed structure and function of rhizosphere microbial communities were closely associated with pepper fruit quality. Our findings indicate that S. pactum Act12-mediated interactions between rhizosphere microbial communities and pepper plants are responsible for intricate fruit metabolic reprogramming patterns, which enhance not only overall fruit quality but also consumer acceptability.

A 21-bp InDel in the promoter of STP1 selected during tomato improvement accounts for soluble solid content in fruits

Domestication and improvement are important processes that generate the variation in genome and phonotypes underlying crop improvement. Unfortunately, during selection for certain attributes, other valuable traits may be inadvertently discarded. One example is the decline in fruit soluble solids content (SSC) during tomato breeding. Several genetic loci for SSC have been identified, but few reports on the underlying mechanisms are available. In this study we performed a genome-wide association study (GWAS) for SSC of the red-ripe fruits in a population consisting of 481 tomato accessions with large natural variations and found a new quantitative trait locus, STP1, encoding a sugar transporter protein. The causal variation of STP1, a 21-bp InDel located in the promoter region 1124 bp upstream of the start codon, alters its expression. STP1 ^Insertion accessions with an 21-bp insertion have higher SSC than STP1 ^Deletion accessions with the 21-bp deletion. Knockout of STP1 in TS-23 with high SSC using CRISPR/Cas9 greatly decreased SSC in fruits. In vivo and in vitro assays demonstrated that ZAT10-LIKE, a zinc finger protein transcription factor (ZFP TF), can specifically bind to the promoter of STP1 ^Insertion to enhance STP1 expression, but not to the promoter of STP1 ^Deletion , leading to lower fruit SSC in modern tomatoes. Diversity analysis revealed that STP1 was selected during tomato improvement. Taking these results together, we identified a naturally occurring causal variation underlying SSC in tomato, and a new role for ZFP TFs in regulating sugar transporters. The findings enrich our understanding of tomato evolution and domestication, and provide a genetic basis for genome design for improving fruit taste.

Genome-wide characterization of the inositol transporters gene family in Populus and functional characterization of PtINT1b in response to salt stress

Inositol transporters (INTs) can mediate the transmembrane transport of inositol, and play crucial roles in plant growth, development and stress resistance. However, the INT gene family in Populus has not been reported. Herein, nine INT genes were identified in the Populus trichocarpa genome and divided into three clades. Tandem duplication and whole-genome duplication events could induce the expansion of PtINT gene family. It was worth noting that PtINT1c* and 1d* formed by twice tandem gene duplication events of PtINT1b, but both had undergone partial structural loss during evolution. PtINT2_p1* and PtINT2_p2* might be originated from one INT2 gene by stop codon- and start codon-gain variants. Different members of PtINTs were localized to the plasma membrane or vacuolar membrane. PtINTs had diversified tissue expression profiles, and many members were significantly induced or suppressed after salt and drought treatments. PtINT1b was induced by drought and salinity stresses, and encoded a vacuolar inositol transporter. Overexpression of PtINT1b rendered the transgenic Arabidopsis plants more resistant to salt stress. In conclusion, this study provides valuable clues for future research on the function of PtINTs, and PtINT1b was identified as a candidate gene for genetic engineering to enhance salinity tolerance in plants.

Identification of sugar transporter genes and their roles in the pathogenicity of Verticillium dahliae on cotton

INTRODUCTION

Verticillium wilt (VW) caused by Verticillium dahliae is a soil-borne vascular fungal disease that severely affects cotton yield and fiber quality. Sugar metabolism plays an important role in the growth and pathogenicity of V. dahliae. However, limited information is known about the sugar transporter genes and their roles in the growth and pathogenicity of V. dahliae.

METHOD

In this study, genome-wide identification of sugar transporter genes in V. dahliae was conducted and the expression profiles of these genes in response to root exudates from cotton varieties susceptible or resistant to V. dahliae were investigated based on RNA-seq data. Tobacco Rattle Virus-based host-induced gene silencing (TRV-based HIGS) and artificial small interfering RNAs (asiRNAs) were applied to investigate the function of candidate genes involved in the growth and pathogenic process of V. dahliae.

RESULTS

A total of 65 putative sugar transporter genes were identified and clustered into 8 Clades. Of the 65 sugar transporter genes, 9 were found to be induced only by root exudates from the susceptible variety, including VdST3 and VdST12 that were selected for further functional study. Silencing of VdST3 or VdST12 in host plants by TRV-based HIGS reduced fungal biomass and enhanced cotton resistance against V. dahliae. Additionally, silencing of VdST12 and VdST3 by feeding asiRNAs targeting VdST12 (asiR815 or asiR1436) and VdST3 (asiR201 or asiR1238) inhibited fungal growth, exhibiting significant reduction in hyphae and colony diameter, with a more significant effect observed for the asiRNAs targeting VdST12. The inhibitory effect of asiRNAs on the growth of V. dahliae was enhanced with the increasing concentration of asiRNAs. Silencing of VdST12 by feeding asiR815+asiR1436 significantly decreased the pathogenicity of V. dahliae.

DISCUSSION

The results suggest that VdST3 and VdST12 are sugar transporter genes required for growth and pathogenicity of V. dahliae and that asiRNA is a valuable tool for functional characterization of V. dahliae genes.

The functional analysis of sugar transporter proteins in sugar accumulation and pollen tube growth in pummelo (Citrus grandis)

Sugar transporter proteins (STPs) play vital roles in sugar transport and allocation of carbon sources in plants. However, the evolutionary dynamics of this important gene family and their functions are still largely unknown in citrus, which is the largest fruit crop in the world. In this study, fourteen non-redundant CgSTP family members were identified in pummelo (Citrus grandis). A comprehensive analysis based on the biochemical characteristics, the chromosomal location, the exon-intron structures and the evolutionary relationships demonstrated the conservation and the divergence of CgSTPs. Moreover, CgSTP4, 11, 13, 14 were proofed to be localized in plasma membrane and have glucose transport activity in yeast. The hexose content were significantly increased with the transient overexpression of CgSTP11 and CgSTP14. In addition, antisense repression of CgSTP4 induced the shorter pollen tube length in vitro, implying the potential role of CgSTP4 in pummelo pollen tube growth. Taken together, this work explored a framework for understanding the physiological role of CgSTPs and laid a foundation for future functional studies of these members in citrus species.

Cowpea NAC1/NAC2 transcription factors improve growth and tolerance to drought and heat in transgenic cowpea through combined activation of photosynthetic and antioxidant mechanisms

NAC (NAM/ATAF1/2/CUC2) transcription factors are central switches of growth and stress responses in plants. However, unpredictable interspecies conservation of function and regulatory targets makes the well-studied NAC orthologs inapt for pulse engineering. The knowledge of suitable NAC candidates in hardy pulses like cowpea (Vigna unguiculata (L.) Walp.) is still in infancy, hence warrants immediate biotechnological intervention. Here, we showed that overexpression of two native NAC genes (VuNAC1 and VuNAC2) promoted germinative, vegetative, and reproductive growth and conferred multiple abiotic stress tolerance in a commercial cowpea variety. The transgenic lines displayed increased leaf area, thicker stem, nodule-rich denser root system, early flowering, higher pod production (∼3.2-fold and ∼2.1-fold), and greater seed weight (10.3% and 6.0%). In contrast, transient suppression of VuNAC1/2 caused severe growth retardation and flower inhibition. The overexpressor lines showed remarkable tolerance to major yield-declining terminal stresses, such as drought, salinity, heat, and cold, and recovered growth and seed production by boosting photosynthetic activity, water use efficiency, membrane integrity, Na⁺ /K⁺ homeostasis, and antioxidant activity. The comparative transcriptome study indicated consolidated activation of genes involved in chloroplast development, photosynthetic complexes, cell division and expansion, cell wall biogenesis, nutrient uptake and metabolism, stress response, abscisic acid, and auxin signaling. Unlike their orthologs, VuNAC1/2 direct synergistic transcriptional tuning of stress and developmental signaling to avoid unwanted trade-offs. Their overexpression governs the favorable interplay of photosynthesis and reactive oxygen species regulation to improve stress recovery, nutritional sufficiency, biomass, and production. This unconventional balance of strong stress tolerance and agronomic quality is useful for translational crop research and molecular breeding of pulses.

Carbohydrate and lipid balances in the positive plant phenotypic response to arbuscular mycorrhiza: increase in sink strength

The exchange of phosphorus (P) and carbon (C) between plants and arbuscular mycorrhizal fungi (AMF) is a major determinant of their mutualistic symbiosis. We explored the C dynamics in tomato (Solanum lycorpersicum) inoculated or not with Rhizophagus irregularis to study their growth response under different NaH₂ PO₄ concentrations (Null P, 0 mM; Low P, 0.065 mM; High P, 1.3 mM). The percentage of AMF colonization was similar in plants under Null and Low P, but severely reduced under High P. However, the AMF mass biomarker 16:1ω5 revealed higher fungal accumulation in inoculated roots under Low P, while more AMF spores were produced in the Null P. Under High P, AMF biomass and spores were strongly reduced. Plant growth response to mycorrhiza was negative under Null P, showing reduction in height, biovolume index, and source leaf (SL) area. Under Low P, inoculated plants showed a positive response (e.g., increased SL area), while inoculated plants under High P were similar to non-inoculated plants. AMF promoted the accumulation of soluble sugars in the SL under all fertilization levels, whereas the soluble sugar level decreased in roots under Low P in inoculated plants. Transcriptional upregulation of SlLIN6 and SlSUS1, genes related to carbohydrate metabolism, was observed in inoculated roots under Null P and Low P, respectively. We conclude that P-limiting conditions that increase AMF colonization stimulate plant growth due to an increase in the source and sink strength. Our results suggest that C partitioning and allocation to different catabolic pathways in the host are influenced by AMF performance.

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.

Tomato growth promotion by the fungal endophytes Serendipita indica and Serendipita herbamans is associated with sucrose de-novo synthesis in roots and differential local and systemic effects on carbohydrate metabolisms and gene expression

Plant growth-promoting and stress resilience-inducing root endophytic fungi represent an additional carbohydrate sink. This study aims to test if such root endophytes affect the sugar metabolism of the host plant to divert the flow of resources for their purposes. Fresh and dry weights of roots and shoots of tomato (Solanum lycopersicum) colonised by the closely related Serendipita indica and Serendipita herbamans were recorded. Plant carbohydrate metabolism was analysed by measuring sugar levels, by determining activity signatures of key enzymes of carbohydrate metabolism, and by quantifying mRNA levels of genes involved in sugar transport and turnover. During the interaction with the tomato plants, both fungi promoted root growth and shifted shoot biomass from stem to leaf tissues, resulting in increased leaf size. A common effect induced by both fungi was the inhibition of phosphofructokinase (PFK) in roots and leaves. This glycolytic-pacing enzyme shows how the glycolysis rate is reduced in plants and, eventually, how sugars are allocated to different tissues. Sucrose phosphate synthase (SPS) activity was strongly induced in colonised roots. This was accompanied by increased SPS-A1 gene expression in S. herbamans-colonised roots and by increased sucrose amounts in roots colonised by S. indica. Other enzyme activities were barely affected by S. indica, but mainly induced in leaves of S. herbamans-colonised plants and decreased in roots. This study suggests that two closely related root endophytic fungi differentially influence plant carbohydrate metabolism locally and systemically, but both induce a similar increase in plant biomass. Notably, both fungal endophytes induce an increase in SPS activity and, in the case of S. indica, sucrose resynthesis in roots. In leaves of S. indica-colonised plants, SWEET11b expression was enhanced, thus we assume that excess sucrose was exported by this transporter to the roots. ‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬.

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

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

JULGI-mediated increment in phloem transport capacity relates to fruit yield in tomato

The continuous growth of the global population and the increase in the amount of arid land has severely constrained agricultural crop production. To solve this problem, many researchers have attempted to increase productivity through the efficient distribution of energy; however, the direct relationship between the plant vasculature, specifically phloem development, and crop yield is not well established. Here, we demonstrate that an optimum increase in phloem-transportation capacity by reducing SIJUL expression leads to improved sink strength in tomato (Solanum lycopersicum L.). SIJUL, a negative regulator of phloem development, suppresses the translation of a positive regulator of phloem development, SlSMXL5. The suppression of SlJUL increases the number of phloem cells and sucrose transport, but only an optimal reduction of SlJUL function greatly enhances sink strength in tomato, improving fruit setting, and yield contents by 37% and 60%, respectively. We show that the increment in phloem cell number confers spare transport capacity. Our results suggest that the control of phloem-transport capacity within the threshold could enhance the commitment of photosynthates to instigate yield improvement.

Genome-wide identification of sugar transporter gene family in Brassicaceae crops and an expression analysis in the radish

BACKGROUND

Sugar not only is an important biomacromolecule that plays important roles in plant growth, development, and biotic and abiotic stress tolerance but also provides a skeleton for other macromolecules, such as proteins and nucleic acids. Sugar transporter proteins (STPs) play essential roles in plant sugar transport and ultimately affect the abovementioned life processes. However, the evolutionary dynamics of this important gene family in Brassicaceae crops are still largely unknown, and the functional differentiation of radish STP genes remains unclear.

RESULTS

In the present study, a comparative genomic study of STP genes in five representative Brassicaceae crops was conducted, and a total of 25, 25, 28, 36 and 49 STP genes were individually identified in Raphanus sativus (Rs), Brassica oleracea (Bo), B. rapa (Br), B. napus (Bn) and B. juncea (Bj), which were divided into four clades by phylogenetic analysis. The number of STP genes was no direct correlation with genome size and the total number of coding genes in Brassicaceae crops, and their physical and chemical properties showed no significant difference. Expression analysis showed that radish STP genes play vital roles not only in flower and seedpod development but also under heavy metal (cadmium, chromium and lead), NaCl and PEG-6000 stresses, Agrobacterium tumefaciens infection, and exogenous sugar treatment. RsSTP13.2 was significantly upregulated in the resistant radish cultivar by A. tumefaciens infection and induced by heavy metal, NaCl and PEG-6000 stress, indicating that it is involved in resistance to both biotic and abiotic stress in radish.

CONCLUSIONS

The present study provides insights into the evolutionary patterns of the STP gene family in Brassicaceae genomes and provides a theoretical basis for future functional analysis of STP genes in Brassicaceae crops.

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.

Triiron Tetrairon Phosphate (Fe7(PO4)6) Nanomaterials Enhanced Flavonoid Accumulation in Tomato Fruits

Flavonoids contribute to fruit sensorial and nutritional quality. They are also highly beneficial for human health and can effectively prevent several chronic diseases. There is increasing interest in developing alternative food sources rich in flavonoids, and nano-enabled agriculture provides the prospect for solving this action. In this study, triiron tetrairon phosphate (Fe₇(PO₄)₆) nanomaterials (NMs) were synthesized and amended in soils to enhance flavonoids accumulation in tomato fruits. 50 mg kg⁻¹ of Fe₇(PO₄)₆ NMs was the optimal dose based on its outstanding performance on promoting tomato fruit flavonoids accumulation. After entering tomato roots, Fe₇(PO₄)₆ NMs promoted auxin (IAA) level by 70.75 and 164.21% over Fe-EDTA and control, and then up-regulated the expression of genes related to PM H⁺ ATPase, leading to root proton ef-flux at 5.87 pmol cm⁻² s⁻¹ and rhizosphere acidification. More Mg, Fe, and Mn were thus taken up into plants. Subsequently, photosynthate was synthesized, and transported into fruits more rapidly to increase flavonoid synthesis potential. The metabolomic and transcriptomic profile in fruits further revealed that Fe₇(PO₄)₆ NMs regulated sucrose metabolism, shi-kimic acid pathway, phenylalanine synthesis, and finally enhanced flavonoid biosynthesis. This study implies the potential of NMs to improve fruit quality by enhancing flavonoids synthesis and accumulation.

Multi-omics approaches identify a key gene, PpTST1, for organic acid accumulation in peach

Organic acid content in fruit is an important determinant of peach organoleptic quality, which undergoes considerable variations during development and maturation. However, its molecular mechanism remains largely unclear. In this study, an integrative approach of genome-wide association studies and comparative transcriptome analysis were applied to identify candidate genes involved in organic acid accumulation in peach. A key gene PpTST1, encoding tonoplast sugar transporter, was identified and the genotype of PpTST1 with a single-base transversion (G1584T) in the third exon which leads to a single amino acid substitution (Q528H) was associated with low level of organic acid content in peach. Overexpression of PpTST1His resulted in reduced organic acid content along with increased sugar content both in peach and tomato fruits, suggesting its dual function in sugar accumulation and organic acid content reduction. Two V-type proton ATPases interact with PpTST1 in yeast two-hybridization assay. In addition, the G1584T transversion appeared and gradually accumulated during domestication and improvement, which indicated that PpTST1 was under selection. The identification and characterization of PpTST1 would facilitate the improvement of peach fruit quality.

Extreme sugar accumulation in late fig ripening is accompanied by global changes in sugar metabolism and transporter gene expression

Female fig (Ficus carica L.) fruit are characterized by a major increase in volume and sugar content during the final week of development. A detailed developmental analysis of water and dry matter accumulation during these final days indicated a temporal separation between the increase in volume due to increasing water content and a subsequent sharp increase in sugar content during a few days. The results present fig as an extreme example of sugar import and accumulation, with calculated import rates that are one order of magnitude higher than those of other sugar-accumulating sweet fruit species. To shed light on the metabolic changes occurring during this period, we followed the expression pattern of 80 genes encoding sugar metabolism enzymes and sugar transporter proteins identified in fig fruit. A parallel comparison with male fig fruits, which do not accumulate sugar during ripening, highlighted the genes specifically related to sugar accumulation. Tissue-specific analysis indicated that the expression of genes involved in sugar metabolism and transport undergoes a global transition.

An Updated Review on the Modulation of Carbon Partitioning and Allocation in Arbuscular Mycorrhizal Plants

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that supply mineral nutrients to the host plant in exchange for carbon derived from photosynthesis. Sucrose is the end-product of photosynthesis and the main compound used by plants to translocate photosynthates to non-photosynthetic tissues. AMF alter carbon distribution in plants by modifying the expression and activity of key enzymes of sucrose biosynthesis, transport, and/or catabolism. Since sucrose is essential for the maintenance of all metabolic and physiological processes, the modifications addressed by AMF can significantly affect plant development and stress responses. AMF also modulate plant lipid biosynthesis to acquire storage reserves, generate biomass, and fulfill its life cycle. In this review we address the most relevant aspects of the influence of AMF on sucrose and lipid metabolism in plants, including its effects on sucrose biosynthesis both in photosynthetic and heterotrophic tissues, and the influence of sucrose on lipid biosynthesis in the context of the symbiosis. We present a hypothetical model of carbon partitioning between plants and AMF in which the coordinated action of sucrose biosynthesis, transport, and catabolism plays a role in the generation of hexose gradients to supply carbon to AMF, and to control the amount of carbon assigned to the fungus.

Deciphering fruit sugar transport and metabolism from tolerant and sensitive tomato plants subjected to simulated field conditions

In the current state of climate change, we must assume that abiotic stresses act together under natural field conditions, these will increase in the coming years. Therefore, in this report we investigated how sugar metabolism was affected under simulated field conditions, where plants faced high ambient temperatures and a low-quality water irrigation. Our studies were carried out on fruits of two tomato recombinant lines, a tolerant and a sensitive one exposed to the combination of heat and salinity. Two ripening stages (mature green and red ripe fruits) were used in our analyzes, where the gene expression levels of the main biosynthetic genes and transporters, enzymatic activities and compounds related to the synthesis, accumulation, and degradation of sugars in plants were analyzed. The tolerant line showed highly significant differences in red ripe fruits in comparison to the sensitive one under the simulated field conditions (35°C + 60 mM NaCl), with an overexpression of the genes SlFBP, SlSPS, SlSUS3, and SlNi. These expression patterns correlated with a higher activity of the enzymes FBP, SPS, SUS3, AI, and G6PDH, which resulted in the accumulation of fructose, glucose and UDP-glucose. Our results showed the advantage of using tomato recombinant lines for rescuing important traits, such as the resistance to some abiotic stresses, and for the identification of important molecular and metabolic markers that could be used to determine fruit quality in green or red maturity stages under detrimental environmental field conditions.

Phylogenetic relationships of sucrose transporters (SUTs) in plants and genome-wide characterization of SUT genes in Orchidaceae reveal roles in floral organ development

Sucrose is the primary form of photosynthetically produced carbohydrates transported long distance in many plant species and substantially affects plant growth, development and physiology. Sucrose transporters (SUTs or SUCs) are a group of membrane proteins that play vital roles in mediating sucrose allocation within cells and at the whole-plant level. In this study, we investigated the relationships among SUTs in 24 representative plant species and performed an analysis of SUT genes in three sequenced Orchidaceae species: Dendrobium officinale, Phalaenopsis equestris, and Apostasia shenzhenica. All the SUTs from the 24 plant species were classified into three groups and five subgroups, subgroups A, B1, B2.1, B2.2, and C, based on their evolutionary relationships. A total of 22 SUT genes were identified among Orchidaceae species, among which D. officinale had 8 genes (DoSUT01-08), P. equestris had eight genes (PeqSUT01-08) and A. shenzhenica had 6 genes (AsSUT01-06). For the 22 OrchidaceaeSUTs, subgroups A, B2.2 and C contained three genes, whereas the SUT genes were found to have significantly expanded in the monocot-specific subgroup B2.1, which contained 12 genes. To understand sucrose partitioning and the functions of sucrose transporters in Orchidaceae species, we analyzed the water-soluble sugar content and performed RNA sequencing of different tissues of D. officinale, including leaves, stems, flowers and roots. The results showed that although the total content of water-soluble polysaccharides was highest in the stems of D. officinale, the sucrose content was highest in the flowers. Moreover, gene expression analysis showed that most of the DoSUTs were expressed in the flowers, among which DoSUT01,DoSUT07 and DoSUT06 had significantly increased expression levels. These results indicated that stems are used as the main storage sinks for photosynthetically produced sugar in D. officinale and that DoSUTs mainly function in the cellular machinery and development of floral organs. Our findings provide valuable information on sucrose partitioning and the evolution and functions of SUT genes in Orchidaceae and other species.

Plasma membrane-localized SlSWEET7a and SlSWEET14 regulate sugar transport and storage in tomato fruits

Sugars, especially glucose and fructose, contribute to the taste and quality of tomato fruits. These compounds are translocated from the leaves to the fruits and then unloaded into the fruits by various sugar transporters at the plasma membrane. SWEETs, are sugar transporters that regulate sugar efflux independently of energy or pH. To date, the role of SWEETs in tomato has received very little attention. In this study, we performed functional analysis of SlSWEET7a and SlSWEET14 to gain insight into the regulation of sugar transport and storage in tomato fruits. SlSWEET7a and SlSWEET14 were mainly expressed in peduncles, vascular bundles, and seeds. Both SlSWEET7a and SlSWEET14 are plasma membrane-localized proteins that transport fructose, glucose, and sucrose. Apart from the resulting increase in mature fruit sugar content, silencing SlSWEET7a or SlSWEET14 resulted in taller plants and larger fruits (in SlSWEET7a-silenced lines). We also found that invertase activity and gene expression of some SlSWEET members increased, which was consistent with the increased availability of sucrose and hexose in the fruits. Overall, our results demonstrate that suppressing SlSWEET7a and SlSWEET14 could be a potential strategy for enhancing the sugar content of tomato fruits.

High-spatiotemporal-resolution transcriptomes provide insights into fruit development and ripening in Citrus sinensis

Citrus fruit has a unique structure with soft leathery peel and pulp containing vascular bundles and several segments with many juice sacs. The function and morphology of each fruit tissue are different. Therefore, analysis at the organ-wide or mixed-tissue level inevitably obscures many tissue-specific phenomena. High-throughput RNA sequencing was used to profile Citrus sinensis fruit development based on four fruit tissue types and six development stages from young fruits to ripe fruits. Using a coexpression network analysis, modules of coexpressed genes and hub genes of tissue-specific networks were identified. Of particular, importance is the discovery of the regulatory network of phytohormones during citrus fruit development and ripening. A model was proposed to illustrate how ABF2 mediates the ABA signalling involved in sucrose transport, chlorophyll degradation, auxin homoeostasis, carotenoid and ABA biosynthesis, and cell wall metabolism during citrus fruit development. Moreover, we depicted the detailed spatiotemporal expression patterns of the genes involved in sucrose and citric acid metabolism in citrus fruit and identified several key genes that may play crucial roles in sucrose and citric acid accumulation in the juice sac, such as SWEET15 and CsPH8. The high spatial and temporal resolution of our data provides important insights into the molecular networks underlying citrus fruit development and ripening.

Population-scale peach genome analyses unravel selection patterns and biochemical basis underlying fruit flavor

A narrow genetic basis in modern cultivars and strong linkage disequilibrium in peach (Prunus persica) has restricted resolution power for association studies in this model fruit species, thereby limiting our understanding of economically important quality traits including fruit flavor. Here, we present a high-quality genome assembly for a Chinese landrace, Longhua Shui Mi (LHSM), a representative of the Chinese Cling peaches that have been central in global peach genetic improvement. We also map the resequencing data for 564 peach accessions to this LHSM assembly at an average depth of 26.34× per accession. Population genomic analyses reveal a fascinating history of convergent selection for sweetness yet divergent selection for acidity in eastern vs. western modern cultivars. Molecular-genetics and biochemical analyses establish that PpALMT1 (aluminum-activated malate transporter 1) contributes to their difference of malate content and that increases fructose content accounts for the increased sweetness of modern peach fruits, as regulated by PpERDL16 (early response to dehydration 6-like 16). Our study illustrates the strong utility of the genomics resources for both basic and applied efforts to understand and exploit the genetic basis of fruit quality in peach.

The Sugar Transporter family in wheat (Triticum aestivum. L): genome-wide identification, classification, and expression profiling during stress in seedlings

The sugar transporter protein (STP) plays a crucial role in regulating plant growth and stress tolerance. We performed genome-wide identification and expression analysis of the STP gene family to investigate the STPSs’ potential roles in the growth of wheat seedlings under stress. Here, a total of 81 TaSTP genes containing the Sugar_tr conserved motif were identified within the wheat genome. Bioinformatic studies including phylogenetic tree, chromosome position, and tandem repeat were performed to analyze the identified genes. The 81 TaSTP genes can be classified into five main groups according to their structural and phylogenetic features, with several subgroups, which were located separately on chromosomes A, B, and D. Moreover, six gene clusters were formed with more than three genes each. The results of three comparative syntenic maps of wheat associated with three representative species suggested that STP genes have strong relationships in monocots. qRT-PCR analysis confirmed that most TaSTP genes displayed different expression profiles after seedlings were subjected to six days of different stress (10% PEG6000, 150 mM NaCl, and their combination, respectively), suggesting that these genes may be involved in regulating plant growth and stress tolerance. In conclusion, 81 TaSTP genes were identified and their expressions changed under stress, indicating TaSTP’s potential roles in wheat growth monosaccharide distribution is regulated.

Plant sugars: Homeostasis and transport under abiotic stress in plants

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

The cytosolic invertase NI6 affects vegetative growth, flowering, fruit set, and yield in tomato

Sucrose metabolism is important for most plants, both as the main source of carbon and via signaling mechanisms that have been proposed for this molecule. A cleaving enzyme, invertase (INV) channels sucrose into sink metabolism. Although acid soluble and insoluble invertases have been largely investigated, studies on the role of neutral invertases (A/N-INV) have lagged behind. Here, we identified a tomato A/N-INV encoding gene (NI6) co-localizing with a previously reported quantitative trait locus (QTL) largely affecting primary carbon metabolism in tomato. Of the eight A/N-INV genes identified in the tomato genome, NI6 mRNA is present in all organs, but its expression was higher in sink tissues (mainly roots and fruits). A NI6-GFP fusion protein localized to the cytosol of mesophyll cells. Tomato NI6-silenced plants showed impaired growth phenotype, delayed flowering and a dramatic reduction in fruit set. Global gene expression and metabolite profile analyses of these plants revealed that NI6 is not only essential for sugar metabolism, but also plays a signaling role in stress adaptation. We also identified major hubs, whose expression patterns were greatly affected by NI6 silencing; these hubs were within the signaling cascade that coordinates carbohydrate metabolism with growth and development in tomato.

Comprehensive Analysis of Evolutionary Characterization and Expression for Monosaccharide Transporter Family Genes in Nelumbo nucifera

Sugar transporters, an important class of transporters for sugar function, regulate many processes associated with growth, maturation, and senescence processes in plants. In this study, a total of 35 NuMSTs were identified in the Nelumbo nucifera genome and grouped by conserved domains and phylogenetic analysis. Additionally, we identified 316 MST genes in 10 other representative plants and performed a comparative analysis with Nelumbo nucifera genes, including evolutionary trajectory, gene duplication, and expression pattern. A large number of analyses across plants and algae indicated that the MST family could have originated from STP and Glct, expanding to form STP and SFP by dispersed duplication. Finally, a quantitative real-time polymerase chain reaction and cis-element analysis showed that some of them may be regulated by plant hormones (e.g., abscisic acid), biotic stress factors, and abiotic factors (e.g., drought, excessive cold, and light). We found that under the four abiotic stress conditions, only NuSTP5 expression was upregulated, generating a stress response, and ARBE and LTR were present in NuSTP5. In summary, our findings are significant for understanding and exploring the molecular evolution and mechanisms of NuMSTs in plants.

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

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

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

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

Integrating Sugar Metabolism With Transport: Elevation of Endogenous Cell Wall Invertase Activity Up-Regulates SlHT2 and SlSWEET12c Expression for Early Fruit Development in Tomato

Early fruit development is critical for determining crop yield. Cell wall invertase (CWIN) and sugar transporters both play important roles in carbon allocation and plant development. However, there is little information about the relationship between CWIN and those functionally related sugar transporters during fruit development. By using transgenic tomato with an elevated CWIN activity, we investigated how an increase in CWIN activity may regulate the expression of sugar transporter genes during fruit development. Our analyses indicate that CWIN activity may be under tight regulation by multiple regulators, including two invertase inhibitors (INVINHs) and one defective CWIN (deCWIN) in tomato ovaries prior to anthesis. Among the sugar transporters, expression of SlSWEET12c for sucrose efflux and SlHT2 for hexose uptake was enhanced by the elevated CWIN activity at 10 and 15 days after anthesis of tomato fruit development, respectively. The findings show that some specific sugars will eventually be exported transporters (SWEETs) and hexose transporters (HTs) respond to elevate CWIN activity probably to promote rapid fruit expansion when sucrose efflux from phloem and hexose uptake by parenchyma cell are in high demand. The analyses provide new leads for improving crop yield by manipulating CWIN-responsive sugar transporters, together with CWIN itself, to enhance fruit development and sugar accumulation.

The GAMYB-like gene SlMYB33 mediates flowering and pollen development in tomato

GAMYBs are positive GA signaling factors that exhibit essential functions in reproductive development, particularly in anther and pollen development. However, there is no direct evidence of the regulation of any GAMYB in these biological processes in tomato (Solanum lycopersicum). Here, we identified a tomato GAMYB-like gene, SlMYB33, and characterized its specific roles. SlMYB33 is predominately expressed in the stamens and pistils. During flower development, high mRNA abundance of SlMYB33 is detected in both male and female organs, such as microspore mother cells, anthers, pollen grains, and ovules. Silencing of SlMYB33 leads to delayed flowering, aberrant pollen viability, and poor fertility in tomato. Histological analyses indicate that SlMYB33 exerts its function in pollen development in the mature stage. Further transcriptomic analyses imply that the knockdown of SlMYB33 significantly inhibits the expression of genes related to flowering in shoot apices, and alters the transcription of genes controlling sugar metabolism in anthers. Taken together, our study suggests that SlMYB33 regulates tomato flowering and pollen maturity, probably by modulating the expression of genes responsible for flowering and sugar metabolism, respectively.

Decreased sink/source ratio enhances hexose transport in the fruits of greenhouse tomatoes: integration of gene expression and biochemical analyses

To examine the physiological role of hexose transporters in determining the sink strength of individual fruits, the regulation of hexose transporters gene expression was studied when the sink/source ratio was artificially altered under the greenhouse condition; this was done in two cultivars of tomato, i.e. Grandella and Isabella. The sink/source ratio treatments included: saving one fruit per truss (1F), two fruits per truss (2F), three fruits per truss (3F) and no fruit pruning (control). The results showed that fruit thinning could increase starch, sucrose, and hexose contents in the fruits; it could also modulate the activity of the key enzymes and the expression of tomato hexose transporter genes (LeHTs). Based on the relative transcript levels, all examined LeHTs were unregulated at the end of cell division and the cell expansion stage of fruit development, but the strongest expression level observed at the onset of ripening was related to LeHT1 and LeHT2. Given the concomitancy of cell wall invertase (EC 3.2.1.26) activity and the LeHTs relative expression cell wall, invertase activity seemed to be involved in the expression level of LeHTs. The increased trends of the LeHTs expression with the decrease of the sink/source ratio confirmed the role of hexose transporters in determining the sink strength of the tomato fruits.

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

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

Genomic Dissection of a Wild Region in a Superior Solanum pennellii Introgression Sub-Line with High Ascorbic Acid Accumulation in Tomato Fruit

The Solanum pennellii introgression lines (ILs) have been exploited to map quantitative trait loci (QTLs) and identify favorable alleles that could improve fruit quality traits in tomato varieties. Over the past few years, ILs exhibiting increased content of ascorbic acid in the fruit have been selected, among which the sub-line R182. The aims of this work were to identify the genes of the wild donor S. pennellii harbored by the sub-line and to detect genes controlling ascorbic acid accumulation by using genomics tools. A Genotyping-By-Sequencing (GBS) approach confirmed that no wild introgressions were present in the sub-line besides one region on chromosome 7. By using a dense single nucleotide polymorphism (SNP) map obtained by RNA sequencing (RNA-Seq), the wild region of the sub-line was finely identified; thus, defining 39 wild genes that replaced 33 genes of the ILs genetic background (cv. M82). The differentially expressed genes mapping in the region and the variants detected among the cultivated and the wild alleles evidenced the potential role of the novel genes present in the wild region. Interestingly, one upregulated gene, annotated as a major facilitator superfamily protein, showed a novel structure in R182, with respect to the parental lines. These genes will be further investigated using gene editing strategies.

Overexpression of sugar transporter gene PbSWEET4 of pear causes sugar reduce and early senescence in leaves

As the main energy source for generating ATP during plant growth and development, sugars are synthesized in leaves, while sugar allocation depends on both intracellular transport between different organelles and source-to-sink transport. However, sugar transport related research is limited in pear. Here, a sugar transporter PbSWEET4 was identified that control sugar content and senescence in leaf. Phylogenetic analysis and multiple sequence alignment results indicated that PbSWEET4 was homologous to AtSWEET15, which contained two conserved domains and could promote senescence. The qRT-PCR and transcriptome database result showed that the expression of PbSWEET4 was positively correlated with leaf development, especially highly expressed in older leaves. Furthermore, the evaluation of promoter-GUS activity also indicated that PbSWEET4 exhibited the highest expression level in older leaves. The subcellular localization revealed that the PbSWEET4 localized in the plasma membrane. Finally, overexpression of the PbSWEET4 in strawberry plants could reduce leaf sugar content and chlorophyll content, while accelerate leaf senescence, which might be due to enhanced export of sugars from leaves. These results enrich the knowledge about the function of sugar exporter in regulating the fruit species development, and provide a novel genetic resource for future improvement in carbohydrate partitioning for pear and other fruit trees.

Coexpression of Sucrose Synthase and the SWEET Transporter, Which Are Associated With Sugar Hydrolysis and Transport, Respectively, Increases the Hexose Content in Vitis vinifera L. Grape Berries

The sugar content of grape berries is affected by many factors. To explore the hexose content in different cultivars, the photosynthesis, vegetative, and reproductive biomass, as well as the enzyme activities and expression levels of genes related to sugar metabolism and sugar contents were measured. Samples were collected 70-110 days after anthesis (DAA), from Riesling (RI), Petit Manseng (PM), and Cabernet Sauvignon (CS) berries cultivated in the field. The results indicated that high expression levels of VvSWEET15 and VvSS3 and a high activity of sucrose synthase (SS) are associated with a higher hexose content in the berries of PM than in the berries of the other two cultivars. These genes promoted hexose accumulation in the berries by regulating sugar hydrolysis and transport. The results of this study indicate that active sugar hydrolysis and transport increase the hexose content of PM berries, which provides insights for grape berry quality improvement and breeding projects in wine production. Main Conclusion: The active VvSS3, sucrose synthase (SS), and VvSWEET15 increases the hexose content in Petit Manseng berries, which are associated with sugar hydrolysis and transport.

Transcriptome Analysis of Carbohydrate Metabolism Genes and Molecular Regulation of Sucrose Transport Gene LoSUT on the Flowering Process of Developing Oriental Hybrid Lily 'Sorbonne' Bulb

The quality of Lily cut flower was determined by the quality of bulbs. During the process of vernalization and flower bud differentiation, sugar massively accumulated in the bulb, which influenced the bulb development. However, the details of sugar genes’ regulation mechanism for these processes were not fully understood. Here, morphological physiology, transcriptomes and gene engineering technology were used to explore this physiological change. Seventy-two genes of 25 kinds of sugar metabolism-related genes were annotated after re-analyzing transcriptome data of Oriental hybrid lily ‘Sorbonne’ bulbs, which were generated on Hiseq Illumina 2000. The results showed that these genes were closely related to lily bulb vernalization and development. Combining gene expression pattern with gene co-expression network, five genes (Contig5669, Contig13319, Contig7715, Contig1420 and Contig87292) were considered to be the most potential signals, and the sucrose transporter gene (SUT) was the focus of this study. Carbohydrate transport pathway and genes’ regulation mechanism were inferred through a physiological and molecular test. SUT seemed to be the sugar sensor that could sense and regulate sugar concentration, which might have effects on other genes, such as FT, LFY and so on. LoSUT2 and LoSUT4 genes were cloned from Oriental hybrid lily ‘Sorbonne’ by RACE, which was the first time for these genes in Oriental hybrid lily ‘Sorbonne’. The physiological properties of these proteins were analyzed such as hydrophobicity and phosphorylation. In addition, secondary and tertiary structures of proteins were predicted, which indicated the two proteins were membrane proteins. Their cellular locations were verified through positioning the experiment of the fluorescent vector. They were highly expressed in cells around phloem, which illustrated the key role of these genes in sugar transport. Furthermore, transient expression assays showed that overexpressed LoSUT2 and LoSUT4 in Arabidopsis thaliana bloomed significantly earlier than the wild type and the expression of FT, SOC1 and LFY were also affected by LoSUT2 and LoSUT4, which indicated that LoSUT2 and LoSUT4 may regulate plants flowering time.

Genome-Wide Identification and Expression Analysis of Sugar Transporter (ST) Gene Family in Longan (Dimocarpus longan L.)

Carbohydrates are nutrients and important signal molecules in higher plants. Sugar transporters (ST) play important role not only in long-distance transport of sugar, but also in sugar accumulations in sink cells. Longan (Dimocarpus longan L.) is one of the most important commercial tropical/subtropical evergreen fruit species in Southeast Asia. In this study, a total of 52 longan sugar transporter (DlST) genes were identified and they were divided into eight clades according to phylogenetic analysis. Out of these 52 DlST genes, many plant hormones (e.g., MeJA and gibberellin), abiotic (e.g., cold and drought), and biotic stress responsive element exist in their promoter region. Gene structure analysis exhibited that each of the clades have closely associated gene architectural features based on similar number or length of exons. The numbers of DlSTs, which exhibited alternative splicing (AS) events, in flower bud is more than that in other tissues. Expression profile analysis revealed that ten DlST members may regulate longan flowerbud differentiation. In silico expression profiles in nine longan organs indicated that some DlST genes were tissue specificity and further qRT-PCR analysis suggested that the transcript level of seven DlSTs (DlINT3, DlpGlcT1, DlpGlcT2, DlPLT4, DlSTP1, DlVGT1 and DlVGT2) was consistent with sugar accumulation in fruit, indicating that they might be involved in sugar accumulations during longan fruit development. Our findings will contribute to a better understanding of sugar transporters in woody plant.

Genome-wide identification, expression, and association analysis of the monosaccharide transporter (MST) gene family in peanut (Arachis hypogaea L.)

In this study, we reported the genome-wide analysis of the whole sugar transporter gene family of a legume species, peanut (Arachis hypogaea L.), including the chromosome locations, gene structures, phylogeny, expression patterns, as well as comparative genomic analysis with Arabidopsis, rice, grape, and soybean. A total of 76 AhMST genes (AhMST1-76) were identified from the peanut genome and located unevenly in 20 chromosomes. Phylogeny analysis indicated that the AhMSTs can be divided into eight groups including two undefined peanut-specific groups. Transcriptional profiles revealed that many AhMST genes showed tissue-specific expression, the majority of the AhMST genes mainly expressed in sink organs and floral organ of peanut. Chromosome distribution pattern and synteny analysis strongly indicated that genome-wide segmental and tandem duplication contributed to the expansion of peanut MST genes. Four common orthologs (AhMST9, AhMST13, AhMST40, and AhMST43) between peanut and the other four species were identified by comparative genomic analysis, which might play important roles in maintaining the growth and development of plant. Furthermore, four polymorphic sites in AhMST11, AhMST13, and AhMST60 were significantly correlated with hundred pod weight (HPW) and hundred seed weight (HSW) by association analysis. In a word, these results will provide new insights for understanding the functions of AhMST family members to sugar transporting and the potential for yield improvement in peanut.

Characterization and Expression Analysis of the Ca2+/Cation Antiporter Gene Family in Tomatoes

The Ca²⁺/cation antiporter (CaCA) superfamily plays an important role in the regulation of the essential element Ca²⁺ and cation concentrations. Characterization and expression analyses of CaCA superfamily genes were performed in the tomato (Solanum lycopersicum) as a representative of dicotyledonous plants and fruit crops. Sixteen CaCA candidate genes were found and identified as tomato CaCA, SlCaCA, by a domain search. In a phylogenetic analysis of the SlCaCA superfamily, the 16 genes were classified into SlCAX, SlNCL, SlCCX, and SlMHX families. Among them, Solyc12g011070, belonging to the SlCAX family, had four splice variants, three of which were predicted to be nonfunctional because of a lack of important motifs. EF-hand domains were only found in SlNCL, in addition to consensus Na_Ca_ex domains, and the region containing EF-hand domains was characteristically long in some members of SlNCL. Furthermore, four genes of the SlCCX family were found to be intronless. As for intracellular localization, one SlCCX member was predicted to be localized to the plasma membrane, while other SlCCXs, SlCAXs, and SlMHXs were predicted to be localized to the vacuolar membrane. The expression patterns of SlCaCAs in various organs, including during several developmental stages of fruit, were classified into four groups. Genes involved in each of the SlCAX, SlNCL, and SlCCX gene families were categorized into three or four groups according to expression patterns, suggesting role sharing within each family. The main member in each subfamily and the members with characteristic fruit expression patterns included genes whose expression was regulated by sugar or auxin and that were highly expressed in a line having metabolite-rich fruit.