Vacuoles release sucrose via tonoplast-localised SUC4-type transporters

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

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

Sugar beet PMT5a and STP13 carriers suitable for proton-driven plasma membrane sucrose and glucose import in taproots

Sugar beet (Beta vulgaris) is the major sugar-producing crop in Europe and Northern America, as the taproot stores sucrose at a concentration of around 20%. Genome sequence analysis together with biochemical and electrophysiological approaches led to the identification and characterization of the TST sucrose transporter driving vacuolar sugar accumulation in the taproot. However, the sugar transporters mediating sucrose uptake across the plasma membrane of taproot parenchyma cells remained unknown. As with glucose, sucrose stimulation of taproot parenchyma cells caused inward proton fluxes and plasma membrane depolarization, indicating a sugar/proton symport mechanism. To decipher the nature of the corresponding proton-driven sugar transporters, we performed taproot transcriptomic profiling and identified the cold-induced PMT5a and STP13 transporters. When expressed in Xenopus laevis oocytes, BvPMT5a was characterized as a voltage- and H+-driven low-affinity glucose transporter, which does not transport sucrose. In contrast, BvSTP13 operated as a high-affinity H+/sugar symporter, transporting glucose better than sucrose, and being more cold-tolerant than BvPMT5a. Modeling of the BvSTP13 structure with bound mono- and disaccharides suggests plasticity of the binding cleft to accommodate the different saccharides. The identification of BvPMT5a and BvSTP13 as taproot sugar transporters could improve breeding of sugar beet to provide a sustainable energy crop.

Vacuolar Sugar Transporter TMT2 Plays Crucial Roles in Germination and Seedling Development in Arabidopsis

Vacuolar sugar transporters transport sugar across the tonoplast, are major players in maintaining sugar homeostasis, and therefore play vital roles in plant growth, development, and biomass yield. In this study, we analyzed the physiological roles of the tonoplast monosaccharide transporter 2 (TMT2) in Arabidopsis. In contrast to the wild type (WT) that produced uniform seedlings, the tmt2 mutant produced three types of offspring: un-germinated seeds (UnG), seedlings that cannot form true leaves (tmt2-S), and seedlings that develop normally (tmt2-L). Sucrose, glucose, and fructose can substantially, but not completely, rescue the abnormal phenotypes of the tmt2 mutant. Abnormal cotyledon development, arrested true leaf development, and abnormal development of shoot apical meristem (SAM) were observed in tmt2-S seedlings. Cotyledons from the WT and tmt2-L seedlings restored the growth of tmt2-S seedlings through micrografting. Moreover, exogenous sugar sustained normal growth of tmt2-S seedlings with cotyledon removed. Finally, we found that the TMT2 deficiency resulted in growth defects, most likely via changing auxin signaling, target of rapamycin (TOR) pathways, and cellular nutrients. This study unveiled the essential functions of TMT2 for seed germination and initial seedling development, ensuring cotyledon function and mobilizing sugars from cotyledons to seedlings. It also expanded the current knowledge on sugar metabolism and signaling. These findings have fundamental implications for enhancing plant biomass production or seed yield in future agriculture.

How pollen tubes fight for food: the impact of sucrose carriers and invertases of Arabidopsis thaliana on pollen development and pollen tube growth

Pollen tubes of higher plants grow very rapidly until they reach the ovules to fertilize the female gametes. This growth process is energy demanding, however, the nutrition strategies of pollen are largely unexplored. Here, we studied the function of sucrose transporters and invertases during pollen germination and pollen tube growth. RT-PCR analyses, reporter lines and knockout mutants were used to study gene expression and protein function in pollen. The genome of Arabidopsis thaliana contains eight genes that encode functional sucrose/H⁺ symporters. Apart from AtSUC2, which is companion cell specific, all other AtSUC genes are expressed in pollen tubes. AtSUC1 is present in developing pollen and seems to be the most important sucrose transporter during the fertilization process. Pollen of an Atsuc1 knockout plant contain less sucrose and have defects in pollen germination and pollen tube growth. The loss of other sucrose carriers affects neither pollen germination nor pollen tube growth. A multiple knockout line Atsuc1Atsuc3Atsuc8Atsuc9 shows a phenotype that is comparable to the Atsuc1 mutant line. Loss of AtSUC1 can`t be complemented by AtSUC9, suggesting a special function of AtSUC1. Besides sucrose carriers, pollen tubes also synthesize monosaccharide carriers of the AtSTP family as well as invertases. We could show that AtcwINV2 and AtcwINV4 are expressed in pollen, AtcwINV1 in the transmitting tissue and AtcwINV5 in the funiculi of the ovary. The vacuolar invertase AtVI2 is also expressed in pollen, and a knockout of AtVI2 leads to a severe reduction in pollen germination. Our data indicate that AtSUC1 mediated sucrose accumulation during late stages of pollen development and cleavage of vacuolar sucrose into monosaccharides is important for the process of pollen germination.

The SnRK2.3-AREB1-TST1/2 cascade activated by cytosolic glucose regulates sugar accumulation across tonoplasts in apple and tomato

Soluble sugars are the core components of fruit quality, and the degree of sugar accumulation is largely determined by tonoplast-localized sugar transporters. We previously showed that two classes of tonoplast sugar transporters, MdERDL6 and MdTST1/2, coordinately regulate sugar accumulation in vacuoles. However, the mechanism underlying this coordination remains unknown. Here we discovered that two transcription factors, MdAREB1.1/1.2, regulate MdTST1/2 expression by binding their promoters in apple. The enhanced MdAREB1.1/1.2 expression in MdERDL6-1-overexpression plants resulted in an increase in MdTST1/2 expression and sugar concentration. Further studies established that MdSnRK2.3, whose expression could be regulated by expressing MdERDL6-1, could interact with and phosphorylate MdAREB1.1/1.2, thereby promoting the MdAREB1.1/1.2-mediated transcriptional activation of MdTST1/2. Finally, the orthologous SlAREB1.2 and SlSnRK2.3 exhibited similar functions in tomato fruit as in their apple counterparts. Together, our findings provide insights into the regulatory mechanism of tonoplast sugar transport exerted by SnRK2.3-AREB1-TST1/2 for fruit sugar accumulation.

Sugar concentrations and expression of SUTs suggest active phloem loading in tall trees of Fagus sylvatica and Quercus robur

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.

Natural variations in the PbCPK28 promoter regulate sugar content through interaction with PbTST4 and PbVHA-A1 in pear

Soluble sugars play an important role in plant growth, development and fruit quality. Pear fruits have demonstrated a considerable improvement in sugar quality during their long history of selection. However, little is known about the underlying molecular mechanisms accompanying the changes in fruit sugar content as a result of selection by horticulturists. Here, we identified a calcium-dependent protein kinase (PbCPK28), which is located on LG15 and is present within a selective sweep region, thus linked to the quantitative trait loci for soluble solids. Association analysis indicates that a single nucleotide polymorphism-13 variation (SNP13^T/C ) in the PbCPK28 regulatory region led to fructose content diversity in pear. Elevated expression of PbCPK28 resulted in significantly increased fructose levels in pear fruits. Furthermore, PbCPK28 interacts with and phosphorylates PbTST4, a proton antiporter, thereby coupling the sugar import into the vacuole with proton export. We demonstrated that residues S277 and S314 of PbTST4 are crucial for its function. Additionally, PbCPK28 interacts with and phosphorylates the vacuolar hydrogen proton pump PbVHA-A1, which could provide proton motive forces for PbTST4. We also found that the T11 and Y120 phosphorylation sites in PbVHA-A1 are essential for its function. Evolution analysis and yeast-two-hybrid results support that the CPK-TST/CPK-VHA-A regulatory network is highly conserved in plants, especially the corresponding phosphorylation sites. Together, our work identifies an agriculturally important natural variation and an important regulatory network, allowing genetic improvement of fruit sugar contents in pears through modulation of PbCPK28 expression and phosphorylation of PbTST4 and PbVHA-A1.

Sec24C mediates a Golgi-independent trafficking pathway that is required for tonoplast localisation of ABCC1 and ABCC2

Protein sorting is an essential biological process in all organisms. Trafficking membrane proteins generally relies on the sorting machinery of the Golgi apparatus. However, many proteins have been found to be delivered to target locations via Golgi-independent pathways, but the mechanisms underlying this delivery system remain unknown. Here, we report that Sec24C mediates the direct secretory trafficking of the phytochelatin transporters ABCC1 and ABCC2 from the endoplasmic reticulum (ER) to prevacuolar compartments (PVCs) in Arabidopsis thaliana. Genetic analysis showed that the sec24c mutants are hypersensitive to cadmium (Cd) and arsenic (As) treatments due to mislocalisation of ABCC1 and ABCC2, which results in defects in the vacuole compartmentalisation of the toxic metals. Furthermore, we found that Sec24C recognises ABCC1 and ABCC2 through direct interactions to mediate their exit from the ER to PVCs, which is independent of brefeldin A-sensitive post-Golgi trafficking pathway. These findings expand our understanding of Golgi-independent trafficking, which also provide key insights regarding the mechanism of tonoplast protein sorting and open a new perspective on the function of Sec24 proteins.

Systematic analysis of the sugar accumulation mechanism in sucrose- and hexose- accumulating cherry tomato fruits

BACKGROUND

Sugar content is an important indicator of fruit quality. Except for a few wild tomato species that accumulate sucrose in the fruits, most cultivated tomato species accumulate hexose. Although several studies have focused on wild sucrose-accumulating tomato, the sucrose accumulation mechanism is still unclear.

RESULTS

Here, two homozygous inbred cherry tomato lines ('TB0023' and 'TB0278', which accumulated sucrose and hexose, respectively) were selected to analyze the sugar accumulation mechanism. Carbohydrate analysis, cytological observation, gene expression and enzyme activity analysis and proteomics methods were used in this study. The results indicated that glucose and fructose were absolutely dominant in the soluble sugar content of hexose-accumulating cherry tomato fruit, while sucrose and a certain proportion of hexose were the main forms of soluble sugar in sucrose-accumulating cherry tomato fruit. The phloem unloading pathway of the hexose-accumulating cherry tomato fruit switched from symplastic to apoplastic during fruit development, and the sucrose-accumulating cherry tomato probably had a mixed unloading pathway involving the symplastic and apoplastic. High activity of acid invertase (AI), sucrose phosphate synthase (SPS), sucrose synthase (SS) and sugar transporters LeSUT1, SlSWEET2a and SlSWEET12c were important factors for hexose accumulation in the hexose-accumulating cherry tomato fruit, while LeSUT2, SPS, SS, SlSWEET1b, SlSWEET5b, SlSWEET11b, SlSWEET7a, SlSWEET14 were responsible for solute sugar accumulation in the sucrose-accumulating cherry tomato.

CONCLUSIONS

This study provides detailed evidence for elucidation of the tomato sugar accumulation mechanism from the perspective of cell structure, physiology and molecular biology, providing a theoretical basis for the improvement of tomato quality and aiding the utilization of tomato genetic resources.

Carbon fluxes and environmental interactions during legume development, with a specific focus on Pisum sativum

Grain legumes are major food crops cultivated worldwide for their seeds with high nutritional content. To answer the growing concern about food safety and protein autonomy, legume cultivation must increase in the coming years. In parallel, current agricultural practices are facing environmental challenges, including global temperature increase and more frequent and severe episodes of drought stress. Crop yield directly relies on carbon allocation and is particularly affected by these global changes. We review the current knowledge on source-sink relationships and carbon resource allocation at all developmental stages, from germination to vegetative growth and seed production in grain legumes, focusing on pea (Pisum sativum). We also discuss how these source-sink relationships and carbon fluxes are influenced by biotic and abiotic factors. Major agronomic traits, including seed yield and quality, are particularly impacted by drought, temperatures, salinity, waterlogging, or pathogens and can be improved through the promotion of beneficial soil microorganisms or through optimized plant carbon resource allocation. Altogether, our review highlights the need for a better understanding of the cellular and molecular mechanisms regulating carbon fluxes from source leaves to sink organs, roots, and seeds. These advancements will further improve our understanding of yield stability and stress tolerance and contribute to the selection of climate-resilient crops.

Contributions of sugar transporters to crop yield and fruit quality

The flux, distribution, and storage of soluble sugars regulate crop yield in terms of starch, oil, protein, and total carbohydrates, and affect the quality of many horticultural products. Sugar transporters contribute to phloem loading and unloading. The mechanisms of phloem loading have been studied in detail, but the complex and diverse mechanisms of phloem unloading and sugar storage in sink organs are less explored. Unloading and subsequent transport mechanisms for carbohydrates vary in different sink organs. Analyzing the transport and storage mechanisms of carbohydrates in important storage organs, such as cereal seeds, fruits, or stems of sugarcane, will provide information for genetic improvements to increase crop yield and fruit quality. This review discusses current research progress on sugar transporters involved in carbohydrate unloading and storage in sink organs. The roles of sugar transporters in crop yield and the accumulation of sugars are also discussed to highlight their contribution to efficient breeding.

Tonoplast inositol transporters: Roles in plant abiotic stress response and crosstalk with other signals

Inositol transporters (INT) are thought to be the pivotal transporters for vital metabolites, in particular lipids, minerals, and sugars. These transporters play an important role in transitional metabolism and various signaling pathways in plants through regulating the transduction of messages from hormones, neurotransmitters, and immunologic and growth factors. Extensive studies have been conducted on animal INT, with promising outcomes. However, only few recent studies have highlighted the importance and complexity of INT genes in the regulation of plant physiology stages, including growth and tolerance to stress conditions. The present review summarizes the most recent findings concerning the role of INT or inositol genes in plant metabolism and the response mechanisms triggered by external stressors. Moreover, we highlight the emerging role of vacuoles and vacuolar INT in plant molecular transition and their related roles in plant growth and development. INTs are the essential mediators of inositol uptake and its intracellular broadcasting for various metabolic pathways where they play crucial roles. Additionally, we report evidence on Na⁺/inositol transporters, which until now have only been characterized in animals, as well as H⁺/inositol symporters and their kinetic functions and physiological role and suggest their roles and operating mode in plants. A more comprehensive understanding of the INT functioning system, in particular the coordinated movement of inositol and the relation between inositol generation and other important plant signaling pathways, would greatly advance the study of plant stress adaptation.

Reimport of carbon from cytosolic and vacuolar sugar pools into the Calvin-Benson cycle explains photosynthesis labeling anomalies

SignificancePhotosynthesis metabolites are quickly labeled when ¹³CO₂ is fed to leaves, but the time course of labeling reveals additional contributing processes involved in the metabolic dynamics of photosynthesis. The existence of three such processes is demonstrated, and a metabolic flux model is developed to explore and characterize them. The model is consistent with a slow return of carbon from cytosolic and vacuolar sugars into the Calvin-Benson cycle through the oxidative pentose phosphate pathway. Our results provide insight into how carbon assimilation is integrated into the metabolic network of photosynthetic cells with implications for global carbon fluxes.

Insights on Fructans and Resistance of Plants to Drought Stress

Drought, one of the major abiotic stresses affecting plants, is characterized by a decrease of water availability, resulting in a decrease of the water potential (Ψ) of the cells. One of the strategies of plants in resisting to this low Ψ and related stresses is regulating their water-plant relation and the interplay between Ψsolutes and the turgor pressure (Ψp). This regulation avoids the dehydration induced by low Ψ and is resulting from the accumulation of specific molecules which induce higher tolerance to water deficit and also other mechanisms that prevent or repair cell damages. In plants, fructans, the non-structural carbohydrates (NSC), have other physiological functions than carbon reserve. Among these roles, fructans have been implicated in protecting plants against water deficit caused by drought. As an efficient strategy to survive to this abiotic stress, plants synthesize fructans in response to osmotic pressure in order to osmoregulate the cellular flux, therefore, protecting the membrane damage and maintaining Ψp. Although different studies have been conducted to elucidate the mechanisms behind this strategy, still the concept itself is not well-understood and many points remain unclear and need to be elucidated in order to understand the causal relation between water deficit and fructans accumulation during water scarcity. This understanding will be a key tool in developing strategies to enhance crop tolerance to stressful dry conditions, particularly under the changing climate prediction. This review aims to give new insights on the roles of fructans in the response and resistance of plants to water deficit and their fate under this severe environmental condition.

Overexpression of the vacuolar sugar importer BvTST1 from sugar beet in Camelina improves seed properties and leads to altered root characteristics

Overexpression of the vacuolar sugar transporter TST1 in Arabidopsis leads to higher seed lipid levels and higher total seed yield per plant. However, effects on fruit biomass have not been observed in crop plants like melon, strawberry, cotton, apple, or tomato with increased tonoplast sugar transporter (TST) activity. Thus, it was unclear whether overexpression of TST in selected crops might lead to increased fruit yield, as observed in Arabidopsis. Here, we report that constitutive overexpression of TST1 from sugar beet in the important crop species Camelina sativa (false flax) resembles the seed characteristics observed for Arabidopsis upon increased TST activity. These effects go along with a stimulation of sugar export from source leaves and not only provoke optimised seed properties like higher lipid levels and increased overall seed yield per plant, but also modify the root architecture of BvTST1 overexpressing Camelina lines. Such mutants grew longer primary roots and showed an increased number of lateral roots, especially when developed under conditions of limited water supply. These changes in root properties result in a stabilisation of total seed yield under drought conditions. In summary, we demonstrate that increased vacuolar TST activity may lead to optimised yield of an oil-seed crop species with high levels of healthy ω3 fatty acids in storage lipids. Moreover, since BvTST1 overexpressing Camelina mutants, in addition, exhibit optimised yield under limited water availability, we might devise a strategy to create crops with improved tolerance against drought, representing one of the most challenging environmental cues today and in future.

Plant SWEET Family of Sugar Transporters: Structure, Evolution and Biological Functions

The SWEET (sugars will eventually be exported transporter) family was identified as a new class of sugar transporters that function as bidirectional uniporters/facilitators and facilitate the diffusion of sugars across cell membranes along a concentration gradient. SWEETs are found widely in plants and play central roles in many biochemical processes, including the phloem loading of sugar for long-distance transport, pollen nutrition, nectar secretion, seed filling, fruit development, plant–pathogen interactions and responses to abiotic stress. This review focuses on advances of the plant SWEETs, including details about their discovery, characteristics of protein structure, evolution and physiological functions. In addition, we discuss the applications of SWEET in plant breeding. This review provides more in-depth and comprehensive information to help elucidate the molecular basis of the function of SWEETs in plants.

Sugar Metabolism and Transcriptome Analysis Reveal Key Sugar Transporters during Camellia oleifera Fruit Development

Camellia oleifera is a widely planted woody oil crop with economic significance because it does not occupy cultivated land. The sugar-derived acetyl-CoA is the basic building block in fatty acid synthesis and oil synthesis in C. oleifera fruit; however, sugar metabolism in this species is uncharacterized. Herein, the changes in sugar content and metabolic enzyme activity and the transcriptomic changes during C. oleifera fruit development were determined in four developmental stages (CR6: young fruit formation; CR7: expansion; CR9: oil transformation; CR10: ripening). CR7 was the key period of sugar metabolism since it had the highest amount of soluble sugar, sucrose, and glucose with a high expression of genes related to sugar transport (four sucrose transporters (SUTs) or and one SWEET-like gene, also known as a sugar, will eventually be exported transporters) and metabolism. The significant positive correlation between their expression and sucrose content suggests that they may be the key genes responsible for sucrose transport and content maintenance. Significantly differentially expressed genes enriched in the starch and sucrose metabolism pathway were observed in the CR6 versus CR10 stages according to KEGG annotation. The 26 enriched candidate genes related to sucrose metabolism provide a molecular basis for further sugar metabolism studies in C. oleifera fruit.

The Gene FvTST1 From Strawberry Modulates Endogenous Sugars Enhancing Plant Growth and Fruit Ripening

Sugar is an important carbon source and contributes significantly to the improvement of plant growth and fruit flavor quality. Sugar transport through the tonoplast is important for intracellular homeostasis and metabolic balance in plant cells. There are four tonoplast sugar transporters (FvTST1-4) in strawberry genome. The qRT-PCR results indicated that FvTST1 has a differential expression pattern in different tissues and developmental stages, and exhibited highest expression level in mature fruits. The yeast complementation assay showed that FvTST1 can mediate the uptake of different sugars, such as fructose, glucose, sucrose, and mannose. Subcellular localization analyses revealed that FvTST1 was mainly targeted to the tonoplast. Transient expression of FvTST1 in strawberry fruits enhanced both fruit ripening and sugar accumulation. Furthermore, FvTST1-transformed tomato plants exhibited higher sucrose and auxin content, enhanced seed germination and vegetative growth, higher photosynthetic rate, early flowering, and bore fruit; fructose and glucose levels were higher in transgenic fruits than those in the control. Transcriptomic analysis indicated that the auxin signaling pathway was highly enriched pathway in up-regulated Gene-ontology terms. In transgenic plants, genes encoding transcription factors, such as phytochrome-interacting factors PIF1, -3, and -4, as well as their potential target genes, were also induced. Collectively, the results show that FvTST1 enhances plant growth and fruit ripening by modulating endogenous sugars, and highlight the biological significance of this gene for future breeding purposes.

Series-Spatial Transcriptome Profiling of Leafy Head Reveals the Key Transition Leaves for Head Formation in Chinese Cabbage

Chinese cabbage is an important leaf heading vegetable crop. At the heading stage, its leaves across inner to outer show significant morphological differentiation. However, the genetic control of this complex leaf morphological differentiation remains unclear. Here, we reported the transcriptome profiling of Chinese cabbage plant at the heading stage using 24 spatially dissected tissues representing different regions of the inner to outer leaves. Genome-wide transcriptome analysis clearly separated the inner leaf tissues from the outer leaf tissues. In particular, we identified the key transition leaf by the spatial expression analysis of key genes for leaf development and sugar metabolism. We observed that the key transition leaves were the first inwardly curved ones. Surprisingly, most of the heading candidate genes identified by domestication selection analysis obviously showed a corresponding expression transition, supporting that key transition leaves are related to leafy head formation. The key transition leaves were controlled by a complex signal network, including not only internal hormones and protein kinases but also external light and other stimuli. Our findings provide new insights and the rich resource to unravel the genetic control of heading traits.

The plant axis as the command centre for (re)distribution of sucrose and amino acids

Along with the increase in size required for optimal colonization of terrestrial niches, channels for bidirectional bulk transport of materials in land plants evolved during a period of about 100 million years. These transport systems are essentially still in operation - though perfected over the following 400 million years - and make use of hydrostatic differentials. Substances are accumulated or released at the loading and unloading ends, respectively, of the transport channels. The intermediate stretch between the channel termini is bifunctional and executes orchestrated release and retrieval of solutes. Analyses of anatomical and physiological data demonstrate that the release/retrieval zone extends deeper into sources and sinks than is commonly thought and covers usually much more than 99% of the translocation stretch. This review sketches the significance of events in the intermediate stretch for distribution of organic materials over the plant body. Net leakage from the channels does not only serve maintenance and growth of tissues along the pathway, but also diurnal, short-term or seasonal storage of reserve materials, and balanced distribution of organic C- and N-compounds over axial and terminal sinks. Release and retrieval are controlled by plasma-membrane transporters at the vessel/parenchyma interface in the contact pits along xylem vessels and by plasma-membrane transporters at the interface between companion cells and phloem parenchyma along sieve tubes. The xylem-to-phloem pathway vice versa is a bifacial, radially oriented system comprising a symplasmic pathway, of which entrance and exit are controlled at specific membrane checkpoints, and a parallel apoplasmic pathway. A broad range of specific sucrose and amino-acid transporters are deployed at the checkpoint plasma membranes. SUCs, SUTs, STPs, SWEETs, and AAPs, LTHs, CATs are localized to the plasma membranes in question, both in monocots and eudicots. Presence of Umamits in monocots is uncertain. There is some evidence for endo- and exocytosis at the vessel/parenchyma interface supplementary to the transporter-mediated uptake and release. Actions of transporters at the checkpoints are equally decisive for storage and distribution of amino acids and sucrose in monocots and eudicots, but storage and distribution patterns may differ between both taxa. While the majority of reserves is sequestered in vascular parenchyma cells in dicots, lack of space in monocot vasculature urges "outsourcing" of storage in ground parenchyma around the translocation path. In perennial dicots, specialized radial pathways (rays) include the sites for seasonal alternation of storage and mobilization. In dicots, apoplasmic phloem loading and a correlated low rate of release along the path would favour supply with photoassimilates of terminal sinks, while symplasmic phloem loading and a correlated higher rate of release along the path favours supply of axial sinks and transfer to the xylem. The balance between the resource acquisition by terminal and axial sinks is an important determinant of relative growth rate and, hence, for the fitness of plants in various habitats. Body enlargement as the evolutionary drive for emergence of vascular systems and mass transport propelled by hydrostatic differentials.

Differential regulation of drought stress by biological membrane transporters and channels

Stress arising due to abiotic factors affects the plant's growth and productivity. Among several existing abiotic stressors like cold, drought, heat, salinity, heavy metal, etc., drought condition tends to affect the plant's growth by inducing two-point effect, i.e., it disturbs the water balance as well as induces toxicity by disturbing the ion homeostasis, thus hindering the growth and productivity of plants, and to survive under this condition, plants have evolved several transportation systems that are involved in regulating the drought stress. The role of membrane transporters has gained interest since genetic engineering came into existence, and they were found to be the important modulators for tolerance, avoidance, ion movements, stomatal movements, etc. Here in this comprehensive review, we have discussed the role of transporters (ABA, protein, carbohydrates, etc.) and channels that aids in withstanding the drought stress as well as the regulatory role of transporters involved in osmotic adjustments arising due to drought stress. This review also provides a gist of hydraulic conductivity by roots that are involved in regulating the drought stress.

Sugar export from Arabidopsis leaves: actors and regulatory strategies

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.

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.

Tuber and Tuberous Root Development

Root and tuber crops have been an important part of human nutrition since the early days of humanity, providing us with essential carbohydrates, proteins, and vitamins. Today, they are especially important in tropical and subtropical regions of the world, where they help to feed an ever-growing population. Early induction and storage organ size are important agricultural traits, as they determine yield over time. During potato tuberization, environmental and metabolic status are sensed, ensuring proper timing of tuberization mediated by phloem-mobile signals. Coordinated cellular restructuring and expansion growth, as well as controlled storage metabolism in the tuber, are executed. This review summarizes our current understanding of potato tuber development and highlights similarities and differences to important tuberous root crop species like sweetpotato and cassava. Finally, we point out knowledge gaps that need to be filled before a complete picture of storage organ development can emerge.

Comparison of carbohydrate partitioning and expression patterns of some genes involved in carbohydrate biosynthesis pathways in annual and biennial species of Cichorium spp

Variation in metabolism and partitioning of carbohydrates, particularly fructans, between annual and perennial Cichorium species remains a challenging topic. To address this problem, an annual (endive, Cichorium endive L. var. Crispum; Asteraceae) and a biennial species (chicory, Cichorium intybus L. var. Witloof; Asteraceae) were compared with in terms of variability in carbohydrate accumulation and expression patterns of fructan-active enzyme genes, as well as sucrose metabolism at various growth and developmental stages. In general, constituents such as 1-kestose, nystose, and inulin were detected only in the root of chicory and were not present in any of the endive tissues. For both species, flower tissue contained maximum levels of both fructose and glucose, while for sucrose, more fluctuations were observed. On the other hand, all the genes under study exhibited variation, not only between the two species but also among different tissues at different sampling times. In endive root compared to endive leaf, the expression of cell wall invertase genes and sucrose accumulation decreased simultaneously, indicating the limited capacity of its roots to absorb sucrose, a precursor to inulin production. In addition, low expression of fructan: fructan fructosyltransferase in endive root compared to chicory root confirmed the inability of endive to inulin synthesis. Overall, annual and biennial species were different in the production of inulin, transport, remobilization, and unloading of sucrose.

Improved resource allocation and stabilization of yield under abiotic stress

Sugars are the main building blocks for carbohydrate storage, but also serve as signaling molecules and protective compounds during abiotic stress responses. Accordingly, sugar transport proteins fulfill multiple roles as they mediate long distance sugar allocation, but also shape the subcellular and tissue-specific carbohydrate profiles by balancing the levels of these molecules in various compartments. Accordingly, transporter activity represents a target by classical or directed breeding approaches, to either, directly increase phloem loading or to increase sink strength in crop species. The relative subcellular distribution of sugars is critical for molecular signaling affecting yield-relevant processes like photosynthesis, onset of flowering and stress responses, while controlled long-distance sugar transport directly impacts development and productivity of plants. However, long-distance transport is prone to become unbalanced upon adverse environmental conditions. Therefore, we highlight the influence of stress stimuli on sucrose transport in the phloem and include the role of stress induced cellular carbohydrate sinks, like raffinose or fructans, which possess important roles to build up tolerance against challenging environmental conditions. In addition, we report on recent breeding approaches that resulted in altered source and sink capacities, leading to increased phloem sucrose shuttling in crops. Finally, we present strategies integrating the need of cellular stress-protection into the general picture of long-distance transport under abiotic stress, and point to possible approaches improving plant performance and resource allocation under adverse environmental conditions, leading to stabilized or even increased crop yield.

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.

GeSUT4 mediates sucrose import at the symbiotic interface for carbon allocation of heterotrophic Gastrodia elata (Orchidaceae)

Gastrodia elata, a fully mycoheterotrophic orchid without photosynthetic ability, only grows symbiotically with the fungus Armillaria. The mechanism of carbon distribution in this mycoheterotrophy is unknown. We detected high sucrose concentrations in all stages of Gastrodia tubers, suggesting sucrose may be the major sugar transported between fungus and orchid. Thick symplasm-isolated wall interfaces in colonized and adjacent large cells implied involvement of sucrose importers. Two sucrose transporter (SUT)-like genes, GeSUT4 and GeSUT3, were identified that were highly expressed in young Armillaria-colonized tubers. Yeast complementation and isotope tracer experiments confirmed that GeSUT4 functioned as a high-affinity sucrose-specific proton-dependent importer. Plasma-membrane/tonoplast localization of GeSUT4-GFP fusions and high RNA expression of GeSUT4 in symbiotic and large cells indicated that GeSUT4 likely functions in active sucrose transport for intercellular allocation and intracellular homeostasis. Transgenic Arabidopsis overexpressing GeSUT4 had larger leaves but were sensitive to excess sucrose and roots were colonized with fewer mutualistic Bacillus, supporting the role of GeSUT4 in regulating sugar allocation. This is not only the first documented carbon import system in a mycoheterotrophic interaction but also highlights the evolutionary importance of sucrose transporters for regulation of carbon flow in all types of plant-microbe interactions.

MdERDL6-mediated glucose efflux to the cytosol promotes sugar accumulation in the vacuole through up-regulating TSTs in apple and tomato

Sugar transport across membranes is essential for maintaining cellular sugar homeostasis and metabolic balance in plant cells. However, it remains unclear how this process is regulated among different classes of sugar transporters. Here, we identified an apple tonoplast H⁺/glucose symporter, MdERDL6-1, that exports glucose to cytosols to up-regulate the expression of H⁺/sugar antiporter genes TST1 and TST2 to import sugars from cytosol to vacuole for accumulation to high concentrations in apples and tomatoes. The findings provide insights into the regulatory mechanism underlying sugar exchange between cytosol and vacuole.

Sugar transport across tonoplasts is essential for maintaining cellular sugar homeostasis and metabolic balance in plant cells. It remains unclear, however, how this process is regulated among different classes of sugar transporters. Here, we identified a tonoplast H⁺/glucose symporter, MdERDL6-1, from apples, which was highly expressed in fruits and exhibited expression patterns similar to those of the tonoplast H⁺/sugar antiporters MdTST1 and MdTST2. Overexpression of MdERDL6-1 unexpectedly increased not only glucose (Glc) concentration but also that of fructose (Fru) and sucrose (Suc) in transgenic apple and tomato leaves and fruits. RNA sequencing (RNA-seq) and expression analyses showed an up-regulation of TST1 and TST2 in the transgenic apple and tomato lines overexpressing MdERDL6-1. Further studies established that the increased sugar concentration in the transgenic lines correlated with up-regulation of TST1 and TST2 expression. Suppression or knockout of SlTST1 and SlTST2 in the MdERDL6-1–overexpressed tomato background reduced or abolished the positive effect of MdERDL6-1 on sugar accumulation, respectively. The findings demonstrate a regulation of TST1 and TST2 by MdERDL6-1, in which Glc exported by MdERDL6-1 from vacuole up-regulates TST1 and TST2 to import sugars from cytosol to vacuole for accumulation to high concentrations. The results provide insight into the regulatory mechanism of sugar accumulation in vacuoles mediated by the coordinated action of two classes of tonoplast sugar transporters.

Sugar and Hormone Dynamics and the Expression Profiles of SUT/SUC and SWEET Sweet Sugar Transporters during Flower Development in Petunia axillaris

Flowering is the first committed step of plant sexual reproduction. While the developing flower is a strong sink requiring large quantity of sugars from photosynthetic source tissues, this process is under-temper-spatially controlled via hormone signaling pathway and nutrient availability. Sugar transporters SUT/SUC and SWEET mediate sugars movement across membranes and play a significant role in various physiological processes, including reproductive organ development. In Petunia axillaris, a model ornamental plant, 5 SUT/SUC and 36 SWEET genes are identified in the current version of the genome. Analysis of their gene structure and chromosomal locations reveal that SWEET family is moderately expanded. Most of the transporter genes are abundantly expressed in the flower than in other organs. During the five flower developmental stages, transcript levels of PaSUT1, PaSUT3, PaSWEET13c, PaSWEET9a, PaSWEET1d, PaSWEET5a and PaSWEET14a increase with the maturation of the flower and reach their maximum in the fully open flowers. PaSWEET9c, the nectar-specific PhNEC1 orthologous, is expressed in matured and fully opened flowers. Moreover, determination of sugar concentrations and phytohormone dynamics in flowers at the five developmental stages shows that glucose is the predominant form of sugar in young flowers at the early stage but depletes at the later stage, whereas sucrose accumulates only in maturated flowers prior to the corolla opening. On the other hand, GA₃ content and to a less extent IAA and zeatin decreases with the flower development; however, JA, SA and ABA display a remarkable peak at mid- or later flower developmental stage.

Defoliation-induced compensatory transpiration is compromised in SUT4-RNAi Populus

The tonoplast sucrose transporter PtaSUT4 is well expressed in leaves of Populus tremula × Populus alba (INRA 717-IB4), and its inhibition by RNA-interference (RNAi) alters leaf sucrose homeostasis. Whether sucrose partitioning between the vacuole and the cytosol is modulated by PtaSUT4 for specific physiological outcomes in Populus remains unexplored. In this study, partial defoliation was used to elicit compensatory increases in photosynthesis and transpiration by the remaining leaves in greenhouse-grown poplar. Water uptake, leaf gas exchange properties, growth and nonstructural carbohydrate abundance in source and sink organs were then compared between wild-type and SUT4-RNAi lines. Partial defoliation increased maximum photosynthesis rates similarly in all lines. There was no indication that source leaf sugar levels changed differently between wild-type and RNAi plants following partial defoliation. Sink levels of hexose (glucose and fructose) and starch decreased similarly in all lines. Interestingly, plant water uptake after partial defoliation was not as well sustained in RNAi as in wild-type plants. While the compensatory increase in photosynthesis was similar between genotypes, leaf transpiration increased less robustly in RNAi than wild-type plants. SUT4-RNAi and wild-type source leaves differed constitutively in their bulk modulus of elasticity, a measure of leaf turgor, and storage water capacitance. The data demonstrate that reduced sucrose partitioning due to PtaSUT4-RNAi altered turgor control and compensatory transpiration capacity more strikingly than photosynthesis and sugar export. The results are consistent with the interpretation that SUT4 may control vacuolar turgor independently of sink carbon provisioning.

Sugar Signaling During Fruit Ripening

Sugars play a key role in fruit quality, as they directly influence taste, and thus consumer acceptance. Carbohydrates are the main resources needed by the plant for carbon and energy supply and have been suggested to be involved in all the important developmental processes, including embryogenesis, seed germination, stress responses, and vegetative and reproductive growth. Recently, considerable progresses have been made in understanding regulation of fruit ripening mechanisms, based on the role of ethylene, auxins, abscisic acid, gibberellins, or jasmonic acid, in both climacteric and non-climacteric fruits. However, the role of sugar and its associated molecular network with hormones in the control of fruit development and ripening is still poorly understood. In this review, we focus on sugar signaling mechanisms described up to date in fruits, describing their involvement in ripening-associated processes, such as pigments accumulation, and their association with hormone transduction pathways, as well as their role in stress-related responses.

Vacuolar sucrose homeostasis is critical for plant development, seed properties, and night-time survival in Arabidopsis

Most cellular sucrose is present in the cytosol and vacuoles of plant cells; however, little is known about the effect of this sucrose compartmentation on plant properties. Here, we examined the effects of altered intracellular sucrose compartmentation in Arabidopsis thaliana leaves by heterologously expressing the sugar beet (Beta vulgaris) vacuolar sucrose loader BvTST2.1 and by generating lines with reduced vacuolar invertase activity (amiR vi1-2). Heterologous expression of BvTST2.1 led to increased monosaccharide levels in leaves, whereas sucrose levels remained constant, indicating that vacuolar invertase activity in mesophyll vacuoles exceeds sucrose uptake. This notion was supported by analysis of tobacco (Nicotiana benthamiana) leaves transiently expressing BvTST2.1 and the invertase inhibitor NbVIF. However, sucrose levels were strongly elevated in leaf extracts from amiR vi1-2 lines, and experiments confirmed that sucrose accumulated in the corresponding vacuoles. The amiR vi1-2 lines exhibited impaired early development and reduced seed weight. When germinated in the dark, amiR vi1-2 seedlings were less able to convert sucrose into monosaccharides than the wild type. Cold temperatures strongly down-regulated both VI genes, but the amiR vi1-2 lines showed normal frost tolerance. These observations indicate that increased vacuolar sucrose levels fully compensate for the effects of low monosaccharide concentrations on frost tolerance.

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.

What determines the composition of the phloem sap? Is there any selectivity filter for macromolecules entering the phloem sieve elements?

In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young sieve elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.

Overexpression of Melon Tonoplast Sugar Transporter CmTST1 Improved Root Growth under High Sugar Content

Sugar allocation is based on the source-to-sink and intracellular transport between different organelles, and sugar transporters are usually involved in these processes. Tonoplast sugar transporters (TST) are responsible for transporting sugar into vacuoles; however, the role of TSTs in root growth and the response to abiotic stress is poorly studied. Here, RNA analysis and promoter-β-glucuronidase staining revealed that a melon TST1 gene (CmTST1) is highly expressed in the roots. The sugar feeding experiment results showed that the expression of CmTST1 in the roots was induced by a relatively high level of sucrose (6%), glucose (3%), and fructose (3%). The ectopic overexpression of CmTST1 in Arabidopsis improved the root and shoot growth of seedlings under high exogenous sugar stress. Furthermore, the ectopic expression of CmTST1 promoted the expression of plasma membrane-located sugar transporters. We proposed that CmTST1 plays a key role in importing sugar transport into the vacuoles of roots in response to metabolic demands to maintain cytosolic sugar homeostasis.

The sucrose transporter MdSUT4.1 participates in the regulation of fruit sugar accumulation in apple

BACKGROUND

Sugar content is an important determinant of fruit sweetness, but details on the complex molecular mechanism underlying fruit sugar accumulation remain scarce. Here, we report the role of sucrose transporter (SUT) family in regulating fruit sugar accumulation in apple.

RESULTS

Gene-tagged markers were developed to conduct candidate gene-based association study, and an SUT4 member MdSUT4.1 was found to be significantly associated with fruit sugar accumulation. MdSUT4.1 encodes a tonoplast localized protein and its expression level had a negative correlation with fruit sugar content. Overexpression of MdSUT4.1 in strawberry and apple callus had an overall negative impact on sugar accumulation, suggesting that it functions to remobilize sugar out of the vacuole. In addition, MdSUT4.1 is located on chromosomal region harboring a previously reported QTL for sugar content, suggesting that it is a candidate gene for fruit sugar accumulation in apple.

CONCLUSIONS

MdSUT4.1 is involved in the regulation of fruit sugar accumulation in apple. This study is not only helpful for understanding the complex mechanism of fruit sugar accumulation, but it also provides molecular tools for genetic improvement of fruit quality in breeding programs of apple.

A novel sucrose transporter gene IbSUT4 involves in plant growth and response to abiotic stress through the ABF-dependent ABA signaling pathway in Sweetpotato

BACKGROUND

To maintain sweetpotato (Ipomoea batatas (L.) Lam) growth and yield, sucrose must be transported from the leaves to the roots. Sucrose transporters or carriers (SUTs or SUCs) transport sucrose and are involved in plant growth and response to abiotic stress. However, the mechanisms of SUTs in sweetpotato abiotic stress resistance remains to be determined.

RESULTS

In the present study, we cloned a novel IbSUT4 gene; the protein encoded by this gene is localized in the tonoplast and plasma membrane. The plant growth was promoted in the IbSUT4 transgenic Arabidopsis thaliana lines, with increased expression of AtFT, a regulator of flowering time in plants. Over-expression of IbSUT4 in Arabidopsis thaliana resulted in higher sucrose content in the roots and lower sucrose content in the leaves, as compared to the wild-type (WT) plants, leading to improved stress tolerance during seedling growth. Moreover, we systematically analyzed the mechanisms of IbSUT4 in response to abiotic stress. The results suggest that the ABRE-motif was localized in the IbSUT4 promoter region, and the expression of the ABA signaling pathway genes (i.e., ABF2, ABF4, SnRK2.2, SnRK2.3, and PYL8/RCAR3) were induced, and the expression of ABI1 was inhibited.

CONCLUSIONS

Our dates provide evidence that IbSUT4 is not only involved in plant growth but also is an important positive regulator in plant stress tolerance through the ABF-dependent ABA signaling pathway.

Expression of Sucrose Transporters from Vitis vinifera Confer High Yield and Enhances Drought Resistance in Arabidopsis

Sucrose is the predominant form of sugar transported from photosynthetic (source) to non-photosynthetic (sink) organs in higher plants relying on the transporting function of sucrose transporters (SUTs or SUCs). Many SUTs have been identified and characterized in both monocots and dicots. However, the function of sucrose transporters (SUTs or SUCs) from Vitis is not clear. As the world’s most planted grape species, Vitis vinifera owns three sucrose transport activity verified SUTs. In this study, we constructed three kinds of VvSUC (Vitis vinifera SUC)-overexpressing transgenic Arabidopsis. VvSUC-overexpressing transgenic Arabidopsis was cultured on sucrose-supplemented medium. VvSUC11- and VvSUC12-overexpressing lines had similar thrived growth phenotypes, whereas the size and number of leaves and roots from VvSUC27-overexpressing lines were reduced compared with that of WT. When plants were cultured in soil, all SUT transgenic seedlings produced more number of leaves and siliques, resulting in higher yield (38.6% for VvSUC12-transformants) than that of WT. Besides, VvSUC27-transformants and VvSUC11-transformants enhanced drought resistance in Arabidopsis, providing a promising target for crop improvement

Exogenous Application of Sucrose Promotes Postharvest Ripening of Kiwifruit

Sucrose is an important component of fruit flavor, but whether sucrose signaling affects the postharvest ripening process of kiwifruit is unclear. The aim of this article was to study the effect of sucrose application on postharvest kiwifruit ripening to clarify the effect of sucrose in this process. Our present study found that exogenous sucrose can promote ethylene synthesis, which increases the ethylene content during fruit ripening, thereby accelerating the ripening and softening of kiwifruit after harvest. A significantly higher expression of AcACS1 and AcACO2 was found in sucrose-treated fruits compared to that in mannitol-treated fruits. Blocking the ethylene signal significantly inhibited the sucrose-modulated expression of most selected ripening-related genes. Sucrose transport is essential for sucrose accumulation in fruits; therefore, we isolated the gene family related to sucrose transport in kiwifruit and analyzed the gene expression of its members. The results show that AcSUT1 and AcTST1 expression increased with fruit ripening and AcSUT4 expression decreased with ripening, indicating that they may have different roles in the regulation of fruit ripening. Additionally, many cis-elements associated with phytohormones and sugar responses were found in the promoter of the three genes, which suggests that they were transcriptionally regulated by sugar signal and phytohormones. This study demonstrates the effect of sucrose on postharvest ripening of kiwifruit, providing a good foundation for further research.

Characterization of a vacuolar sucrose transporter, HbSUT5, from Hevea brasiliensis: involvement in latex production through regulation of intracellular sucrose transport in the bark and laticifers

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.

Sugar transporters in Fabaceae, featuring SUT MST and SWEET families of the model plant Medicago truncatula and the agricultural crop Pisum sativum

Sugar transporters play a crucial role for plant productivity, as they coordinate sugar fluxes from source leaf towards sink organs (seed, fruit, root) and regulate the supply of carbon resources towards the microorganisms of the rhizosphere (bacteria and fungi). Thus, sugar fluxes mediated by SUT (sucrose transporters), MST (monosaccharide transporters) and SWEET (sugar will eventually be exported transporters) families are key determinants of crop yield and shape the microbial communities living in the soil. In this work, we performed a systematic search for sugar transporters in Fabaceae genomes, focusing on model and agronomical plants. Here, we update the inventory of sugar transporter families mining the latest version of the Medicago truncatula genome and identify for the first time SUT MST and SWEET families of the agricultural crop Pisum sativum. The sugar transporter families of these Fabaceae species comprise respectively 7 MtSUT 7 PsSUT, 72 MtMST 59 PsMST and 26 MtSWEET 22 PsSWEET. Our comprehensive phylogenetic analysis sets a milestone for the scientific community, as we propose a new and simple nomenclature to correctly name SUT MST and SWEET families. Then, we searched for transcriptomic data available for our gene repertoire. We show that several clusters of homologous genes are co-expressed in different organs, suggesting that orthologous sugar transporters may have a conserved function. We focused our analysis on gene candidates that may be involved in remobilizing resources during flowering, grain filling and in allocating carbon towards roots colonized by arbuscular mycorrhizal fungi and Rhizobia. Our findings open new perspectives for agroecological applications in legume crops, as for instance improving the yield and quality of seed productions and promoting the use of symbiotic microorganisms.

Measuring Sucrose Transporter Activities Using a Protoplast-Esculin Assay

Sucrose transport across membranes requires the activity of transport proteins. Sucrose-specific SWEET proteins mediate sugar efflux out of the cytosol and SUC proteins catalyze the uptake of sucrose from the apoplast. Both transport processes are involved in phloem loading in source leaves as well as in the post-phloem pathway in sink tissues. An important step during the characterization of new sucrose transporters is to analyze their transport activity. This is usually achieved by heterologous expression of the respective gene in yeast cells or Xenopus oocytes and subsequent uptake measurements. However, in some cases, mistargeting to internal membranes or the lack of protein modifications and/or interaction partners in the heterologous system can interfere with uptake analyses. Therefore, a new in planta method was developed that is based on mesophyll protoplasts as expression system and the fluorescent sucrose analog esculin to monitor uptake activities by confocal microscopy. In this chapter we describe the design of constructs required to analyze sucrose transporters in protoplasts, the experimental setup of the protoplast-esculin assay, and the quantitative evaluation of the obtained data. The quantification of esculin uptake allows the application of the new assay to a variety of questions, e.g., by comparison of point mutants, splice variants, or transporters with and without interaction partners.

An apple sucrose transporter MdSUT2.2 is a phosphorylation target for protein kinase MdCIPK22 in response to drought

Sugars increase with drought stress in plants and accumulate in the vacuole. However, the exact molecular mechanism underlying this process is not clear yet. In this study, protein interaction and phosphorylation experiments were conducted for sucrose transporter and CIPK kinase in apple. The specific phosphorylation site of sucrose transporter was identified with mass spectrometry. Transgenic analyses were performed to characterize their biological function. It was found that overexpression of sucrose transporter gene MdSUT2.2 in apple plants promoted sugar accumulation and drought tolerance. MdSUT2.2 protein was phosphorylated at Ser³⁸¹ site in response to drought. A DUALmembrane system using MdSUT2.2 as bait through an apple cDNA library got a protein kinase MdCIPK22. Bimolecular fluorescence complementary (BiFC), pull-down and co-immunoprecipitation (Co-IP) assays further demonstrated that MdCIPK22 interacted with MdSUT2.2. A series of transgenic analysis showed that MdCIPK22 was required for the drought-induced phosphylation at Ser³⁸¹ site of MdSUT2.2 protein, and that it enhanced the stability and transport activity of MdSUT2.2 protein. Finally, it was found that MdCIPK22 overexpression promoted sugar accumulation and improved drought tolerance in an MdSUT2.2-dependent manner in transgenic apple plants. MdCIPK22-MdSUT2.2 regulatory module shed light on the molecular mechanism by which plant accumulates sugars and enhances tolerance in response to drought stress.

Seasonal changes of sucrose transporter expression and sugar partitioning in common European tree species

In temperate woody species, carbon transport from source to sink tissues is a striking physiological process, particularly considering seasonal changes. The functions of different tissues can also alternate across the seasons. In this regard, phloem loading and sugar distribution are important aspects of carbon partitioning, and sucrose uptake transporters (SUTs) play a key role in these processes. Therefore, the influence of seasons and different light-dark conditions on the expression of SUTs from 3-year-old Fagus sylvatica L., Quercus robur L. and Picea abies (L.) Karst. trees were analyzed. In addition, tissue-specific sugar and starch contents under these different environmental conditions were determined. Putative SUTs were identified in the gymnosperms (Picea abies, Ginkgo biloba L.), here for the first time, and also in the angiosperms (Q. robur, F. sylvatica). The identified SUT sequences of the different tree species cluster into three types, similar to other SUTs from herbaceous and tree species. Furthermore, the sequences from angiosperm and those from gymnosperm species form distinct clusters within the three types of SUTs. In F. sylvatica, Q. robur and P. abies, the expression levels of the different SUTs during seasons showed marked variations. Because of the high expression levels of type I SUTs in bark, wood and leaves during active growing phases in spring and summer, it can be assumed that they are involved in phloem loading, sucrose retrieval and possibly in further physiological processes. The expression patterns also indicate a flexible expression in all tissues depending on physiological requirements and environmental conditions. Compared with type I SUTs, the seasonal variations of type II SUT expression were less pronounced, whereas the seasonal variations of the type III SUT expression patterns were partly reverse. In addition to the seasonal regulation, the expressions of the different SUTs were also regulated by light in a diurnal manner.

AtALMT3 is Involved in Malate Efflux Induced by Phosphorus Deficiency in Arabidopsis thaliana Root Hairs

Under phosphorus (P)-deficient conditions, organic acid secretion from roots plays an important role in P mobilization from insoluble P in the soil. In this study, we characterized AtALMT3, a homolog of the Arabidopsis thaliana aluminum-activated malate transporter family gene. Among the 14 AtALMT family genes, only AtALMT3 was significantly up-regulated in P-deficient roots. AtALMT3 promoter::β-glucuronidase is expressed in the epidermis in roots, especially in root hair cells. AtALMT3 protein was localized in the plasma membrane and in small vesicles. Fluorescence of AtALMT3::GFP was not observed on the vacuole membrane of protoplast after lysis, indicating that AtALMT3 localizes mainly in the plasma membrane. Compared with the wild-type (WT) line, malate exudation in the AtALMT3-knockdown line (atalmt3-1) and overexpression line (atalmt3-2) under P deficiency were, respectively, 37% and 126%. In contrast, no significant difference was found in citrate exudation among these lines. The complementation of the atalmt3-1 line with AtALMT3 recovered the malate exudation to the level of the WT. Taken together, these results suggest that AtALMT3 localized in root hair membranes is involved in malate efflux in response to P deficiency.

The Tonoplastic Inositol Transporter INT1 From Arabidopsis thaliana Impacts Cell Elongation in a Sucrose-Dependent Way

The tonoplastic inositol transporter INT1 is the only known transport protein in Arabidopsis that facilitates myo-inositol import from the vacuole into the cytoplasm. Impairment of the release of vacuolar inositol by knockout of INT1 results in a severe inhibition of cell elongation in roots as well as in etiolated hypocotyls. Importantly, a more strongly reduced cell elongation was observed when sucrose was supplied in the growth medium, and this sucrose-dependent effect can be complemented by the addition of exogenous myo-inositol. Comparing int1 mutants (defective in transport) with mutants defective in myo-inositol biosynthesis (mips1 mutants) revealed that the sucrose-induced inhibition in cell elongation does not just depend on inositol depletion. Secondary effects as observed for altered availability of inositol in biosynthesis mutants, as disturbed membrane turnover, alterations in PIN protein localization or alterations in inositol-derived signaling molecules could be ruled out to be responsible for impairing the cell elongation in int1 mutants. Although the molecular mechanism remains to be elucidated, our data implicate a crucial role of INT1-transported myo-inositol in regulating cell elongation in a sucrose-dependent manner and underline recent reports of regulatory roles for sucrose and other carbohydrate intermediates as metabolic semaphores.

Isolation of Intact Vacuoles from Petunia Petals and Extraction of Sequestered Glycosylated Phenylpropanoid Compounds

Plant vacuoles are the largest compartment in plant cells, occupying more than 80% of the cell volume. A variety of proteins, sugars, pigments and other metabolites are stored in these organelles ( Paris et al., 1996 ; Olbrich et al., 2007 ). Flowers produce a variety of specialized metabolites, some of which are unique to this organ, such as components of pollination syndromes, i.e., scent volatiles and flavonoids ( Hoballah et al., 2007; Cna'ani et al., 2015). To study the compounds stored in floral vacuoles, this compartment must be separated from the rest of the cell. To enable isolation of vacuoles, protoplasts were first generated by incubating pierced corollas with cellulase and macrozyme enzymes. After filtering and several centrifugation steps, protoplasts were separated from the debris and damaged/burst protoplasts, as revealed by microscopic observation. Concentrated protoplasts were lysed, and vacuoles were extracted by Ficoll-gradient centrifugation. Vacuoles were used for quantitative GC-MS analyses of sequestered metabolites. This method allowed us to identify vacuoles as the subcellular accumulation site of glycosylated volatile phenylpropanoids and to hypothesize that conjugated scent compounds are sequestered in the vacuoles en route to the headspace (Cna'ani et al., 2017).