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SUMMARY The formation and development of storage roots is the most important physiological process in sweetpotato production. Sucrose transporters (SUTs) regulate sucrose transport from source to sink organs and play important roles in growth and development of plants. However, whether SUTs involved in sweetpotato storage roots formation is so far unknown. In this study, we show that IbSUT1, a SUT, is localized to the plasma membrane. Overexpression of IbSUT1 in sweetpotato promotes the sucrose efflux rate from leaves, leading to increased sucrose levels in roots, thus induces lignin deposition in the stele, which inhibits the storage roots formation and compromises the yield. Heterologous expression of IbSUT1 in Arabidopsis significantly increases the sucrose accumulation and promotes lignification in the inflorescence stems. RNA‐seq and biochemical analysis further demonstrated that IbMYB1 negatively regulates the expression of IbSUT1. Overexpression of IbMYB1 in Arabidopsis reduces the sucrose accumulation and lignification degree in the inflorescence stems. Moreover, co‐overexpression of IbMYB1 and IbSUT1 restores the phenotype of lignin over‐deposition in Arabidopsis. Collectively, our results reveal that IbSUT1 regulates source‐sink sucrose transport and participates in the formation of sweetpotato storage roots and highlight the potential application of IbSUT1 in improving sweetpotato yield in the future. [ABSTRACT FROM AUTHOR]

Photosynthetic products in most fleshy fruits are unloaded via the apoplasmic pathway. Sugar transporters play an important role in the apoplasmic unloading pathway and are involved in sugar transport for fruit development. The MdSWEET23, cloned from "Hanfu" apple (Malus × domestica Borkh.) fruits, belongs to Clade III of the SWEET family. Subcellular localization revealed that MdSWEET23 is localized on the plasma membrane. b-glucuronidase activity assays showed that MdSWEET23 was primarily expressed in the sepal and carpel vascular bundle of apple fruits. Heterologous expression assays in yeast showed that MdSWEET23 functions in sucrose transport. The overexpression of MdSWEET23 in the "Orin" calli increased the soluble sugar content. The silencing of MdSWEET23 significantly reduced the contents of sucrose and sorbitol in apple fruits. Ectopic overexpression of MdSWEET23 in tomato altered sugar metabolism and distribution in leaves and fruits, causing a reduction in photosynthetic rates and plant height, enhanced cold stress tolerance, and increased the content of sucrose, fructose, and glucose in breaking color fruits, but did not increase sugar sink potency of tomato fruits. [ABSTRACT FROM AUTHOR]

Background: Sucrose transporters (SUTs) mediate sucrose phloem loading in source tissue and sucrose unloading into sink tissue in potatoes and higher plants, thus playing a crucial role in plant growth and development. In potatoes, the physiological function of the sucrose transporters StSUT1 and StSUT4 has been clarified, whereas the physiological role of StSUT2 is not yet fully understood. Methods and Results: This study analyzed the relative expression of StSUT2 compared to that of StSUT1 and StSUT4 in different tissues from potatoes and its impact on different physiological characteristics by using StSUT2-RNA interference lines. Here, we report a negative effect of StSUT2-RNA interference on plant height, fresh weight, internodes number, leaf area, flowering time, and tuber yield. However, our data indicate that StSUT2 is not involved in carbohydrate accumulation in potato leaves and tubers. In addition, the data of the RNA-seq between the StSUT2-RNA interference line and WT showed that 152 genes were differentially expressed, of which 128 genes were upregulated and 24 genes were downregulated, and the GO and KEGG analyses revealed that differentially expressed genes were mainly related to cell wall composition metabolism. Conclusions: Thus, StSUT2 functions in potato plant growth, flowering time, and tuber yield without affecting carbohydrate accumulation in the leaves and tubers but may be involved in cell wall composition metabolism.

In the developing seeds of all higher plants, filial cells are symplastically isolated from the maternal tissue supplying photosynthate to the reproductive structure. Photoassimilates must be transported apoplastically, crossing several membrane barriers, a process facilitated by sugar transporters. Sugars Will Eventually be Exported Transporters (SWEETs) have been proposed to play a crucial role in apoplastic sugar transport during phloem unloading and the post-phloem pathway in sink tissues. Evidence for this is presented here for developing seeds of the C4 model grass Setaria viridis. Using immunolocalization, SvSWEET4 was detected in various maternal and filial tissues within the seed along the sugar transport pathway, in the vascular parenchyma of the pedicel, and in the xylem parenchyma of the stem. Expression of SvSWEET4a in Xenopus laevis oocytes indicated that it functions as a high-capacity glucose and sucrose transporter. Carbohydrate and transcriptional profiling of Setaria seed heads showed that there were some developmental shifts in hexose and sucrose content and consistent expression of SvSWEET4 homologues. Collectively, these results provide evidence for the involvement of SWEETs in the apoplastic transport pathway of sink tissues and allow a pathway for post-phloem sugar transport into the seed to be proposed. [ABSTRACT FROM AUTHOR]

Phloem loading and sugar distribution are key steps for carbon partitioning in herbaceous and woody species. Although the phloem loading mechanisms in herbs are well studied, less is known for trees. It was shown for saplings of Fagus sylvatica L. and Quercus robur L. that the sucrose concentration in the phloem sap was higher than in the mesophyll cells, which suggests that phloem loading of sucrose involves active steps. However, the question remains whether this also applies for tall trees. To approach this question, tissue-specific sugar and starch contents of small and tall trees of F. sylvatica and Q. robur as well as the sugar concentration in the subcellular compartments of mesophyll cells were examined. Moreover, sucrose uptake transporters (SUTs) were analyzed by heterology expression in yeast and the tissue-specific expressions of SUTs were investigated. Sugar content in leaves of the canopy (11 and 26 m height) was up to 25% higher compared with that of leaves of small trees of F. sylvatica and Q. robur (2 m height). The sucrose concentration in the cytosol of mesophyll cells from tall trees was between 120 and 240 mM and about 4- to 8-fold lower than the sucrose concentration in the phloem sap of saplings. The analyzed SUT sequences of both tree species cluster into three types, similar to SUTs from other plant species. Heterologous expression in yeast confirmed that all analyzed SUTs are functional sucrose transporters. Moreover, all SUTs were expressed in leaves, bark and wood of the canopy and the expression levels in small and tall trees were similar. The results show that the phloem loading in leaves of tall trees of F. sylvatica and Q. robur probably involves active steps, because there is an uphill concentration gradient for sucrose. SUTs may be involved in phloem loading. [ABSTRACT FROM AUTHOR]

Starch is an important reserve of sugar, and starch-sugar conversion in plants plays an important role in the response of plants to various abiotic stresses. Nicosulfuron is a post-emergence herbicide commonly applied to maize fields. However, it is unclear how sucrose and starch in sweet corn are converted to accommodate nicosulfuron stress. Field and pot experiments were conducted to study the effects of nicosulfuron on the sugar metabolism enzymes, starch metabolism enzymes, non-enzyme substances, and expression of key enzyme genes in leaves and roots of sweet maize seedlings. Accordingly, this research compared the responses of the sister lines HK301 and HK320, which are nicosulfuron tolerant and sensitive, respectively. Under nicosulfuron stress, compared with HK301 seedlings, the accumulation of stem and root dry matter of HK320 seedlings was significantly reduced, resulting in a lower root-to-shoot ratio. Compared with HK320 seedlings, nicosulfuron stress significantly increased the sucrose, soluble sugar, and starch contents in HK301 leaves and roots. This may be related to the enhanced carbohydrate metabolism under nicosulfuron stress, including significant changes in sugar metabolism enzyme activity and the levels of SPS and SuSys expression. Further, under nicosulfuron stress, sucrose transporter genes (SUC 1, SUC 2, SWEET 13a, and SWEET 13b) in the leaves and roots of HK301 seedlings were significantly upregulated. Our results emphasize that changes in sugar distribution, metabolism, and transport can improve the adaptability of sweet maize to nicosulfuron stress. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Interdependent metabolic and transport processes of carbon (C) and nitrogen (N) regulate plant growth and development, while the regulatory pathways remain poorly defined. We previously reported that rice circadian clock N-mediated heading date-1 (Nhd1) regulates growth duration-dependent N use efficiency. Here, we report that knockout of Nhd1 in rice reduced the rate of photosynthesis and the sucrose ratio of sheaths to blades, but increased the total C to N ratio and free amino acids. Leaf RNA-seq analysis indicated that mutation of Nhd1 dramatically altered expression of the genes linked to starch and sucrose metabolism, circadian rhythm, and amino acid metabolic pathways. We identified that Nhd1 can directly activate the transcriptional expression of sucrose transporter-1 (OsSUT1). Knockout of Nhd1 suppressed OsSUT1 expression, and both nhd1 and ossut1 mutants showed similar shorter height, and lower shoot biomass and sucrose concentration in comparison with the wild type, while overexpression of OsSUT1 can restore the defective sucrose transport and partially ameliorate the reduced growth of nhd1 mutants. The Nhd1-binding site of the OsSUT1 promoter is conserved in all known rice genomes. The positively related variation of Nhd1 and OsSUT1 expression among randomly selected indica and japonica varieties suggests a common regulatory module of Nhd1– OsSUT1- mediated C and N balance in rice. [ABSTRACT FROM AUTHOR]

At the end of the summer season, grapevine buds (Vitis vinifera L) grown in temperate climates enter a state of winter recess or endodormancy (ED), which is induced by the shortening of the photoperiod, and during this period, the buds accumulate sucrose. In this study, we investigated whether the shortening of the photoperiod regulates the accumulation of sucrose in the buds in the same way as it regulates its entry into the ED. Because sucrose accumulation is regulated by genes that control its transport and degradation, the effect of the SD photoperiod and the transition of buds from paradormancy (PD) to ED on the expression of sucrose transporter (VvSUTs) and invertase genes (VvINVs) was studied. To analyze the possible role of sucrose during ED development, its effect on bud swelling and sprouting was studied on dormant and nondormant buds under forced growth conditions. The results showed that the SD photoperiod upregulates the expression of the VvSUT genes and downregulates that of the VvINV genes in grapevine buds. Additionally, during the transition of buds from PD to ED, the sucrose content increased, the expression of the VvINV genes decreased, and the expression of the VvSUT genes did not change significantly. Sucrose delayed bud swelling and sprouting when applied to dormant buds but had no effect when applied to nondormant buds. Therefore, we concluded that ED development and sucrose accumulation were synchronized events induced by the SD photoperiod and that a sucrose peak marks the end of ED development in grapevine buds. [Display omitted] • Sucrose accumulation and ED are triggered by the SD photoperiod. • Sucrose accumulated in the bud during ED is supplied by the cane. • Exogenous sucrose delay bud swelling and bud sprouting. [ABSTRACT FROM AUTHOR]

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

Sucrose transporter (SUT) is a type of transmembrane protein that exists widely in plants and plays a significant role in the transportation of sucrose and the specific signal sensing process of sucrose. Therefore, identifying sucrose transporter is significant to the study of seed development and plant flowering and growth. In this study, a random forest-based model named ISTRF was proposed to identify sucrose transporter. First, a database containing 382 SUT proteins and 911 non-SUT proteins was constructed based on the UniProt and PFAM databases. Second, k-separated-bigrams-PSSM was exploited to represent protein sequence. Third, to overcome the influence of imbalance of samples on identification performance, the Borderline-SMOTE algorithm was used to overcome the shortcoming of imbalance training data. Finally, the random forest algorithm was used to train the identification model. It was proved by 10-fold cross-validation results that k-separated-bigrams-PSSM was the most distinguishable feature for identifying sucrose transporters. The Borderline-SMOTE algorithm can improve the performance of the identification model. Furthermore, random forest was superior to other classifiers on almost all indicators. Compared with other identification models, ISTRF has the best general performance and makes great improvements in identifying sucrose transporter proteins. [ABSTRACT FROM AUTHOR]

The ever-increasing human population alongside environmental deterioration has presented a pressing demand for increased food production per unit area. As a consequence, considerable research effort is currently being expended in assessing approaches to enhance crop yields. One such approach is to harness the allelic variation lost in domestication. This is of particular importance since crop wild relatives often exhibit better tolerance to abiotic stresses. Here, we wanted to address the question as to why wild rice species have decreased grain production despite being characterized by enhanced rates of photosynthesis. In order to do so, we selected ten rice species on the basis of the presence of genome information, life span, the prominence of distribution, and habitat type and evaluated the expression of genes in photosynthesis, photorespiration, sucrose and starch synthesis, sucrose transport, and primary and secondary cell walls. We additionally measured the levels of a range of primary metabolites via gas chromatography–mass spectrometry. The results revealed that the wild rice species exhibited not only higher photosynthesis but also superior CO2 recovery by photorespiration; showed greater production of photosynthates such as soluble sugars and starch and quick transportation to the sink organs with a possibility of transporting forms such as RFOs, revealing the preferential consumption of soluble sugars to develop both primary and secondary cell walls; and, finally, displayed high glutamine/glutamic acid ratios, indicating that they likely exhibited high N-use efficiency. The findings from the current study thus identify directions for future rice improvement through breeding. [ABSTRACT FROM AUTHOR]

The transport, compartmentation and allocation of sugar are critical for plant growth and development, as well as for stress resistance, but sugar transporter genes have not been comprehensively characterized in soybean. Here, we performed a genome-wide identification and expression analyses of sugar transporter genes in soybean in order to reveal their putative functions. A total of 122 genes encoding sucrose transporters (SUTs) and monosaccharide transporters (MSTs) were identified in soybean. They were classified into 8 subfamilies according to their phylogenetic relationships and their conserved motifs. Comparative genomics analysis indicated that whole genome duplication/segmental duplication and tandem duplication contributed to the expansion of sugar transporter genes in soybean. Expression analysis by retrieving transcriptome datasets suggested that most of these sugar transporter genes were expressed in various tissues, and a number of genes exhibited tissue-specific expression patterns. Several genes including GmSTP21, GmSFP8, and GmPLT5/6/7/8/9 were predominantly expressed in nodules, and GmPLT8 was significantly induced by rhizobia inoculation in root hairs. Transcript profiling and qRT-PCR analyses suggested that half of these sugar transporter genes were significantly induced or repressed under stresses like salt, drought, and cold. In addition, GmSTP22 was found to be localized in the plasma membrane, and its overexpression promoted plant growth and salt tolerance in transgenic Arabidopsis under the supplement with glucose or sucrose. This study provides insights into the evolutionary expansion, expression pattern and functional divergence of sugar transporter gene family, and will enable further understanding of their biological functions in the regulation of growth, yield formation and stress resistance of soybean., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Houqing Zeng reports financial support was provided by Hangzhou Normal University. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Masson SAS. All rights reserved.)

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

Introduction: Photosynthesis provides the energy basis for the life activities of plants by producing organic compounds, mainly sugar. As the main energy form of photosynthesis, sugar affects the growth and development of plants. During long-distance transportation, sucrose is the main form of transportation. The rate of sugar transport and the allocation of carbohydrates affect the biomass of crops and are closely related to the reproductive growth of crops. Main text: The transportation of sugar is divided into active transportation and passive transportation. So how does the sucrose transporters (SUT) genes, which are the main carriers of sucrose in active transportation, affect the performance of rice agronomic traits is still to be explored. In this article, we describe the structure of inflorescence and review the transport forms and metabolic processes of sucrose in rice, such as how CO2 is fixed, carbohydrate assimilation, and transport of organic matter. Sucrose transporters exhibited remarkable effects on the development of reproductive organs in rice. Conclusions: Here, the effects of different factors, such as the effects of anthers morphology on starch enrichment of pollen, effects of biotic and abiotic factors on sucrose transporters, effects of changes in trace elements on sucrose transporters, were discussed. Moreover, the regulation of transcription or translation level provides ideas for future research on sucrose transporters. [ABSTRACT FROM AUTHOR]

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

Peach is an important fruit tree worldwide. Sucrose transporters (SUTs) have been extensively studied in relation to plant growth, development, and stress responses. In this study, we cloned a putative SUT, PpSUT2, from the peach cultivar 'New Kawanakajima.' Bioinformatics analysis predicted that the protein had functional domains typical of the GPH_sucrose family. Phylogenetic analysis revealed high sequence similarity between PpSUT2 and SUTs from other plant species. The qRT-PCR suggested that PpSUT2 was expressed concurrently in the petals, sepals, pistils, stamen, phloem, and leaves. Different trends and different relative expression levels were observed during the flowering and fruit development periods. Western blot analysis produced similar results. The heterologous expression of SUTs in the defective yeast SUSY7/ura3 showed that PpSUT2 had a complementary function on SD sucrose medium. This indicated that it had sucrose transport activity and might be a signal protein. Then, 11PpSUT2 transgenic tobacco plants were obtained.After exposure to low temperature and drought stress conditions, the transgenic overexpression plants exhibited increased proline and soluble sugar content, reduced malondialdehyde (MDA) content and reduced relative conductivity, and less membrane injury. These results suggest that the overexpression of PpSUT2 in tobacco promoted greater resistance to cold and drought. [ABSTRACT FROM AUTHOR]

SWEET (Sugars Will Eventually be Exported Transporter) proteins are well known to play pivotal roles in the growth and development of plants. Here, we report the presence of 43 members of the AhSWEET family in peanut (Arachis hypogaea) and determine their general characteristics including chromosomal localization, gene structure, and numerous physical and chemical features of the proteins. We found that the AhSWEET genes were unevenly distributed among the peanut’s 20 chromosomes. The AhSWEET proteins were hydrophobic with a grand average hydropathicity >0 while a majority of the proteins were basic with isoelectric points >7.0. Additionally, most of the AhSWEET genes contained 6 exons and 5 introns. The expression profiles of the AhSWEET genes were explored based on the previous transcriptome atlas. Interestingly, we found that the AhSWEET genes exhibited differential expression patterns across various organs and tissues during the growth and development of peanut plants. Our study provides a solid foundation of the AhSWEET gene family for further functional characterization of AhSWEET genes in the regulation of peanut growth and development.

Abstract Background Sucrose (Suc), as the precursor molecule for rubber biosynthesis in Hevea brasiliensis, is transported via phloem-mediated long-distance transport from leaves to laticifers in trunk bark, where latex (cytoplasm of laticifers) is tapped for rubber. In our previous report, six Suc transporter (SUT) genes have been cloned in Hevea tree, among which HbSUT3 is verified to play an active role in Suc loading to the laticifers. In this study, another latex-abundant SUT isoform, HbSUT5, with expressions only inferior to HbSUT3 was characterized especially for its roles in latex production. Results Both phylogenetic analysis and subcellular localization identify HbSUT5 as a tonoplast-localized SUT protein under the SUT4-clade (=type III). Suc uptake assay in baker’s yeast reveals HbSUT5 to be a typical Suc-H+ symporter, but its high affinity for Suc (Km = 2.03 mM at pH 5.5) and the similar efficiency in transporting both Suc and maltose making it a peculiar SUT under the SUT4-clade. At the transcript level, HbSUT5 is abundantly and preferentially expressed in Hevea barks. The transcripts of HbSUT5 are conspicuously decreased both in Hevea latex and bark by two yield-stimulating treatments of tapping and ethephon, the patterns of which are contrary to HbSUT3. Under the ethephon treatment, the Suc level in latex cytosol decreases significantly, but that in latex lutoids (polydispersed vacuoles) changes little, suggesting a role of the decreased HbSUT5 expression in Suc compartmentalization in the lutoids and thus enhancing the Suc sink strength in laticifers. Conclusions Our findings provide insights into the roles of a vacuolar sucrose transporter, HbSUT5, in Suc exchange between lutoids and cytosol in rubber-producing laticifers.

The ever-increasing human population alongside environmental deterioration has presented a pressing demand for increased food production per unit area. As a consequence, considerable research effort is currently being expended in assessing approaches to enhance crop yields. One such approach is to harness the allelic variation lost in domestication. This is of particular importance since crop wild relatives often exhibit better tolerance to abiotic stresses. Here, we wanted to address the question as to why wild rice species have decreased grain production despite being characterized by enhanced rates of photosynthesis. In order to do so, we selected ten rice species on the basis of the presence of genome information, life span, the prominence of distribution, and habitat type and evaluated the expression of genes in photosynthesis, photorespiration, sucrose and starch synthesis, sucrose transport, and primary and secondary cell walls. We additionally measured the levels of a range of primary metabolites via gas chromatography–mass spectrometry. The results revealed that the wild rice species exhibited not only higher photosynthesis but also superior CO2 recovery by photorespiration; showed greater production of photosynthates such as soluble sugars and starch and quick transportation to the sink organs with a possibility of transporting forms such as RFOs, revealing the preferential consumption of soluble sugars to develop both primary and secondary cell walls; and, finally, displayed high glutamine/glutamic acid ratios, indicating that they likely exhibited high N-use efficiency. The findings from the current study thus identify directions for future rice improvement through breeding.

Plant acclimation and stress responses depend on the dynamic optimization of carbon balance between source and sink organs. This optimization also applies to the leaf export rate of photosynthetically produced sugars. So far, investigations into the molecular mechanisms of how the rate is controlled have focused on sugar transporters responsible for loading sucrose into the phloem sieve element–companion cell complex of leaf veins. Here, we take a broader view of the various proteins with potential direct influence on the leaf sugar export rate in the model plant Arabidopsis thaliana , helped by the cell type-specific transcriptome data that have recently become available. Furthermore, we integrate current information on the regulation of these potential target proteins. Our analysis identifies putative control points and units of transcriptionally and post-transcriptionally co-regulated genes. Most notable is the potential regulatory unit of sucrose transporters (SUC2, SWEET11, SWEET12, and SUC4) and proton pumps (AHA3 and AVP1). Our analysis can guide future research aimed at understanding the regulatory network controlling leaf sugar export by providing starting points for characterizing regulatory strategies and identifying regulatory factors that link sugar export rate to the major signaling pathways. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

Phloem loading is the first step in sucrose transport from source leaves to sink organs. The phloem loading strategy in rice remains unclear. To determine the potential phloem loading mechanism in rice, yeast invertase (INV) was overexpressed by a 35S promoter specifically in the cell wall to block sugar transmembrane loading in rice. The transgenic lines exhibited obvious phloem loading suppression characteristics accompanied by the accumulation of sucrose and starch, restricted vegetative growth and decreased grain yields. The decreased sucrose exudation rate with p-chloromercuribenzenesulfonic acid (PCMBS) treatment also indicated that rice actively transported sucrose into the phloem. OsSUT1 (SUCROSE TRANSPORTER 1) showed the highest mRNA levels of the plasma membrane-localized OsSUT s in source leaves. Cross sections of the OsSUT::GUS transgenic plants showed that the expression of OsSUT1 and OsSUT5 occurred in the phloem companion cells. Rice ossut1 mutants showed reduced growth and grain yield, supporting the hypothesis of OsSUT1 acting in phloem loading. Based on these results, we conclude that apoplastic phloem loading plays a major role in the export of sugar from rice leaves. [ABSTRACT FROM AUTHOR]

Sucrose is an important carbon source for plant tissue repairing and also a signal involved in defense responses to against insect herbivores. Allocation of sucrose toward to undamaged tissues is an herbivory tolerance mechanism. Sucrose transporters (SUTs) are key factors to control sucrose flow in plants. In this study, herbivory effects caused by rice leaffolder [Cnaphalocrocis medinalis (Guenée)] on OsSUT4 expressions were investigated in rice plants, and the results showed OsSUT4 expression was significantly upregulated by larvae infestation and mechanical wounding. The wounding-induced OsSUT4 expression was not systemically demonstrated in undamaged leaves. The larval chewing-induced OsSUT4 expression was obviously reduced by blocking jasmonic acid (JA) and abscisic acid (ABA) biosynthesis. In addition to JA and ABA, data showed H2O2 generated in wounded tissues also role a signal in the regulatory pathway of herbivore-induced OsSUT4 expressions. Our finding provide insights for further study in signal network of herbivory-induced sucrose translocation.

In most plants, sucrose is the predominant form of carbohydrate for long-distance transport, and sucrose transporters (SUTs) mediate its transportation. In present study, we cloned a new sucrose transporter gene RuSUT2 from blackberry (Rubus spp.) cultivar Boysenberry fruit. The RuSUT2 cDNA contained an open reading frame (ORF) of 1986 nucleotides encoding a 631 amino acids protein. It was predicted that the protein exhibited 11 transmembrane spanning domain and the histidine residue (H) was conserved in the extracellular loop. Subcellular localization investigation indicated that RuSUT2 protein was localized in the plasma membrane. The phylogenetic comparisons revealed that RuSUT2 was a member of SUT2 clade and shared high identity with RcSUC3 in Rosa chinensis, FvSUC3 in Fragaria vesca, and PpSUT2 in Prunus persica. The quantitative real-time PCR (qRT-PCR) was performed to investigate the expression patterns of RuSUT2. The expression of RuSUT2 was detected in all test organs, and showed much higher expression in roots and leaves. During the fruit ripening stage, RuSUT2 was highly expressed at the 33 days after flowering (DAF). Overexpression of RuSUT2 enhanced sucrose content in the mature leaves of the transgenic tobacco. Our findings suggested that RuSUT2 has transport activity and could be involved in the sucrose transportation during blackberry plant growth and development. [ABSTRACT FROM AUTHOR]

Mucoromycotina is one of the earliest fungi to establish a mutualistic relationship with plants in the ancient land. However, the detailed information on their carbon supply from the host plants is largely unknown. In this research, a free-living Mucoromycotina called Gongronella sp. w5 (w5) was employed to explore its effect on Medicago truncatula growth and carbon source utilization from its host plant during the interaction process. W5 promoted M. truncatula growth and caused the sucrose accumulation in M. truncatula root tissue at 16 days post-inoculation (dpi). The transportation of photosynthetic product sucrose to the rhizosphere by M. truncatula root cells seemed accelerated by upregulating the SWEET gene. A predicted cytoplasmic invertase (GspInv) gene and a sucrose transporter (GspSUT1) homology gene in the w5 genome upregulated significantly at the transcriptional level during w5–M. truncatula interaction at 16 dpi, indicating the possibility of utilizing plant sucrose directly by w5 as the carbon source. Further investigation showed that the purified GspInv displayed an optimal pH of 5.0 and a specific activity of 3380 ± 26 U/mg toward sucrose. The heterologous expression of GspInv and GspSUT1 in Saccharomyces cerevisiae confirmed the function of GspInv as invertase and GspSUT1 as sugar transporter with high affinity to sucrose in vivo. Phylogenetic tree analysis showed that the ability of Mucoromycotina to utilize sucrose from its host plant underwent a process of “loss and gain.” These results demonstrated the capacity of Mucoromycotina to interact with extant land higher plants and may employ a novel strategy of directly up-taking and assimilating sucrose from the host plant during the interaction.

Mucoromycotina is one of the earliest fungi to establish a mutualistic relationship with plants in the ancient land. However, the detailed information on their carbon supply from the host plants is largely unknown. In this research, a free-living Mucoromycotina called Gongronella sp. w5 (w5) was employed to explore its effect on Medicago truncatula growth and carbon source utilization from its host plant during the interaction process. W5 promoted M. truncatula growth and caused the sucrose accumulation in M. truncatula root tissue at 16 days post-inoculation (dpi). The transportation of photosynthetic product sucrose to the rhizosphere by M. truncatula root cells seemed accelerated by upregulating the SWEET gene. A predicted cytoplasmic invertase (GspInv) gene and a sucrose transporter (GspSUT1) homology gene in the w5 genome upregulated significantly at the transcriptional level during w5– M. truncatula interaction at 16 dpi, indicating the possibility of utilizing plant sucrose directly by w5 as the carbon source. Further investigation showed that the purified GspInv displayed an optimal pH of 5.0 and a specific activity of 3380 ± 26 U/mg toward sucrose. The heterologous expression of GspInv and GspSUT1 in Saccharomyces cerevisiae confirmed the function of GspInv as invertase and GspSUT1 as sugar transporter with high affinity to sucrose in vivo. Phylogenetic tree analysis showed that the ability of Mucoromycotina to utilize sucrose from its host plant underwent a process of "loss and gain." These results demonstrated the capacity of Mucoromycotina to interact with extant land higher plants and may employ a novel strategy of directly up-taking and assimilating sucrose from the host plant during the interaction. [ABSTRACT FROM AUTHOR]

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.

Abstract Plant sucrose transporters are required for phloem loading, and therefore are essential for plant growth and development. In common beans (Phaseolus vulgaris) there are only two sucrose transporters functionally characterized. Through a previous RNA‐seq study, we identified a putative sucrose transporter in common bean, which we hypothesize to function in import of sucrose into plant cells. In silico analysis revealed that PvSUT1.1 is a putative sucrose‐proton co‐transporter distinct from other characterized sucrose transporters in common bean indicating that this is a previously undescribed transporter protein in beans. Further analysis revealed that PvSUT1.1 shares high protein sequence homology to the phloem loader Arabidopsis SUC2; both have 12 transmembrane domains, a typical characteristic of plant sucrose transporters. Heterologous expression in yeast further showed PvSUT1.1 to be functional and it imported sucrose into yeast cells with a Km of 0.7 mM sucrose. Import of sucrose through PvSUT1.1 is also pH‐dependent with highest uptake at pH 4.0, and activity is lost in the presence of the uncoupler carbonyl cyanide 3‐chlorophenylhydrazone. Consistent with identification of PvSUT1.1 as a Type I transporter, PvSUT1.1 also transports esculin. Finally, PvSUT1.1 showed expression in multiple tissues and the protein was localized to the plasma membrane. The results show that PvSUT1.1 is a sucrose transporter that is probably involved in the uptake of sucrose into source and sink cells. The potential role of PvSUT1.1 in leaf phloem loading of sucrose in common beans and its importance in heat tolerance of reproductive tissues are further discussed.

The sugar distribution mechanism in fruits has been the focus of research worldwide; however, it remains unclear. In order to elucidate the relevant mechanisms in grape berries, the expression, localization, function, and regulation of three sucrose transporters were studied in three representative Vitis varieties. Both SUC11 and SUC12 expression levels were positively correlated with sugar accumulation in grape berries, whereas SUC27 showed a negative relationship. The alignment analysis and sucrose transport ability of isolated SUCs were determined to reflect coding region variations among V. vinifera, V. amurensis Ruper, and V. riparia, indicating that functional variation existed in one SUT from different varieties. Furthermore, potentially oligomerized abilities of VvSUCs colocalized in the sieve elements of the phloem as plasma membrane proteins were verified. The effects of oligomerization on transport properties were characterized in yeast. VvSUC11 and VvSUC12 are high-affinity/low-capacity types of SUTs that stimulate each other by upregulating Vmax and Km, inhibiting sucrose transport, and downregulating the Km of VvSUC27. Thus, changes in the distribution of different SUTs in the same cell govern functional regulation. The activation and inhibition of sucrose transport could be achieved in different stages and tissues of grape development to achieve an effective distribution of sugar.

Plant sucrose transporters are required for phloem loading, and therefore are essential for plant growth and development. In common beans (Phaseolus vulgaris) there are only two sucrose transporters functionally characterized. Through a previous RNA‐seq study, we identified a putative sucrose transporter in common bean, which we hypothesize to function in import of sucrose into plant cells. In silico analysis revealed that PvSUT1.1 is a putative sucrose‐proton co‐transporter distinct from other characterized sucrose transporters in common bean indicating that this is a previously undescribed transporter protein in beans. Further analysis revealed that PvSUT1.1 shares high protein sequence homology to the phloem loader Arabidopsis SUC2; both have 12 transmembrane domains, a typical characteristic of plant sucrose transporters. Heterologous expression in yeast further showed PvSUT1.1 to be functional and it imported sucrose into yeast cells with a Km of 0.7 mM sucrose. Import of sucrose through PvSUT1.1 is also pH‐dependent with highest uptake at pH 4.0, and activity is lost in the presence of the uncoupler carbonyl cyanide 3‐chlorophenylhydrazone. Consistent with identification of PvSUT1.1 as a Type I transporter, PvSUT1.1 also transports esculin. Finally, PvSUT1.1 showed expression in multiple tissues and the protein was localized to the plasma membrane. The results show that PvSUT1.1 is a sucrose transporter that is probably involved in the uptake of sucrose into source and sink cells. The potential role of PvSUT1.1 in leaf phloem loading of sucrose in common beans and its importance in heat tolerance of reproductive tissues are further discussed. [ABSTRACT FROM AUTHOR]

Short and concentrated natural fluorescence hinders tree peony (Paeonia suffruticosa) annual production, and defoliation and gibberellin (GA) application is used to induce its reflowering in autumn. Here, the individual roles of defoliation and GA treatment were determined by monitoring morphological and soluble sugar changes in buds and leaves, and by investigating carbon allocation- and metabolism-related gene expression. Both defoliation and GA treatment induced early bud development, but induction was faster using the GA treatment. Only defoliation, not GA treatment, induced the final reflowering, although their combination accelerated it. Furthermore, defoliation decreased the sucrose content in buds much faster than the GA treatment. This sucrose reduction may play a key role in tree peony reflowering, and the higher carbon metabolism activity in young leaves after defoliation may further help the reflowering process. Defoliation enhanced the expression of sucrose transporters PsSUT4 and PsSWEET12 in buds, and their expression in young leaves was greater than after GA treatment. This indicated that PsSUT4 and PsSWEET12 may help transport carbon into buds after defoliation. In addition, the invertases, PsCIN2 and PsCWIN1 in young leaves were more highly expressed after defoliation, indicating that they may contribute to reflowering after defoliation by accelerating sucrose hydrolysis in young leaves. In addition, the expression levels of PsVIN1 and PsVIN2 in leaves, and PsVIN2 in buds were more highly induced by GA treatment than by defoliation, indicating that PsVINs may mainly respond to GA treatment. These results may help improve the tree peony forcing culture technology and related industrial production. • Defoliation, not GA treatment, induced tree peony reflowering in autumn. • Defoliation induced faster sucrose hydrolysis in buds during reflowering process. • Defoliation also increased the sucrose supply and hydrolysis in young leaves. • After defoliation, PsSUT4 and PsSWEET12 may help transport carbon into buds. • PsCIN2 and PsCWIN1 may be more involved in sucrose hydrolysis in the young leaves. [ABSTRACT FROM AUTHOR]

All multicellular organisms keep a balance between sink and source activities by controlling nutrient transport at strategic positions. In most plants, photosynthetically produced sucrose is the predominant carbon and energy source, whose transport from leaves to carbon sink organs depends on sucrose transporters. In the model plant Arabidopsis thaliana, transport of sucrose into the phloem vascular tissue by SUCROSE TRANSPORTER 2 (SUC2) sets the rate of carbon export from source leaves, just like the SUC2 homologs of most crop plants. Despite their importance, little is known about the proteins that regulate these sucrose transporters. Here, identification and characterization of SUC2-interaction partners revealed that SUC2 activity is regulated via its protein turnover rate and phosphorylation state. UBIQUITIN-CONJUGATING ENZYME 34 (UBC34) was found to trigger turnover of SUC2 in a light-dependentmanner. The E2 enzyme UBC34 could ubiquitinate SUC2 in vitro, a function generally associated with E3 ubiquitin ligases. ubc34 mutants showed increased phloem loading, as well as increased biomass and yield. In contrast, mutants of another SUC2-interaction partner, WALLASSOCIATED KINASE LIKE 8 (WAKL8), showed decreased phloem loading and growth. An in vivo assay based on a fluorescent sucrose analog confirmed that SUC2 phosphorylation byWAKL8 can increase transport activity. Both proteins are required for the up-regulation of phloemloading in response to increased light intensity. The molecular mechanism of SUC2 regulation elucidated here provides promising targets for the biotechnological enhancement of source strength. [ABSTRACT FROM AUTHOR]

Sucrose is the main photosynthetic product in plants, and acts as a major energy substrate and signaling regulator of plant growth. Furthermore, sucrose is involved in the responses to various abiotic stresses. However, the role of sucrose in soybean (Glycine max L.) growth and development under drought stress remains largely unknown. In this study, the two soybean cultivars, Shennong8 (CV.SN8) and Shennong12 (CV.SN12), were grown in pot culture and subjected to three water treatments for 15 days: soil moisture contents of 75 ± 5% (CK), 45 ± 5% (MD), and 30 ± 5% (SD) of field capacity. Under drought stress, the reduction in shoot biomass was more pronounced than the reduction of biomass in the root of both soybean cultivars, resulting in higher root/shoot (R/S) ratio. Drought stress increased the contents of soluble sugar and sucrose in the leaves, but decreased starch content; in the roots, all of these parameters were increased. This may be related to the enhanced carbohydrate metabolism activity under drought stress, including notable changes in the activities of sugar metabolism enzymes and expression levels of GmSPS , GmSuSy , GmC-INV , GmA-INV , GmAMY3 , and GmBAM1. Furthermore, the expression levels of sucrose transporter genes (GmSUC2 , GmSWEET6 , and GmSWEET15) in leaves and roots of soybean seedlings were up-regulated under drought stress. In conclusion, our results highlight that the increase in R/S ratio caused by the changes of sugar allocation, metabolism, and transport under drought stress contributes towards drought resistance of soybean. • Drought stress caused stronger growth inhibition in seedling shoots than in roots. • Drought stress affected sugar allocation, metabolism, and transport. • Soybean root sugar content increased in response to drought stress. [ABSTRACT FROM AUTHOR]

Sugar content and cold tolerance are important apple (Malus × domestica) characteristics targeted by breeding programs. Here, a vacuolar membrane sucrose transporter, MdSWEET16, was cloned and characterized. A phylogenetic tree analysis found that MdSWEET16 was on the same polygenetic branch as Arabidopsis thaliana SWEET16 and SWEET17. MdSWEET16 was located on chromosome 2 and consists of six exons and five introns. The recombinant protein was obtained by prokaryotic induction, and the amino acid sequence and transmembrane domain were analyzed by bioinformatics. The encoded ~ 30-kDa protein has seven transmembrane domains and is localized on the tonoplasts of 'Orin' callus protoplasts. The expression of MdSWEET16 changed in response to sucrose and low temperature in 'Orin' calli. In addition, we also analyzed the expression level of MdSWEET16 during different fruit developmental stages using qRT-PCR. MdSWEET16 was highly expressed in young fruit, and its expression level during fruit development was significantly negatively correlated with sucrose content as assessed by a quantitative fluorescence analysis. Overexpression of MdSWEET16 in 'Orin' calli could reduce their sucrose content but increased their cold tolerance compared with MdSWEET16 RNA interference calli, which indicated that MdSWEET16 is involved in the sucrose transport and cold tolerance of apple. Key message: MdSWEET16's expression level was significantly correlated with the sucrose content and low temperature, and was induced by sucrose and low temperatures. Overexpression of MdSWEET16 increased 'Orin' calli cold tolerance. The prokaryotic expression of its recombinant protein can be used to further study the function of the MdSWEET16 protein in apple. [ABSTRACT FROM AUTHOR]

Background: Sucrose (Suc), as the precursor molecule for rubber biosynthesis in Hevea brasiliensis, is transported via phloem-mediated long-distance transport from leaves to laticifers in trunk bark, where latex (cytoplasm of laticifers) is tapped for rubber. In our previous report, six Suc transporter (SUT) genes have been cloned in Hevea tree, among which HbSUT3 is verified to play an active role in Suc loading to the laticifers. In this study, another latex-abundant SUT isoform, HbSUT5, with expressions only inferior to HbSUT3 was characterized especially for its roles in latex production. Results: Both phylogenetic analysis and subcellular localization identify HbSUT5 as a tonoplast-localized SUT protein under the SUT4-clade (=type III). Suc uptake assay in baker's yeast reveals HbSUT5 to be a typical Suc-H+ symporter, but its high affinity for Suc (Km = 2.03 mM at pH 5.5) and the similar efficiency in transporting both Suc and maltose making it a peculiar SUT under the SUT4-clade. At the transcript level, HbSUT5 is abundantly and preferentially expressed in Hevea barks. The transcripts of HbSUT5 are conspicuously decreased both in Hevea latex and bark by two yield-stimulating treatments of tapping and ethephon, the patterns of which are contrary to HbSUT3. Under the ethephon treatment, the Suc level in latex cytosol decreases significantly, but that in latex lutoids (polydispersed vacuoles) changes little, suggesting a role of the decreased HbSUT5 expression in Suc compartmentalization in the lutoids and thus enhancing the Suc sink strength in laticifers. Conclusions: Our findings provide insights into the roles of a vacuolar sucrose transporter, HbSUT5, in Suc exchange between lutoids and cytosol in rubber-producing laticifers. [ABSTRACT FROM AUTHOR]

Sucrose is an important carbon source for plant tissue repairing and also a signal involved in defense responses to against insect herbivores. Allocation of sucrose toward to undamaged tissues is an herbivory tolerance mechanism. Sucrose transporters (SUTs) are key factors to control sucrose flow in plants. In this study, herbivory effects caused by rice leaffolder [Cnaphalocrocis medinalis (Guenée)] on OsSUT4 expressions were investigated in rice plants, and the results showed OsSUT4 expression was significantly upregulated by larvae infestation and mechanical wounding. The wounding-induced OsSUT4 expression was not systemically demonstrated in undamaged leaves. The larval chewing-induced OsSUT4 expression was obviously reduced by blocking jasmonic acid (JA) and abscisic acid (ABA) biosynthesis. In addition to JA and ABA, data showed H2O2 generated in wounded tissues also role a signal in the regulatory pathway of herbivore-induced OsSUT4 expressions. Our finding provide insights for further study in signal network of herbivory-induced sucrose translocation. [ABSTRACT FROM AUTHOR]

Longan fruit pulp sugar content and sweetness decline after ripening, which affects fruit quality and postharvest storage. Fruit sugar decline research has focused mainly on fruit physiology and biochemistry. To explore the molecular mechanism underlying sugar decline, we conducted transcriptome and metabolome analyses of 'Shixia' longan fruit aril (pulp) tissues at different stages of development. In total, 873 metabolites and 26584 annotated unigenes were identified. Sugar content changes in aril tissues were closely related to gene expression patterns related to starch and sucrose metabolism, such as sucrose phosphate synthase (SPS), invertase (INV), sucrose synthase (SUS), and glucose-6-phosphate isomerase (GPI). Notably, glycolytic pathway gene expression and cellulose and lignin contents increased during the sugar decline stage compared to the sugar accumulation stage. Conversely, sucrose transporter genes, such as DlSWEET2b , DlSWEET10 , and DlSWEET12 , showed the opposite expression pattern, pointing to reduced sucrose transport during sugar decline. These results indicate that longan fruit sugar decline is likely related to an interruption of sugar input (cessation of sucrose transport to the aril) and continued glycolysis and cellulose/lignin biosynthesis. This provides a new way to delay longan fruit sugar decline, by regulating sugar transport instead of inhibiting respiration. • A sudden decrease in sucrose content results in longan sugar receding. • Glycolysis and cellulose and lignin synthesis reduce the sucrose content of longan. • Inhibition of expression of sucrose transporter genes blocks sucrose transport. • Input interruption and continuous consumption of sugar lead to sugar receding. [ABSTRACT FROM AUTHOR]

Sucrose transporters (SUTs) are involved in the transmembrane transport of sucrose. The roles of SUTs in regulating stress tolerance of plants have been reported, but differential results were observed in different plants, implying the need for more studies in different plants. Moreover, the mechanism of SUTs in regulating stress tolerance is still poorly understood. Here, we investigated the role of SlSUT4 from tomato in regulating drought tolerance by using SlSUT4 -overexpressed and -RNAi plants. The results showed that SlSUT4 overexpression decreased the drought tolerance of plants, as demonstrated by reduced seed germination, decreased plant growth, photosynthesis and relative water content, and higher oxidative damage under osmotic stress or drought stress; whereas silencing of SlSUT4 increased the stress tolerance. In the control conditions, the leaf sucrose export rate was reduced in the overexpression plants, but promoted in the RNAi plants, and all transgenic lines had higher leaf sucrose concentrations than the wild type. Accumulation of soluble sugars was more obvious in SlSUT4 -overexpressed plants and the wild type than the RNAi plants under drought. Overexpressing SlSUT4 decreased the proportion of sucrose in the vacuole in the control conditions, whereas the RNAi plants showed an opposite change. These results suggest that alteration in SlSUT4 expression modulates sucrose vacuole-cytosol compartmentation, thus affecting the responses of tomato plants to drought stress. • Silencing SlSUT4 increased drought tolerance of tomato plants. • Overexpressing SlSUT4 decreased drought tolerance of tomato plants. • Silencing SUT4 increased leaf sucrose export while overexpression reduced the export. • Silencing SlSUT4 increased the sucrose proportion in the vacuole. • Overexpressing SlSUT4 increased the sucrose proportion in the cytosol. [ABSTRACT FROM AUTHOR]

In higher plants, sugars (mainly sucrose) are produced by photosynthetically assimilated carbon in mesophyll cells of leaves and translocated to heterotrophic organs to ensure plant growth and development. Sucrose transporters, or sucrose carriers (SUCs), play an important role in the long-distance transportation of sucrose from source organs to sink organs, thereby affecting crop yield and quality. The identification, characterization, and molecular function analysis of sucrose transporter genes have been reported for monocot and dicot plants. However, no relevant study has been reported on sucrose transporter genes in Brassica rapa var. rapa, a cruciferous root crop used mainly as vegetables and fodder. We identified and cloned 12 sucrose transporter genes from turnips, named BrrSUC1.1 to BrrSUC6.2 according to the SUC gene sequences of B. rapa pekinensis. We constructed a phylogenetic tree and analyzed conserved motifs for all 12 sucrose transporter genes identified. Real-time quantitative polymerase chain reaction was conducted to understand the expression levels of SUC genes in different tissues and developmental phases of the turnip. These findings add to our understanding of the genetics and physiology of sugar transport during taproot formation in turnips.

In potato plants, the phloem-mobile miR172 is involved in the sugar-dependent transmission of flower and tuber inducing signal transduction pathways and a clear link between solute transport and the induction of flowering and tuberization was demonstrated. The sucrose transporter StSUT4 seems to play an important role in the photoperiod-dependent triggering of both developmental processes, flowering and tuberization, and the phenotype of StSUT4-inhibited potato plants is reminiscent to miR172 overexpressing plants. The first aim of this study was the determination of the level of miR172 in sink and source leaves of StSUT4-silenced as well as StSUT4-overexpressing plants in comparison to Solanum tuberosum ssp. Andigena wild type plants. The second aim was to investigate the effect of sugars on the level of miRNA172 in whole cut leaves, as well as in whole in vitro plantlets that were supplemented with exogenous sugars. Experiments clearly show a sucrose-dependent induction of the level of mature miR172 in short time as well as long time experiments. A sucrose-dependent accumulation of miR172 was also measured in mature leaves of StSUT4-silenced plants where sucrose export is delayed and sucrose accumulates at the end of the light period.

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Summary: Phloem loading, as the first step of transporting photoassimilates from mesophyll cells to sieve element‐companion cell complex, creates a driving force for long‐distance nutrient transport. Three loading strategies have been proposed: passive symplastic loading, apoplastic loading and symplastic transfer followed by polymer‐trapping of stachyose and raffinose. Although individual species are generally referred to as using a single phloem loading mechanism, it has been suggested that some plants may use more than one, i.e. 'mixed loading'. Here, by using a combination of electron microscopy, reverse genetics and 14C labeling, loading strategies were studied in cucumber, a polymer‐trapping loading species. The results indicate that intermediary cells (ICs), which mediate polymer‐trapping, and ordinary companion cells, which mediate apoplastic loading, were mainly found in the fifth and third order veins, respectively. Accordingly, a cucumber galactinol synthase gene (CsGolS1) and a sucrose transporter gene (CsSUT2) were expressed mainly in the fifth/third and the third order veins, respectively. Immunolocalization analysis indicated that CsGolS1 was localized in companion cells (CCs) while CsSUT2 was in CCs and sieve elements (SEs). Suppressing CsGolS1 significantly decreased the stachyose level and increased sucrose content, while suppressing CsSUT2 decreased the sucrose level and increased the stachyose content in leaves. After 14CO2 labeling, [14C]sucrose export increased and [14C]stachyose export reduced from petioles in CsGolS1i plants, but [14C]sucrose export decreased and [14C]stachyose export increased into petioles in CsSUT2i plants. Similar results were also observed after pre‐treating the CsGolS1i leaves with PCMBS (transporter inhibitor). These results demonstrate that cucumber phloem loading depends on both polymer‐trapping and apoplastic loading strategies. Significance Statement: This study demonstrates a mixed‐loading strategy (active symplastic and apoplastic strategies) in cucumber leaves based on the ultrastructure of companion cells, 14C labeling and reverse genetics, and provides comprehensive evidence from physiology and molecular biology that the apoplastic loading pathway is present in a putative symplastic phloem loader. [ABSTRACT FROM AUTHOR]

Key message: The homologous genes to OsSUT1-5 in wheat were identified and detailed analysed. TaSUT1 was the predominant sucrose transporter group and it illustrated the genotypic variations towards drought during grain filling.Abstract: Sucrose transporters (SUT) play crucial roles in wheat stem water soluble carbohydrate (WSC) remobilization to grain. To determine the major functional SUT gene groups in shoot parts of wheat during grain development, drought tolerant varieties, Westonia and Kauz, were investigated in field drought experiments. Fourteen homologous genes to OsSUT1-5 were identified on five homeologous groups, namely TaSUT1_4A, TaSUT1_4B, TaSUT1_4D; TaSUT2_5A, TaSUT2_5B, TaSUT2_5D; TaSUT3_1A, TaSUT3_1D; TaSUT4_6A, TaSUT4_6B, TaSUT4_6D; TaSUT5_2A, TaSUT5_2B, and TaSUT5_2D, and their gene structures were analysed. Wheat plants above the ground were harvested from pre-anthesis to grain maturity and the stem, leaf sheath, rachis, lemma and developing grain were used for analysing TaSUT gene expression. Grain weight, thousand grain weight, kernel number per spike, biomass and stem WSC were characterized. The study showed that among the five TaSUT groups, TaSUT1 was the predominant sucrose transporting group in all organs sampled, and the expression was particularly high in the developing grain. In contrast to TaSUT1, the gene expression levels of TaSUT2, TaSUT3 and TaSUT4 were lower, except for TaSUT3 which showed preferential expression in the lemma before anthesis. The TaSUT5 gene group was very weakly expressed in all tissues. The upregulated gene expression of TaSUT1 Westonia type in stem and grain reveal a crucial role in stem WSC remobilization to grain under drought. The high TaSUT1 gene expression and the significant correlations with thousand grain weight (TGW) and kernel number per spike demonstrated the contribution in Kauz’s high grain yield in an irrigated environment and high TGW in Westonia under drought stress. Further molecular level identification is required for gene marker development. [ABSTRACT FROM AUTHOR]

Post-translational regulation of sucrose transporters represents one possibility to adapt transporter activity in a very short time frame. This can occur either via phosphorylation/dephosphorylation, oligomerization, protein−protein interactions, endocytosis/exocytosis, or degradation. It is also known that StSUT1 can change its compartmentalization at the plasma membrane and concentrate in membrane microdomains in response to changing redox conditions. A systematic screen for protein−protein-interactions of plant sucrose transporters revealed that the interactome of all three known sucrose transporters from the Solanaceous species Solanum tuberosum and Solanum lycopersicum represents a specific subset of interaction partners, suggesting different functions for the three different sucrose transporters. Here, we focus on factors that affect the subcellular distribution of the transporters. It was already known that sucrose transporters are able to form homo- as well as heterodimers. Here, we reveal the consequences of homo- and heterodimer formation and the fact that the responses of individual sucrose transporters will respond differently. Sucrose transporter SlSUT2 is mainly found in intracellular vesicles and several of its interaction partners are involved in vesicle traffic and subcellular targeting. The impact of interaction partners such as SNARE/VAMP proteins on the localization of SlSUT2 protein will be investigated, as well as the impact of inhibitors, excess of substrate, or divalent cations which are known to inhibit SUT1-mediated sucrose transport in yeast cells. Thereby we are able to identify factors regulating sucrose transporter activity via a change of their subcellular distribution.

Sucrose is the main photosynthesis product of plants and the fundamental carbon skeleton monomer and energy supply for seed formation and development. Drought stress induces decreased photosynthetic carbon assimilation capacity, and seriously affects seed weight in soybean. However, little is known about the relationship between decreases in soybean seed yield and disruption of sucrose metabolism and transport balance in leaves and seeds during the reproductive stages of crop growth. Three soybean cultivars with similar growth periods, “Shennong17”, “Shennong8”, and “Shennong12”, were subjected to drought stress during reproductive growth for 45 days. Drought stress significantly reduced leaf photosynthetic rate, shoot biomass, and seed weight by 63.93, 33.53, and 41.65%, respectively. Drought stress increased soluble sugar contents, the activities of sucrose phosphate synthase, sucrose synthase, and acid invertase enzymes, and up-regulated the expression levels of GmSPS1, GmSuSy2, and GmA-INV, but decreased starch content by 15.13% in leaves. Drought stress decreased the contents of starch, fructose, and glucose in seeds during the late seed filling stages, while it induced sucrose accumulation, which resulted in a decreased hexose-to-sucrose ratio. In developing seeds, the activities of sucrose synthesis and degradation enzymes, the expression levels of genes related to metabolism, and the expression levels of sucrose transporter genes were enhanced during early seed development under drought stress; however, under prolonged drought stress, all of them decreased. These results demonstrated that drought stress enhances the capacity for unloading sucrose into seeds and activated sucrose metabolism during early seed development. At the middle and late seed filling stages, sucrose flow from leaves to seeds was diminished, and the balance of sucrose metabolism was impaired in seeds, resulting in seed mass reduction. The different regulation strategies in sucrose allocation, metabolism, and transport during different seed development stages may be one of the physiological mechanisms for soybean plants to resist drought stress.

Sucrose transporters (SUTs) play key roles in allocating the translocation of assimilates from source to sink tissues. Although the characteristics and biological roles of SUTs have been intensively investigated in higher plants, this gene family has not been functionally characterized in cotton. In this study, we performed a comprehensive analysis of SUT genes in the tetraploid cotton Gossypium hirsutum. A total of 18 G. hirsutum SUT genes were identified and classified into three groups based on their evolutionary relationships. Up to eight SUT genes in G. hirsutum were placed in the dicot-specific SUT1 group, while four and six SUT genes were, respectively, clustered into SUT4 and SUT2 groups together with members from both dicot and monocot species. The G. hirsutum SUT genes within the same group displayed similar exon/intron characteristics, and homologous genes in G. hirsutum At and Dt subgenomes, G. arboreum, and G. raimondii exhibited one-to-one relationships. Additionally, the duplicated genes in the diploid and polyploid cotton species have evolved through purifying selection, suggesting the strong conservation of SUT loci in these species. Expression analysis in different tissues indicated that SUT genes might play significant roles in cotton fiber elongation. Moreover, analyses of cis-acting regulatory elements in promoter regions and expression profiling under different abiotic stress and exogenous phytohormone treatments implied that SUT genes, especially GhSUT6A/D, might participate in plant responses to diverse abiotic stresses and phytohormones. Our findings provide valuable information for future studies on the evolution and function of SUT genes in cotton.