The predominance of the apoplasmic phloem-unloading pathway is interrupted by a symplasmic pathway during Chinese jujube fruit development

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.

MdSWEET23, a sucrose transporter from apple (Malus × domestica Borkh.), influences sugar metabolism and enhances cold tolerance in tomato

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. β-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.

An update on sugar allocation and accumulation in fruits

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

Interaction between sugar transport and plant development

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

Genome-Wide Identification and Expression Patterns of AcSWEET Family in Pineapple and AcSWEET11 Mediated Sugar Accumulation

Pineapple (Ananas comosus L.) is an important fruit crop in tropical regions, and it requires efficient sugar allocation during fruit development. Sugars Will Eventually be Exported Transporters (SWEETs) are a group of novel sugar transporters which play critical roles in seed and fruit development. However, the function of AcSWEETs remains unknown in the sugar accumulation. Herein, 17 AcSWEETs were isolated and unevenly located in 11 chromosomes. Analysis of a phylogenetic tree indicated that 17 genes were classified into four clades, and the majority of AcSWEETs in each clade shared similar conserved motifs and gene structures. Tissue-specific gene expression showed that expression profiles of AcSWEETs displayed differences in different tissues and five AcSWEETs were strongly expressed during fruit development. AcSWEET11 was highly expressed in the stage of mature fruits in 'Tainong16' and 'Comte de paris', which indicates that AcSWEET11 was important to fruit development. Subcellular localization analysis showed that AcSWEET11 was located in the cell membrane. Notably, overexpression of AcSWEET11 could improve sugar accumulation in pineapple callus and transgenic tomato, which suggests that AcSWEET11 might positively contribute to sugar accumulation in pineapple fruit development. These results may provide insights to enhance sugar accumulation in fruit, thus improving pineapple quality in the future.

Phloem Unloading in Developing Rice Caryopses and its Contribution to Non-Structural Carbohydrate Translocation from Stems and Grain Yield Formation

Phloem unloading plays an important role in photoassimilate partitioning and grain yield improvements in cereal crops. The phloem unloading strategy and its effects on photoassimilate translocation and yield formation remain unclear in rice. In this study, plasmodesmata were observed at the interface between the sieve elements (SEs) and companion cells (CCs), and between the SE-CC complex and surrounding parenchyma cells (PCs) in phloem of the dorsal vascular bundle in developing caryopses. Carboxyfluorescein (CF) signal was detected in the phloem of caryopses, which showed that CF was unloaded into caryopses. These results indicated that the SE-CC complex was symplasmically connected with adjacent PCs by plasmodesmata. Gene expression for sucrose transporter (SUT) and cell wall invertase (CWI), and OsSUT1 and OsCIN1 proteins were detected in developing caryopses, indicating that rice plants might actively unload sucrose into caryopses by the apoplasmic pathway. Among three rice recombinant inbred lines, R201 exhibited lower plasmodesmal densities at the boundaries between cell types (SE-CC, SE-PC and CC-PC) in developing caryopses than R91 and R156. R201 also had lower expression of SUT and CWI genes and lower protein levels of OsSUT1 and OsCIN1, as well as CWI activity, than R91 and R156. These data agreed with stem non-structural carbohydrate (NSC) translocation and grain yields for the three lines. The nitrogen application rate had no significant effect on plasmodesmal densities at the interfaces between different cells types, and did not affect CF unloading in the phloem of developing caryopses. Low nitrogen treatment enhanced expression levels of OsSUT and OsCIN genes in the three lines. These results suggested that nitrogen application had no substantial effect on symplasmic unloading but affected apoplasmic unloading. Therefore, we concluded that poor symplasmic and apoplasmic unloading in developing caryopses might result in low stem NSC translocation and poor grain yield formation of R201.

Low Nitrogen Enhances Apoplastic Phloem Loading and Improves the Translocation of Photoassimilates in Rice Leaves and Stems

The grain filling of rice depends on photoassimilates from leaves and stems. Phloem loading is the first crucial step for the transportation of sucrose to grains. However, phloem loading mechanisms in rice leaves and stems and their response to nitrogen (N) remain unclear. Here, using a combination of electron microscopy, transportation of a phloem tracer and 13C labeling, phloem loading was studied in rice leaves and stems. The results showed that the sieve element-companion cell complex lacked a symplastic connection with surrounding parenchyma cells in leaves and stems. The genes expression and protein levels of sucrose transporter (SUTs) and sugars will eventually be exported transporters (SWEETs) were detected in the vascular bundle of leaves and stems. A decrease in the 13C isotope remobilization from leaves to stems and panicles following treatment with p-chloromercuribenzenesulfonic acid indicated that rice leaves and stems actively transport sucrose into the phloem. Under low-N (LN) treatment, the activities of α-amylase, β-amylase and sucrose phosphate synthase (SPS) in stems and activity of SPS in leaves increased; genes expression and protein levels of SUTs and SWEETs in leaves and stems increased; the 13C isotope reallocation in panicles increased. These indicated that LN enhanced apoplastic phloem loading in stems and leaves. This improved the translocation of photoassimilates and consequently increased grain filling percentage, grain weight and harvest index. This study provides evidence that rice leaves and stems utilize an apoplastic loading strategy and respond to N stimuli by regulating the genes expression and protein levels of SUTs and SWEETs.

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.

Stem small vascular bundles have greater accumulation and translocation of non-structural carbohydrates than large vascular bundles in rice

Phloem unloading and loading are associated with stem non-structural carbohydrates (NSCs) accumulation and remobilization in rice (Oryza sativa L.). Four rice recombinant inbred lines (R032, R191, R046, and R146) derived from a cross between Zhenshan 97 and Minghui 63 were used to investigate the contributions of stem large and small vascular bundles (SVBs) to NSCs accumulation and translocation. Before heading, the parenchyma cells in stem cortex tissues (PCs) surrounding SVBs had higher starch density than those surrounding large vascular bundles (LVBs). Moreover, the protein levels of sucrose transporters (SUTs), cell wall invertase, sucrose synthase, and adenosine diphosphate glucose pyrophosphorylase, as well as the phloem plasmodesma densities were higher in SVBs than those in LVBs. After heading, starch density decreased more in PCs surrounding SVBs than in LVBs. Also, the protein levels of SUTs, α-amylase, sucrose phosphate synthase and sucrose synthase, the phloem plasmodesma densities in SVBs were higher than those in LVBs. The correlations of the number and total cross-sectional area of SVBs with mass and contribution to yield of transferred NSCs were higher than those of LVBs. Our results suggest that SVBs may have higher contributions to pre-anthesis stem NSCs accumulation and post-anthesis translocation than LVBs, which is potentially attributed to the high level of protein and enzyme involved in stem unloading and loading via apoplastic and symplastic pathways.

Integrative Physiological and Transcriptomic Analysis Reveals the Transition Mechanism of Sugar Phloem Unloading Route in Camellia oleifera Fruit

Sucrose phloem unloading plays a vital role in photoassimilate distribution and storage in sink organs such as fruits and seeds. In most plants, the phloem unloading route was reported to shift between an apoplasmic and a symplasmic pattern with fruit development. However, the molecular transition mechanisms of the phloem unloading pathway still remain largely unknown. In this study, we applied RNA sequencing to profile the specific gene expression patterns for sucrose unloading in C. oleifera fruits in the apo- and symplasmic pathways that were discerned by CF fluoresce labelling. Several key structural genes were identified that participate in phloem unloading, such as PDBG11, PDBG14, SUT8, CWIN4, and CALS10. In particular, the key genes controlling the process were involved in callose metabolism, which was confirmed by callose staining. Based on the co-expression network analysis with key structural genes, a number of transcription factors belonging to the MYB, C2C2, NAC, WRKY, and AP2/ERF families were identified to be candidate regulators for the operation and transition of phloem unloading. KEGG enrichment analysis showed that some important metabolism pathways such as plant hormone metabolism, starch, and sucrose metabolism altered with the change of the sugar unloading pattern. Our study provides innovative insights into the different mechanisms responsible for apo- and symplasmic phloem unloading in oil tea fruit and represents an important step towards the omics delineation of sucrose phloem unloading transition in crops.

Sugar conundrum in plant-pathogen interactions: roles of invertase and sugar transporters depend on pathosystems

It has been increasingly recognized that CWIN (cell wall invertase) and sugar transporters including STP (sugar transport protein) and SWEET (sugar will eventually be exported transporters) play important roles in plant-pathogen interactions. However, the information available in the literature comes from diverse systems and often yields contradictory findings and conclusions. To solve this puzzle, we provide here a comprehensive assessment of the topic. Our analyses revealed that the regulation of plant-microbe interactions by CWIN, SWEET, and STP is conditioned by the specific pathosystems involved. The roles of CWINs in plant resistance are largely determined by the lifestyle of pathogens (biotrophs versus necrotrophs or hemibiotrophs), possibly through CWIN-mediated salicylic acid or jasmonic acid signaling and programmed cell death pathways. The up-regulation of SWEETs and STPs may enhance or reduce plant resistance, depending on the cellular sites from which pathogens acquire sugars from the host cells. Finally, plants employ unique mechanisms to defend against viral infection, in part through a sugar-based regulation of plasmodesmatal development or aperture. Our appraisal further calls for attention to be paid to the involvement of microbial sugar metabolism and transport in plant-pathogen interactions, which is an integrated but overlooked component of such interactions.

Switch from symplasmic to aspoplasmic phloem unloading in Xanthoceras sorbifolia fruit and sucrose influx XsSWEET10 as a key candidate for Sugar transport

The process of phloem unloading and post-unloading transport of photoassimilate is critical to crop output. Xanthoceras sorbifolia is a woody oil species with great biomass energy prospects in China; however, underproduction of seeds seriously restricts its development. Here, our cytological studies by ultrastructural observation revealed that the sieve element-companion cell complex in carpellary bundle was symplasmically interconnected with surrounding parenchyma cells at the early and late fruit developmental stages, whereas it was symplasmically isolated at middle stage. Consistently, real-time imaging showed that fluorescent tracer 6(5)carboxyfluorescein was confined to phloem strands at middle stage but released into surrounding parenchymal cells at early and late stages. Enzymatic assay showed that sucrose synthase act as the key enzyme catalyzing the progress of Suc degradation post-unloading pathway whether in pericarp or in seed, while vacuolar acid invertase and neutral invertase play compensation roles in sucrose decomposition. Sugar transporter XsSWEET10 had a high expression profile in fruit, especially at middle stage. XsSWEET10 is a plasma membrane-localized protein and heterologous expression in SUC2-deficient yeast strain SUSY7/ura3 confirmed its ability to uptake sucrose. These findings approved the transition from symplasmic to apoplasmic phloem unloading in Xanthoceras sorbifolia fruit and XsSWEET10 as a key candidate in sugar transport.

Predominantly symplastic phloem unloading of photosynthates maintains efficient starch accumulation in the cassava storage roots (Manihot esculenta Crantz)

BACKGROUND

Cassava (Manihot esculenta Crantz) efficiently accumulates starch in its storage roots. However, how photosynthates are transported from the leaves to the phloem (especially how they are unloaded into parenchymal cells of storage roots) remains unclear.

RESULTS

Here, we investigated the sucrose unloading pattern and its impact on cassava storage root development using microstructural and physiological analyses, namely, carboxyfluorescein (CF) and C¹⁴ isotope tracing. The expression profiling of genes involved in symplastic and apoplastic transport was performed, which included enzyme activity, protein gel blot analysis, and transcriptome sequencing analyses. These finding showed that carbohydrates are transported mainly in the form of sucrose, and more than 54.6% was present in the stem phloem. Sucrose was predominantly unloaded symplastically from the phloem into storage roots; in addition, there was a shift from apoplastic to symplastic unloading accompanied by the onset of root swelling. Statistical data on the microstructures indicated an enrichment of plasmodesmata within sieve, companion, and parenchyma cells in the developing storage roots of a cultivar but not in a wild ancestor. Tracing tests with CF verified the existence of a symplastic channel, and [¹⁴C] Suc demonstrated that sucrose could rapidly diffuse into root parenchyma cells from phloem cells. The relatively high expression of genes encoding sucrose synthase and associated proteins appeared in the middle and late stages of storage roots but not in primary fibrous roots, or secondary fibrous roots. The inverse expression pattern of sucrose transporters, cell wall acid invertase, and soluble acid invertase in these corresponding organs supported the presence of a symplastic sucrose unloading pathway. The transcription profile of genes involved in symplastic unloading and their significantly positive correlation with the starch yield at the population level confirmed that symplastic sucrose transport is vitally important in the development of cassava storage roots.

CONCLUSIONS

In this study, we revealed that the cassava storage root phloem sucrose unloading pattern was predominantly a symplastic unloading pattern. This pattern is essential for efficient starch accumulation in high-yielding varieties compared with low-yielding wild ancestors.

Sugar Metabolism in Stone Fruit: Source-Sink Relationships and Environmental and Agronomical Effects

The partitioning of assimilates in fruits, which are economically important sink organs, is ruled by different physiological processes and affected by both environmental and agronomical factors. The bulk of the water and solutes, required for growth, is imported into fruits and seeds through xylem and phloem. In the stone fruits, five vascular bundles enter the base of the fruit, then dividing to supply either the flesh or the seed. The main sugars accumulated in stone fruits include fructose, glucose, and sucrose, along with other minor saccharides. The mechanisms of phloem loading in these fruit species have not been fully elucidated yet, but the available data hint either an apoplastic or a symplastic type or possibly a combination of both, depending on the species and the sugar considered. Similarly, phloem unloading mechanisms, elucidated for a small number of species, depend on genotype and developmental stage. Remarkably, key enzymes and transporters involved in the main sugars-conversion and transport pathways have received considerable attention. In stone fruit trees, the presence of an elevated number of fruits alters the source-sink balance, with a consequent intensification of competition among them and between vegetative and reproductive growth. The main environmental factors affecting this balance and the agronomical/artificial manipulations of source-sink relationships to achieve adequate fruit production and quality are reviewed.

Localization shift of a sugar transporter contributes to phloem unloading in sweet watermelons

Unloading sugar from sink phloem by transporters is complex and much remains to be understood about this phenomenon in the watermelon fruit. Here, we report a novel vacuolar sugar transporter (ClVST1) identified through map-based cloning and association study, whose expression in fruit phloem is associated with accumulation of sucrose (Suc) in watermelon fruit. ClVST1⁹⁷ knockout lines show decreased sugar content and total biomass, whereas overexpression of ClVST1⁹⁷ increases Suc content. Population genomic and subcellular localization analyses strongly suggest a single-base change at the coding region of ClVST1⁹⁷ as a major molecular event during watermelon domestication, which results in the truncation of 45 amino acids and shifts the localization of ClVST1⁹⁷ to plasma membranes in sweet watermelons. Molecular, biochemical and phenotypic analyses indicate that ClVST1⁹⁷ is a novel sugar transporter for Suc and glucose efflux and unloading. Functional characterization of ClVST1 provides a novel strategy to increase sugar sink potency during watermelon domestication.

Developmental patterns of enzyme activity, gene expression, and sugar content in sucrose metabolism of two broomrape species

A better understanding of broomrape physiological features opens up new perspectives for developing specific management strategies. For this purpose, activities of key enzymes involved in osmoregulation (SAI1, CWI, M6PR, and SUS1) were considered at developmental stages of two important broomrape species (Egyptian and branched broomrape) on tomato. While Egyptian broomrape tubercles had high activities of invertases, branched broomrape shoots revealed high activities of M6PR and SUS1 during both pre- and post-emergence stages except for M6PR at post-emergence stages of P. aegyptiaca. Interestingly, the main accumulation of total reducing sugars was detected in tubercle during pre- and in shoot during post-emergence. Unlike low levels of genes expression (except for CWI) before parasite emergence, significantly higher expression levels of SAI1, SUS1 and M6PR were detected after parasite emergence. Matching the expression levels of SAI1 and SUS1 genes with their corresponding enzymes activities makes them as the suitable candidates for gene silencing strategies.

Primary Metabolism in Citrus Fruit as Affected by Its Unique Structure

Citrus is one of the world's most important fruit crops, contributing essential nutrients, such as vitamin C and minerals, to the human diet. It is characterized by two important traits: first, its major edible part is composed of juice sacs, a unique structure among fruit, and second, relatively high levels of citric acid are accumulated in the vacuole of the juice sac cell. Although the major routes of primary metabolism are generally the same in citrus fruit and other plant systems, the fruit's unique structural features challenge our understanding of carbon flow into the fruit and its movement through all of its parts. In fact, acid metabolism and accumulation have only been summarized in a few reviews. Here we present a comprehensive view of sugar, acid and amino acid metabolism and their connections within the fruit, all in relation to the fruit's unique structure.

Sugar transport played a more important role than sugar biosynthesis in fruit sugar accumulation during Chinese jujube domestication

Sugar transport, including the symplasmic pathway in plasmodesmata and apoplasmic pathway mediated by sugar transporters, accelerated sugar accumulation in cultivated jujube, while sugar metabolism-related genes played weak roles in jujube domestication. The fruit of Chinese jujube (Ziziphus jujuba Mill.) is high in sugar concentration. By contrast, wild type-sour jujube (Z. jujuba Mill. var. spinosa Hu) contains markedly less sugar. It is unknown whether sugar transport or sugar metabolism drove sugar accumulation during jujube domestication. Using a combination of ultrastructural observations, phylogenetic analysis, testing for soluble sugars, and transcriptional analysis, the sugar accumulation mechanism was studied in the developmental stages of cultivated jujube and sour jujube. Our results indicate that the symplasmic transport pathway in plasmodesmata is present in cultivated jujube, but not in sour jujube. Sugar transporter genes have higher frequencies of duplication than sugar metabolism-related genes. Gene expression patterns indicate that sugar transporter genes, especially ZjSUT2, ZjSWEET1, ZjSWEET7, ZjSWEET11, ZjSTP3, and ZjSTP13a, rather than sugar metabolism-related genes showed higher expression levels in cultivated jujube versus sour jujube during fruit sugar accumulation. These findings suggest that sugar transport, including apoplasmic and symplasmic transport, rather than sugar biosynthesis, is associated with the difference in sugar accumulation between jujube and sour jujube, and that it may drive jujube domestication. This study provides valuable genetic information for jujube improvement, and offers new insights into fruit tree domestication related to sugar accumulation.

Sugars in peach fruit: a breeding perspective

The last decade has been characterized by a decrease in peach (Prunus persica) fruit consumption in many countries, foremost due to unsatisfactory quality. The sugar content is one of the most important quality traits perceived by consumers, and the development of novel peach cultivars with sugar-enhanced content is a primary objective of breeding programs to revert the market inertia. Nevertheless, the progress reachable through classical phenotypic selection is limited by the narrow genetic bases of peach breeding material and by the complex quantitative nature of the trait, which is deeply affected by environmental conditions and agronomical management. The development of molecular markers applicable in MAS or MAB has become an essential strategy to boost the selection efficiency. Despite the enormous advances in 'omics' sciences, providing powerful tools for plant genotyping, the identification of the genetic bases of sugar-related traits is hindered by the lack of adequate phenotyping methods that are able to address strong within-plant variability. This review provides an overview of the current knowledge of the metabolic pathways and physiological mechanisms regulating sugar accumulation in peach fruit, the main advances in phenotyping approaches and genetic background, and finally addressing new research priorities and prospective for breeders.

Sucrose transport and phloem unloading in peach fruit: potential role of two transporters localized in different cell types

Several complex physiological processes, which include long-distance translocation in the phloem and unloading in sink tissues, govern the partitioning of sugars in economically important organs, such as peach fruit. In this study, we took advantage of a symplastic tracer, carboxyfluorescein (CF), providing evidence for an apoplastic sucrose transfer in the early (SI) and middle (SIII) phases of peach fruit development. Moreover, using a combination of in situ hybridization and laser microdissection-assisted expression analysis, three putative sucrose transporters encoding genes (PpSUT1, PpSUT2, PpSUT4) were transcriptionally analyzed to relate their expression with sucrose storage in this organ. Our study revealed that PpSUT2 and PpSUT4 are the genes predominantly expressed in fruit flesh, and the detailed analysis of their expression pattern in the different cell types enabled us to suggest a specialized role in sucrose distribution. Both PpSUTs transporters could be involved in the retrieval of sucrose lost from the symplastic continuum of the phloem and, when expressed in parenchyma cells, they could be active in the import of sucrose into sink tissues, via symport from the apoplast. An alternative hypothesis has been proposed and discussed for PpSUT4 because of its putative tonoplastic localization. Taken together, our results provide new insights into the molecular mechanisms underpinning sucrose unloading and accumulation in peach fruit.

Sugar uptake in the Aril of litchi fruit depends on the apoplasmic post-phloem transport and the activity of proton pumps and the putative transporter LcSUT4

The post-phloem unloading pathway and the mechanism of sugar accumulation remain unclear in litchi fruit. A combination of electron microscopy, transport of phloem-mobile symplasmic tracer (carboxyfluorescein, CF) and biochemical and molecular assays was used to explore the post-phloem transport pathway and the mechanism of aril sugar accumulation in litchi. In the funicle, where the aril originates, abundant plasmodesmata were observed, and CF introduced from the peduncle diffused to the parenchyma cells. In addition, abundant starch and pentasaccharide were detected and the sugar concentration was positively correlated with activities of sucrose hydrolysis enzymes. These results clearly showed that the phloem unloading and post-phloem transport in the funicle were symplastic. On the other hand, imaging of CF showed that it remained confined to the parenchyma cells in funicle tissues connecting the aril. Infiltration of both an ATPase inhibitor [eosin B (EB)] and a sucrose transporter inhibitor [p-chloromercuribenzene sulfonate (PCMBS)] inhibited sugar accumulation in the aril. These results indicated an apoplasmic post-phloem sugar transport from the funicle to the aril. Although facilitated diffusion might help sucrose uptake from the cytosol to the vacuole in cultivars with high soluble invertase, membrane ATPases in the aril, especially tonoplast ATPase, are crucial for aril sugar accumulation. The expression of a putative aril vacuolar membrane sucrose transporter gene (LcSUT4) was highly correlated with the sugar accumulation in the aril of litchi. These data suggest that apoplasmic transport is critical for sugar accumulation in litchi aril and that LcSUT4 is involved in this step.

Understanding and manipulating sucrose phloem loading, unloading, metabolism, and signalling to enhance crop yield and food security

Sucrose is produced in, and translocated from, photosynthetically active leaves (sources) to support non-photosynthetic tissues (sinks), such as developing seeds, fruits, and tubers. Different plants can utilize distinct mechanisms to transport sucrose into the phloem sieve tubes in source leaves. While phloem loading mechanisms have been extensively studied in dicot plants, there is less information about phloem loading in monocots. Maize and rice are major dietary staples, which have previously been proposed to use different cellular routes to transport sucrose from photosynthetic cells into the translocation stream. The anatomical, physiological, and genetic evidence supporting these conflicting hypotheses is examined. Upon entering sink cells, sucrose often is degraded into hexoses for a wide range of metabolic and storage processes, including biosynthesis of starch, protein, and cellulose, which are all major constituents for food, fibre, and fuel. Sucrose, glucose, fructose, and their derivate, trehalose-6-phosphate, also serve as signalling molecules to regulate gene expression either directly or through cross-talk with other signalling pathways. As such, sugar transport and metabolism play pivotal roles in plant development and realization of crop yield that needs to be increased substantially to meet the projected population demand in the foreseeable future. This review will discuss the current understanding of the control of carbon partitioning from the cellular to whole-plant levels, focusing on (i) the pathways employed for phloem loading in source leaves, particularly in grasses, and the routes used in sink organs for phloem unloading; (ii) the transporter proteins responsible for sugar efflux and influx across plasma membranes; and (iii) the key enzymes regulating sucrose metabolism, signalling, and utilization. Examples of how sugar transport and metabolism can be manipulated to improve crop productivity and stress tolerance are discussed.

In-silico analysis of water and carbon relations under stress conditions. A multi-scale perspective centered on fruit

Fruit development, from its early stages, is the result of a complex network of interacting processes, on different scales. These include cell division, cell expansion but also nutrient transport from the plant, and exchanges with the environment. In the presence of nutrient limitation, in particular, the plant reacts as a whole, by modifying its architecture, metabolism, and reproductive strategy, determining the resources available for fruit development, which in turn affects the overall source-sink balance of the system. Here, we present an integrated model of tomato that explicitly accounts for early developmental changes (from cell division to harvest), and use it to investigate the impact of water deficit and carbon limitation on nutrient fluxes and fruit growth, in both dry and fresh mass. Variability in fruit response is analyzed on two different scales: among trusses at plant level, and within cell populations at fruit level. Results show that the effect of stress on individual cells strongly depends on their age, size, and uptake capabilities, and that the timing of stress application, together with the fruit position on the plant, is crucial in determining the final phenotypic outcome. Water deficit and carbon depletion impacted either source size, source activity, or sink strength with contrasted effects on fruit growth. An important prediction of the model is the major role of symplasmic transport of carbon in the early stage of fruit development, as a catalyst for cell and fruit growth.

Trafficking of plant vacuolar invertases: from a membrane-anchored to a soluble status. Understanding sorting information in their complex N-terminal motifs

Vacuolar invertases (VIs) are highly expressed in young tissues and organs. They may have a substantial regulatory influence on whole-plant metabolism as well as on photosynthetic efficiency. Therefore, they are emerging as potentially interesting biotechnological targets to increase plant biomass production, especially under stress. On the one hand, VIs are well known as soluble and extractable proteins. On the other hand, they contain complex N-terminal propeptide (NTPP) regions with a basic region (BR) and a transmembrane domain (TMD). Here we analyzed in depth the Arabidopsis thaliana VI2 (AtVI2) NTPP by mutagenesis. It was found that correct sorting to the lytic vacuole (LV) depends on the presence of intact dileucine (SSDALLPIS), BR (RRRR) and TMD motifs. AtVI2 remains inserted into membranes on its way to the LV, and the classical sorting pathway (endoplasmic reticulum→Golgi→LV) is followed. However, our data suggest that VIs might follow an alternative, adaptor protein 3 (AP3)-dependent route as well. Membrane-anchored transport and a direct recognition of the dileucine motif in the NTPP of VIs might have evolved as a simple and more efficient sorting mechanism as compared with the vacuolar sorting receptor 1/binding protein of 80 kDa (VSR1/BP80)-dependent sorting mechanism followed by those proteins that travel to the vacuole as soluble proteins.

Does Don Fisher's high-pressure manifold model account for phloem transport and resource partitioning?

The pressure flow model of phloem transport envisaged by Münch (1930) has gained wide acceptance. Recently, however, the model has been questioned on structural and physiological grounds. For instance, sub-structures of sieve elements may reduce their hydraulic conductances to levels that impede flow rates of phloem sap and observed magnitudes of pressure gradients to drive flow along sieve tubes could be inadequate in tall trees. A variant of the Münch pressure flow model, the high-pressure manifold model of phloem transport introduced by Donald Fisher may serve to reconcile at least some of these questions. To this end, key predicted features of the high-pressure manifold model of phloem transport are evaluated against current knowledge of the physiology of phloem transport. These features include: (1) An absence of significant gradients in axial hydrostatic pressure in sieve elements from collection to release phloem accompanied by transport properties of sieve elements that underpin this outcome; (2) Symplasmic pathways of phloem unloading into sink organs impose a major constraint over bulk flow rates of resources translocated through the source-path-sink system; (3) Hydraulic conductances of plasmodesmata, linking sieve elements with surrounding phloem parenchyma cells, are sufficient to support and also regulate bulk flow rates exiting from sieve elements of release phloem. The review identifies strong circumstantial evidence that resource transport through the source-path-sink system is consistent with the high-pressure manifold model of phloem transport. The analysis then moves to exploring mechanisms that may link demand for resources, by cells of meristematic and expansion/storage sinks, with plasmodesmal conductances of release phloem. The review concludes with a brief discussion of how these mechanisms may offer novel opportunities to enhance crop biomass yields.

Phloem unloading follows an extensive apoplasmic pathway in cucumber (Cucumis sativus L.) fruit from anthesis to marketable maturing stage

The phloem unloading pathway remains unclear in fruits of Cucurbitaceae, a classical stachyose-transporting species with bicollateral phloem. Using a combination of electron microscopy, transport of phloem-mobile symplasmic tracer carboxyfluorescein, assays of acid invertase and sucrose transporter, and [(14)C]sugar uptake, the phloem unloading pathway was studied in cucumber (Cucumis sativus) fruit from anthesis to the marketable maturing stage. Structural investigations showed that the sieve element-companion cell (SE-CC) complex of the vascular bundles feeding fruit flesh is apparently symplasmically restricted. Imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the vascular bundles in the whole fruit throughout the stages examined. A 37 kDa acid invertase was located predominantly in the cell walls of SE-CC complexes and parenchyma cells. Studies of [(14)C]sugar uptake suggested that energy-driven transporters may be functional in sugar trans-membrane transport within symplasmically restricted SE-CC complex, which was further confirmed by the existence of a functional plasma membrane sucrose transporter (CsSUT4) in cucumber fruit. These data provide a clear evidence for an apoplasmic phloem unloading pathway in cucumber fruit. A presumption that putative raffinose or stachyose transporters may be involved in soluble sugars unloading was discussed.

A dual switch in phloem unloading during ovule development in Arabidopsis

Developing flowers are important sinks in Arabidopsis thaliana. Their energy demand is covered by assimilates which are synthesized in source leaves and transported via the vasculature. Assimilates are unloaded either symplastically through plasmodesmata or apoplastically by specific transport proteins. Here we studied the pathway of phloem unloading and post-phloem transport in developing gynoecia. Using phloem-mobile fluorescent tracers, we show that phloem unloading into cells of ovule primordia followed a symplastic pathway. Subsequently, the same tracers could not move out of phloem cells into mature ovules anymore. A further change in the mode of phloem unloading occurred after anthesis. In open flowers as well as in outgrowing siliques, the phloem was again unloaded via the symplast. This observed onset of symplastic phloem unloading was accompanied by a change in frequency of MP17-GFP-labeled plasmodesmata. We could also show that the change in cell-cell connectivity was independent of fertilization and increasing sink demand. The presented results indicate that symplastic connectivity is highly regulated and varies not only between different sink tissues but also between different developmental stages.