Expression of peach sucrose transporters in heterologous systems points out their different physiological role

Local endocytosis of sucrose transporter 2 in duckweed reveals the role of sucrose transporter 2 in guard cells

The local endocytosis of membrane proteins is critical for many physiological processes in plants, including the regulation of growth, development, nutrient absorption, and osmotic stress response. Much of our knowledge on the local endocytosis of plasma membrane (PM) protein only focuses on the polar growth of pollen tubes in plants and neuronal axon in animals. However, the role of local endocytosis of PM proteins in guard cells has not yet been researched. Here, we first cloned duckweed SUT2 (sucrose transporter 2) protein and then conducted subcellular and histological localization of the protein. Our results indicated that LpSUT2 (Landoltia punctata 0202 SUT2) is a PM protein highly expressed on guard cells. In vitro experiments on WT (wild type) lines treated with high sucrose concentration showed that the content of ROS (reactive oxygen species) in guard cells increased and stomatal conductance decreased. We observed the same results in the lines after overexpression of the LpSUT2 gene with newfound local endocytosis of LpSUT2. The local endocytosis mainly showed that LpSUT2 was uniformly distributed on the PM of guard cells in the early stage of development, and was only distributed in the endomembrane of guard cells in the mature stage. Therefore, we found the phenomenon of guard cell LpSUT2 local endocytosis through the changes of duckweed stomata and concluded that LpSUT2 local endocytosis might be dependent on ROS accumulation in the development of duckweed guard cells. This paper might provide future references for the genetic improvement and water-use efficiency in other crops.

Metabolomic and transcriptomic analyses reveal the mechanism of sweet-acidic taste formation during pineapple fruit development

Pineapple (Ananas comosus L.) is one of the most valuable subtropical fruit crop in the world. The sweet-acidic taste of the pineapple fruits is a major contributor to the characteristic of fruit quality, but its formation mechanism remains elusive. Here, targeted metabolomic and transcriptomic analyses were performed during the fruit developmental stages in two pineapple cultivars ("Comte de Paris" and "MD-2") to gain a global view of the metabolism and transport pathways involved in sugar and organic acid accumulation. Assessment of the levels of different sugar and acid components during fruit development revealed that the predominant sugar and organic acid in mature fruits of both cultivars was sucrose and citric acid, respectively. Weighted gene coexpression network analysis of metabolic phenotypes and gene expression profiling enabled the identification of 21 genes associated with sucrose accumulation and 19 genes associated with citric acid accumulation. The coordinated interaction of the 21 genes correlated with sucrose irreversible hydrolysis, resynthesis, and transport could be responsible for sucrose accumulation in pineapple fruit. In addition, citric acid accumulation might be controlled by the coordinated interaction of the pyruvate-to-acetyl-CoA-to-citrate pathway, gamma-aminobutyric acid pathway, and tonoplast proton pumps in pineapple. These results provide deep insights into the metabolic regulation of sweetness and acidity in pineapple.

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.

Fruit transpiration drives interspecific variability in fruit growth strategies

Fruit growth is a complex mechanism resulting from biochemical and biophysical events leading water and dry matter to accumulate in the fruit tissues. Understanding how fruits choose their growth strategies can help growers optimizing their resource management for a more sustainable production and a higher fruit quality. This paper compares the growth strategies adopted by different fruit crops, at different times during the season and relates their fruit surface conductance to key physiological parameters for fruit growth such as phloem and xylem inflows as well transpiration losses. Our results show how fruits capacity to transpire (determined by their surface conductance) is a key driver in determining the growth strategy adopted by a species and explains the inter-species variability existing among different crops. Indeed, fruits change their surface conductance depending on the species and the phenological stage. This has an impact on the fruit's ability to lose water due to transpiration, affecting fruit pressure potential and increasing the force with which the fruit is able to attract xylem and phloem flows, with a considerable impact on fruit growth rate.

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.

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.

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.

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.

Apoplasmic and simplasmic phloem unloading mechanisms: Do they co-exist in Angeleno plums under demanding environmental conditions?

Biophysical fruit growth depends on a balance among the vascular and transpiration flows entering/exiting the fruit via phloem, xylem and through the epidermis. There is no information on vascular flows of Japanese plums, a species characterized by high-sugar content of its fruit at harvest. Vascular flows of Angeleno plums were monitored by fruit gauges during late fruit development, under the dry environment of the Goulburn Valley, Victoria, Australia. Phloem, xylem flows and skin transpiratory losses were determined, as well as diurnal leaf, stem and fruit pressure potentials. Fruit seasonal development, skin conductance and dry matter accumulation were also monitored. Fruit grew following a double-sigmoid pattern, but fruit size increased only 3.1 g over the last 3 weeks of development. Fruit grew very little in the morning, primarily due to phloem inflows (0.05 g fruit^-1hr^-1), while water left the fruit via the xylem. Negligible skin transpiration was recorded for vapour pressure deficit (VPD) values below 3 kPa. This growth pattern, in the absence of skin transpiration, suggests apoplastic phloem unloading. However, at VPD values over 3 kPa (e.g. from early afternoon to a peak around 18:00 h), transpiratory losses through the skin (up to 0.25 g fruit^-1hr^-1) caused fruit to shrink, leading to enhanced phloem and xylem inflows (ca. 0.15 g fruit^-1hr^-1), a scenario that would correspond to symplastic phloem unloading. Over 24 h the fruit showed a slightly negative total growth, consistent with fruit growth measured in situ during the season at weekly intervals. A few fruit species are known to alter their phloem unloading mechanism, switching from symplastic to apoplastic during the season. Our data support the coexistence in Japanese plum of different phloem unloading strategies within the same day.

Overexpression of a Grapevine Sucrose Transporter (VvSUC27) in Tobacco Improves Plant Growth Rate in the Presence of Sucrose In vitro

The import of sugar from source leaves and it further accumulation in grape berries are considerably high during ripening, and this process is mediated via sucrose transporters. In this study, a grape sucrose transporter (SUT) gene, VvSUC27, located at the plasma membrane, was transferred to tobacco (Nicotiana tabacum). The transformants were more sensitive to sucrose and showed more rapid development, especially roots, when cultured on MS agar medium containing sucrose, considering that the shoot/root dry weight ratio was only half that of the control. Moreover, all transformed plants exhibited light-colored leaves throughout their development, which indicated chlorosis and an associated reduction in photosynthesis. The total sugar content in the roots and stems of transformants was higher than that in control plants. No significant difference was observed in the leaves between the transformants and control plants. The levels of growth-promoting hormones were increased, and those of stress-mediating hormones were reduced in transgenic tobacco plants. The qRT-PCR analysis revealed that the expression of VvSUC27 was 1,000 times higher than that of the autologous tobacco sucrose transporter, which suggested that the markedly increased growth rate of transformants was because of the heterogeneously expressed gene. The transgenic tobacco plants showed resistance to abiotic stresses. Strikingly, the overexpression of VvSUC27 leaded to the up regulation of most reactive oxygen species scavengers and abscisic acid-related genes that might enable transgenic plants to overcome abiotic stress. Taken together, these results revealed an important role of VvSUC27 in plant growth and response to abiotic stresses, especially in the presence of sucrose in vitro.

Assessment of Sugar Components and Genes Involved in the Regulation of Sucrose Accumulation in Peach Fruit

Soluble sugar contents in mature fruits of 45 peach accessions were quantified using gas chromatography analysis. Sucrose is the predominant sugar in mature fruit, followed by glucose and fructose, which have similar concentrations. Overall, sucrose metabolism and accumulation are crucial determinants of sugar content in peach fruit, and there is a wide range of sucrose concentrations among peach genotypes. To understand the mechanisms regulating sucrose accumulation in peach fruit, expression profiles of genes involved in sucrose metabolism and transport were compared among four genotypes. Two sucrose-cleaving enzyme genes (SUS4 and NINV8), one gene involved in sucrose resynthesis (SPS3), and three sugar transporter genes (SUT2, SUT4, and TMT2) were prevalently expressed in peach fruit, and their expression levels are significantly correlated with sucrose accumulation. In contrast, the VAINV genes responsible for sucrose cleavage in the vacuole were weakly expressed in mature fruit, suggesting that the sucrose-cleaving reaction is not active in the vacuole of sink cells of mature peach fruit. This study suggests that sucrose accumulation in peach fruit involves the coordinated interaction of genes related to sucrose cleavage, resynthesis, and transport, which could be helpful for future peach breeding.

A sucrose transporter-interacting protein disulphide isomerase affects redox homeostasis and links sucrose partitioning with abiotic stress tolerance

Sucrose accumulation in leaves in response to various abiotic stresses suggests a specific role of this disaccharide for stress tolerance and adaptation. The high-affinity transporter StSUT1 undergoes substrate-induced endocytosis presenting the question as to whether altered sucrose accumulation in leaves in response to stresses is also related to enhanced endocytosis or altered activity of the sucrose transporter. StSUT1 is known to interact with several stress-inducible proteins; here we investigated whether one of the interacting candidates, StPDI1, affects its subcellular localization in response to stress: StPDI1 expression is induced by ER-stress and salt. Both proteins, StSUT1 and StPDI1, were found in the detergent resistant membrane (DRM) fraction, and this might affect internalization. Knockdown of StPDI1 expression severely affects abiotic stress tolerance of transgenic potato plants. Analysis of these plants does not reveal modified subcellular localization or endocytosis of StSUT1, but rather a disturbed redox homeostasis, reduced detoxification of reactive oxygen species and effects on primary metabolism. Parallel observations with other StSUT1-interacting proteins are discussed. The redox status in leaves seems to be linked to the sugar status in response to various stress stimuli and to play a role in stress tolerance.

New Insights into Phloem Unloading and Expression of Sucrose Transporters in Vegetative Sinks of the Parasitic Plant Phelipanche ramosa L. (Pomel)

The plant-parasitic plant interaction is a interesting model to study sink-source relationship and phloem unloading. The parasitic plants, such as the achlorophyllous plant Phelipanche ramosa, connect to the host phloem through the haustorium and act as supernumerary sinks for the host-derived photoassimilates, primarily sucrose. The application of the fluorescent symplastic tracer, carboxyfluorescein (CF) derived from carboxyfluorescein diacetate (CFDA), to the leaves of the host plant (Brassica napus) showed direct phloem connections at the host-parasite interface. These experiments also evidenced the dominant apoplastic pathway for phloem unloading in major vegetative sinks of the parasite, including tubercles and shoots, except the adventitious root apices. The CF experiments showed also the symplastic isolation of the phloem tissues from the sink tissues in tubercle and shoot of the parasite, then suggesting the pivotal role of sucrose transporters in sucrose unloading in P. ramosa sinks. Three cDNAs encoding sucrose transporters (PrSUT) were isolated from the parasitic plant. PrSUT1 transcripts accumulated at the same level in the tubercle throughout the parasite growth while a significant increase in transcript accumulation occurred after emergence in the flowering shoot, notably in the growing apical part. The in situ hybridization experiments revealed the PrSUT1 transcript accumulation in the mature phloem cells of both subterranean and flowering shoots, as well as in shoot terminal sinks corresponding to apical meristem, scale leaf primordia and immature vasculature. The transient expression experiments in Arabidopsis protoplasts showed that PrSUT1 was localized at the plasma membrane, suggesting its role in phloem functioning and sucrose uptake by the sink cells in P. ramosa. Conversely, the PrSUT2 transcript accumulation was constantly low in tubercles and shoots but PrSUT3 transcripts accumulated markedly in the subterranean and flowering shoots, in concordance with the PrSUT3 mRNA accumulation in multiple sink areas including apical meristem, scale-leaf primordia, immature vasculature and even storage parenchyma. However, the PrSUT3 transcripts did not accumulate in the mature phloem cells. The transient expression experiments in Arabidopsis protoplasts suggested a tonoplast localization of PrSUT3, for which nevertheless the involvement in intracellular sucrose transport needs clarification.