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

MdNAC5: a key regulator of fructose accumulation in apple fruit

The sweetness of apple fruit is a key factor in the improvement of apple varieties, with fructose being the sweetest of the soluble sugars, playing a crucial role in determining the overall sweetness of the apple. Therefore, uncovering the key genes controlling fructose accumulation and deciphering the regulatory mechanisms of fructose are vitally important for the improvement of apple varieties. In this study, through BSA-seq and transcriptome analysis of the 'Changfu 2' × 'Golden Delicious' F1 hybrid population, MdNAC5 was identified as a key regulatory gene for fructose content. MdNAC5 was shown to significantly influence fructose accumulation in both apples and tomatoes. Furthermore, we conducted a detailed identification of sugar transporters and metabolic enzymes in apples, discovering that MdNAC5 can enhance fructose accumulation in vacuoles and the conversion of sucrose to fructose by binding to and activating the promoters of the vacuolar sugar transporter MdTST2 and the neutral invertase MdNINV6. Additionally, MdNAC5 regulated the MdEIN3.4-MdSWEET15a module, strengthening the unloading of sucrose in the phloem of the fruit. Our results reveal a new mechanism by which MdNAC5 regulates fructose accumulation in apples and provide theoretical foundations for improving apple sweetness through genetic modification.

Characterization and Expression Analysis of Sugar Transporters through Partial Least Square Structural Equation Model (PLS-SEM) Revealed Their Role in Pepper (Capsicum annuum L.)

Pepper (Capsicum annuum L.) is one of the most important economic crops in the world. By controlling the transport and distribution of photosynthetic products between cells and organs, sugar transporters are widely involved in growth and development, environmental adaptation, and microbial interactions. The present study was carried out at the genome-wide level to systematically characterize sugar transporters. As a result, 50 MST, 3 SUT, and 29 SWEET genes were identified and classified. The expression pattern of sugar transporters in pepper was analyzed by transcriptomic data. The expression properties of sugar transporters were further explored in pepper varieties with significant differences in weight, shape, and pungency. It was shown that the pepper sugar transporter genes had obvious spatiotemporal specific expression characteristics and exhibited variety-specific expression preferences. We focus on analyzing candidate genes that may be involved in fruit development and expansion. We further explore the response of pepper sugar transporters to adversity stress using a structural equation model. Finally, we found that the MST, SUT, and SWEET families are collectively involved in balancing pepper resistance to abiotic stress by coordinating the expression strengths of different family members. Our study may contribute to the functional study of pepper sugar transporter genes and create the prospect of utilizing sugar transporter gene resources to improve pepper variety.

Epigenomic mechanism regulating the quality and ripeness of apple fruit with differing harvest maturity

Harvest maturity significantly affects the quality of apple fruit in post-harvest storage process. Although the regulatory mechanisms underlying fruit ripening have been studied, the associated epigenetic modifications remain unclear. Thus, we compared the DNA methylation changes and the transcriptional responses of mature fruit (MF) and immature fruit (NF). There were significant correlations between DNA methylation and gene expression. Moreover, the sugar contents (sucrose, glucose, and fructose) were higher in MF than in NF, whereas the opposite pattern was detected for the starch content. The expression-level differences were due to DNA methylations and ultimately resulted in diverse fruit textures and ripeness. Furthermore, the higher ethylene, auxin, and abscisic acid levels in MF than in NF, which influenced the fruit texture and ripening, were associated with multiple differentially expressed genes in hormone synthesis, signaling, and response pathways (ACS, ACO, ZEP, NCED, and ABA2) that were regulated by DNA methylations. Multiple transcription factor genes involved in regulating fruit ripening and quality via changes in DNA methylation were identified, including MIKCC-type MADS-box genes and fruit ripening-related genes (NAP, SPL, WRKY, and NAC genes). These findings reflect the diversity in the epigenetic regulation of gene expression and may be relevant for elucidating the epigenetic regulatory mechanism underlying the ripening and quality of apple fruit with differing harvest maturity.

Transcription factors PuPRE6/PuMYB12 and histone deacetylase PuHDAC9-like regulate sucrose levels in pear

The improvement of fruit quality, in particular sugar content, has been a major goal of plant breeding programmes for many years. Here, 2 varieties of the Ussurian pear (Pyrus ussuriensis), Nanguo, and its high-sucrose accumulation bud sport, Nanhong, were used to study the molecular mechanisms regulating sucrose transport in fruits. Comparative transcriptome analysis showed that in Nanhong fruit, an MYB transcription factor, PuMYB12, and a sucrose transporter protein, PuSUT4-like, were expressed at higher levels, while a paclobutrazol resistance transcription factor, PuPRE6, and a histone deacetylase (HDAC), PuHDAC9-like, were expressed at lower levels in Nanguo fruit. PuSUT4-like silencing and overexpression experiments in Nanguo pear showed that PuSUT4-like is essential for sucrose transportation. PuPRE6 and PuMYB12 act as antagonistic complexes to regulate PuSUT4-like transcription and sucrose accumulation. The histone deacetylation levels of the PuMYB12 and PuSUT4-like promoters were higher in Nanguo fruit than in Nanhong fruit, and Y1H assays showed that HDAC PuHDAC9-like bound directly to the promoters of PuMYB12 and PuSUT4-like. Our results uncovered transcription regulation and epigenetic mechanisms underlying sucrose accumulation in pears.

The apple transcription factor MdZF-HD11 regulates fruit softening by promoting Mdβ-GAL18 expression

Fruit ripening and the associated softening are major determinants of fruit quality and post-harvest shelf life. Although the mechanisms underlying fruit softening have been intensively studied, there are limited reports on the regulation of fruit softening in apples (Malus domestica). Here, we identified a zinc finger homeodomain transcription factor MdZF-HD11that trans-activates the promoter of Mdβ-GAL18, which encodes a pectin-degradation enzyme associated with cell wall metabolism. Both MdZF-HD11 and Mdβ-GAL18 genes were up-regulated by exogenous ethylene treatment and repressed by 1-methylcyclopropene treatment. Further experiments revealed that MdZF-HD11 binds directly to the Mdβ-GAL18 promoter and up-regulates its transcription. Moreover, using transgenic apple fruit calli, we found that overexpression of Mdβ-GAL18 or MdZF-HD11 significantly enhanced β-galactosidase activity, and overexpression of MdZF-HD11 induced the expression of Mdβ-GAL18. We also discovered that transient overexpression of Mdβ-GAL18 or MdZF-HD11 in 'Golden Delicious' apple significantly increased the release of ethylene, reduced fruit firmness, promoted the transformation of skin color from green to yellow, and accelerated ripening and softening of the fruit. Finally, the overexpression of MdZF-HD11 in tomato also promoted fruit softening. Collectively, these results indicate that ethylene-induced MdZF-HD11 interacts with Mdβ-GAL18 to promote the post-harvest softening of apple.

Effect of sucrose as an elicitor in increasing quercetin-3-O-rhamnoside (quercitrin) content of chrysanthemum (Chrysanthemum morifolium Ramat) callus culture based on harvest time differences

Chrysanthemum (Chrysanthemum morifolium) contains secondary metabolites, such as flavonoid compounds, especially luteolin-7-glucoside and quercetin-3-O-rhamnoside (quercitrin), in its tissues. Utilizing sucrose as an elicitor through callus culture presents an alternative method to enhance the production of secondary metabolites. This research aimed to determine the best sucrose concentration and harvest time for maximizing quercitrin content in chrysanthemum callus culture. The research employed a completely randomized design with four treatment groups: 0, 30, 45, and 60 g/l of sucrose added to MS medium containing 4 ppm 2,4-dichlorophenoxyacetic acid (2,4-D). Callus samples were harvested on the 15th and 30th days of culture. The observed parameters included callus morphology (color and texture), fresh weight, dry weight, the diameter of the callus, and quercitrin content analyzed using high-performance liquid chromatography. The results showed that all callus cultures exhibited intermediate textures and varied colors, predominantly shades of brown. The treatment involving 45 g/l of sucrose with a 30th-day harvest yielded the highest fresh weight, dry weight, and quercitrin content, namely 2.108 g, 0.051 g, and 0.437 mg/g DW, respectively. Notably, the quercitrin content exhibited a 63.67% increase compared to the control.

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.

QTL Mapping and Genome-Wide Association Study Reveal Genetic Loci and Candidate Genes Related to Soluble Solids Content in Melon

Melon (Cucumis melo L.) is an economically important Cucurbitaceae crop grown around the globe. The sweetness of melon is a significant factor in fruit quality and consumer appeal, and the soluble solids content (SSC) is a key index of melon sweetness. In this study, 146 recombinant inbred lines (RILs) derived from two oriental melon materials with different levels of sweetness containing 1427 bin markers, and 213 melon accessions containing 1,681,775 single nucleotide polymorphism (SNP) markers were used to identify genomic regions influencing SSC. Linkage mapping detected 10 quantitative trait loci (QTLs) distributed on six chromosomes, seven of which were overlapped with the reported QTLs. A total of 211 significant SNPs were identified by genome-wide association study (GWAS), 138 of which overlapped with the reported QTLs. Two new stable, co-localized regions on chromosome 3 were identified by QTL mapping and GWAS across multiple environments, which explained large phenotypic variance. Five candidate genes related to SSC were identified by QTL mapping, GWAS, and qRT-PCR, two of which were involved in hydrolysis of raffinose and sucrose located in the new stable loci. The other three candidate genes were involved in raffinose synthesis, sugar transport, and production of substrate for sugar synthesis. The genomic regions and candidate genes will be helpful for molecular breeding programs and elucidating the mechanisms of sugar accumulation.

Phloem unloading in cultivated melon fruits follows an apoplasmic pathway during enlargement and ripening

Melon (Cucumis melo L.) has a long history of cultivation worldwide. During cultivation, domestication, and selection breeding, the sugar content of mature melon fruits has been significantly increased. Compared with unsweet melon and wild melon, rapid sucrose accumulation can occur in the middle and late stages of sweet melon fruit development. The phloem unloading pathway during the evolution and development of melon fruit has not been identified and analyzed. In this study, the phloem unloading pathway and the function of related sugar transporters in cultivated and wild melon fruits were analyzed by CFDA [5(6)-carbofluorescein diacetate] and esculin tracing, cytological pathway observation, qRT-PCR, and gene function analysis, etc. Results show that the phloem unloading pathway of wild melon fruit is largely symplastic, whereas the phloem unloading pathway of cultivated melon fruit shifts from symplastic to apoplasmic during development. According to a fruit grafting experiment, the fruit sink accumulates sugars independently. Correlation analysis showed that the expression amounts of several sucrose transporter genes were positively correlated with the sucrose content of melon fruit. Furthermore, CmSWEET10 was proved to be a sucrose transporter located on the plasma membrane of the phloem and highly expressed in the premature stage of sweet melon fruits, which means it may be involved in phloem apoplast unloading and sucrose accumulation in sweet melon fruits. Finally, we summarize a functional model of related enzymes and sugar transporters involved in the apoplast unloading of sweet melon fruits during enlargement and sucrose accumulation.

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.

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

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

Genome-wide identification and expression analysis of SUT gene family members in sugar beet (Beta vulgaris L.)

Sucrose transporters (SUTs) play an important role in the transmembrane transport and distribution of sucrose, and their activity has an important impact on plant growth and crop yield. In this study, the SUT gene family was identified in the whole beet genome using bioinformatics methods, and gene characteristics, subcellular localization prediction, phylogenetic evolution, promoter cis-elements and expression patterns were systematically analyzed. A total of 9 SUT gene family members were identified from in beet genome and divided into 3 different groups (group 1, group 2, and Group 3), which were unevenly distributed on 4 chromosomes. Most SUT family members contained photoresponsive and hormone-regulated response elements. Subcellular localization prediction showed that the BvSUT genes are all located in the inner membrane, and most of the terms identified through GO enrichment analysis are classified as "membrane" related. The results of RT-qPCR showed that the expression level of the BvSUT gene was significantly higher in the tuber enlargement stage (100-140 d) than in other stages. This study is the first to analyze the BvSUT gene family in sugar beet, and it provides a theoretical basis for the functional exploration and application of SUT genes in crop improvement, especially in sugar crops.

Effects of Magnesium on nitrate uptake and sorbitol synthesis and translocation in apple seedlings

Both magnesium (Mg) and nitrogen (N) play many important roles in plant physiological and biochemical processes. Plants usually exhibit low nitrogen utilization efficiency (NUE) under Mg deficiency conditions, but the mechanisms by which Mg regulates NUE are not well understood. Herein, we investigated biomass, nutrient uptake, sorbitol and sucrose transport, and relative gene expression in apple seedlings under various concentrations of Mg and N treatments in hydroponic cultures. We first observed that low Mg supply significantly limited plant growth and N, Mg concentrations. Increasing the supply of N, but not Mg, partially alleviated the inhibition of plant growth under low Mg stress, which indicated that Mg deficiency had a negative impact on plant growth because it inhibits N absorption. Moreover, we found that the expression of nitrate transporter genes MdNRT2.1 and MdNRT2.4 was significantly downregulated by low Mg stress, and sufficient Mg significantly promoted sucrose and sorbitol synthesis and transport from leaves to roots by regulating relevant enzyme activity and genes expression. Further experiments showed that exogenous sorbitol could rapidly restore MdNRT2.1/2.4 expression and nitrate uptake under low Mg availability without increasing internal Mg level, suggesting that Mg may regulate MdNRT2.1/2.4 expression by regulating more sorbitol transport to roots, the effect of Mg on N was indirect, sorbitol played a key role during this process. Taken together, Mg promoted sorbitol synthesis and transport into roots, thus upregulating the expression of MdNRT2.1/2.4 and increasing the absorption of nitrate.

Magnesium improved fruit quality by regulating photosynthetic nitrogen use efficiency, carbon-nitrogen metabolism, and anthocyanin biosynthesis in 'Red Fuji' apple

INTRODUCTION

Both nitrogen (N) and magnesium (Mg) play important roles in biochemical and physiological processes in plants. However, the application of excessive N and insufficient Mg may be the factor leading to low nitrogen utilization rate (NUE) and fruit quality degradation in apple production.

METHODS

In this study, we analyzed the effects of different application rates of Mg (0, 50, 100, 150, 200 kg/ha) on the photosynthetic nitrogen use efficiency (PNUE), the accumulation and distribution of carbon (C), N metabolism, anthocyanin biosynthesis and fruit quality of the 'Red Fuji' apple in 2018 and 2019.

RESULTS

The results showed that the application of Mg significantly increased the ¹⁵NUE and increased the allocation rate of ¹⁵N in the leaves whereas the ¹⁵N allocation rate in the perennial organs and fruits was decreased. With the increase in Mg supply, the activities of N metabolism enzymes (NiR, GS, and GOGAT) were significantly promoted and the content of intermediate products in N metabolism ( NO 2 - , NH 4 + , and free amino acid) was significantly decreased. Furthermore, an appropriate rate of Mg significantly promoted the net photosynthetic rate (P_n) and photosynthetic nitrogen use efficiency (PNUE), enhanced the enzyme activities of C metabolism (SS, SPS, S6PDH), and increased the contents of sorbitol and sucrose in leaves. In addition, Mg upregulated the gene expression of sugar transporters (MdSOT1, MdSOT3, MdSUT1, and MdSUT4) in fruit stalk and fruit fresh; ¹³C isotope tracer technology also showed that Mg significantly increased the ¹³C allocation in the fruits. Mg also significantly increased the expression of anthocyanin biosynthesis genes (MdCHS and MdF3H) and transcription factors (MdMYB1 and MdbZIP44) and the content of anthocyanin in apple peel.

CONCLUSION

The comprehensive analysis showed that the appropriate application of Mg (150 kg/ha) promoted PNUE, C-N metabolism, and anthocyanin biosynthesis in apple trees.

Plasma Membrane-Localized Transporter MdSWEET12 Is Involved in Sucrose Unloading in Apple Fruit

Sugar content is an important factor determining the flavor in apple fruit. Sugar unloading is a prerequisite step for sugar accumulation. However, little is known about sugar unloading mechanisms in apple. Transcriptomic sequencing of two apple varieties, "Envy" and "Pacific Rose," with significantly different sugar content was performed. MdSWEET12a from the SWEET transporter family was differentially expressed. Further study of the MdSWEET12a showed that this plasma membrane-localized transporter protein-encoding gene was mainly expressed in sieve element-companion cells (SE-CC) in the fruit, which was positively correlated with the sucrose accumulation during the development of "Envy" apple. Consistently manipulating the gene expression through either transient overexpression or silencing significantly increased or decreased the sugar content in apple fruit, respectively. Complementary growth experiments in mutant yeast cells indicated that MdSWEET12a transported sucrose. Heterologous expression of MdSWEET12a in tomato increased the expression of genes related to sugar metabolism and transport, leading to increased sugar content. These findings underpin the involvement of MdSWEET12a in sugar unloading in apple fruit.

Characterization of the genetic and regulatory networks associated with sugar and acid metabolism in apples via an integrated strategy

Although sugars and acids have a substantial influence on the taste of apple fruits, the genetic and regulatory networks underlying their metabolism in fruit remain insufficiently determined. To fully decipher the genetic basis of the accumulation of sugars and acids in apple fruits, we adopted an integrated strategy that included time-course RNA-seq, QTL mapping, and whole-genome sequencing to examine two typical cultivars ('HanFu' and 'Huahong') characterized by distinctive flavors. Whole-genome sequencing revealed substantial genetic variation between the two cultivars, thereby providing an indication of the genetic basis of the distinct phenotypes. Constructed co-expression networks yielded information regarding the intra-relationships among the accumulation of different types of metabolites, and also revealed key regulatory nodes associated with the accumulation of sugars and acids, including the genes MdEF2, MdPILS5, and MdGUN8. Additionally, on the basis of QTL mapping using a high-density genetic map, we identified a series of QTLs and functional genes underlying vital traits, including sugar and acid contents. Collectively, our methodology and observations will provide an important reference for further studies focusing on the flavor of apples.

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.

Research Progress on Genetic Basis of Fruit Quality Traits in Apple (Malus × domestica)

Identifying the genetic variation characteristics of phenotypic traits is important for fruit tree breeding. During the long-term evolution of fruit trees, gene recombination and natural mutation have resulted in a high degree of heterozygosity. Apple (Malus × domestica Borkh.) shows strong ecological adaptability and is widely cultivated, and is among the most economically important fruit crops worldwide. However, the high level of heterozygosity and large genome of apple, in combination with its perennial life history and long juvenile phase, complicate investigation of the genetic basis of fruit quality traits. With continuing augmentation in the apple genomic resources available, in recent years important progress has been achieved in research on the genetic variation of fruit quality traits. This review focuses on summarizing recent genetic studies on apple fruit quality traits, including appearance, flavor, nutritional, ripening, and storage qualities. In addition, we discuss the mapping of quantitative trait loci, screening of molecular markers, and mining of major genes associated with fruit quality traits. The overall aim of this review is to provide valuable insights into the mechanisms of genetic variation and molecular breeding of important fruit quality traits in apple.

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.

Novel Role of AaMYBC1 in Regulating Actinidia arguta Vine Architecture by Elongating Internode Based on Multi-Omics Analysis of Transgenic Tobacco

The internode length affects the status of fruiting branches and shapes the vine architecture. MYB TFs (transcription factors) have been widely studied and reported to control many biological processes including secondary metabolism, abiotic stresses, growth and development, etc. However, the roles of MYB TFs in regulating internode length remain poorly understood. Here, we demonstrated that a secondary metabolism-related R2R3-MYB TF AaMYBC1 from Actinidia arguta was involved in the regulation of internode length by combined analysis of transcriptome and metabolome of transgenic tobacco plants. The metabolome analysis of OE (over-expressed tobacco) and WT (wild-typed tobacco) showed that there were a total of 1000 metabolites, 176 of which had significant differences. A key metabolite pme1651 annotated as indole 3-acetic acid belonged to phytohormone that was involved in internode length regulation. The RNA-seq analysis presented 446 differentially expressed genes (DEGs) between OE and WT, 14 of which were common DEGs in KEGG and GO enrichment. Through the combined analysis of metabolome and transcriptome in transgenic and wild-type tobacco, three key genes including two SAUR and a GH3 gene were possibly involved in internode elongation. Finally, a regulatory module was deduced to show the role of AaMYBC1 in internode elongation. Our results proposed a molecular mechanism of AaMYBC1 regulating internode length by mediated auxin signaling, implying the potential role in regulating the vine architecture.

Comparative physiological and transcriptomic analysis reveal MdWRKY75 associated with sucrose accumulation in postharvest 'Honeycrisp' apples with bitter pit

BACKGROUND

Calcium (Ca) deficiency can cause apple bitter pit, reduce the quality and shelf life. WRKY transcription factors play essential role in plant response to multiple disorders. However, the underlying mechanisms causing bitter pit in apple fruit due to Ca deficiency during storage is extremely limited.

RESULTS

In the present study, the nutritional metabolites and reactive oxygen species (ROS) were compared in Ca-deficient and healthy apple fruit (CK) during storage. Results showed that Ca-deficient apples sustained significantly higher production of ROS, PPO activity, flavonoids, total phenol, total soluble solids (TSS), and sucrose contents, but the contents of Ca, H₂O₂, titratable acids (TA), glucose and fructose were significantly lower than those of CK during storage. Principal component analysis (PCA) showed that TSS, •O₂^-, PPO, malondialdehyde (MDA) and Ca were the main factors, and TSS had a positive correlation with sucrose. Furthermore, transcriptome analysis revealed that WRKYs were co-expressed with sucrose metabolism-related enzymes (SWEETs, SS, SPS). qRT-PCR and correlation analysis indicated that MdWRKY75 was correlated positively with MdSWEET1. Moreover, transient overexpression of MdWRKY75 could significantly increase the sucrose content and promote the expression of MdSWEET1 in apple fruit.

CONCLUSIONS

Calcium deficiency could decrease antioxidant capacity, accelerate nutritional metabolism and up-regulate the expression of WRKYs in apple with bitter pit. Overexpression of MdWRKY75 significantly increased sucrose accumulation and the expression of MdSWEET1. These findings further strengthened knowledge of the basic molecular mechanisms in calcium deficiency apple flesh and contributed to improving the nutritional quality of apple fruit.

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.

Roles of abscisic acid in regulating ripening and quality of strawberry, a model non-climacteric fruit

Abscisic acid (ABA) is a dominant regulator of ripening and quality in non-climacteric fruits. Strawberry is regarded as a model non-climacteric fruit due to its extensive genetic studies and proven suitability for transgenic approaches to understanding gene function. Strawberry research has contributed to studies on color, flavor development, and fruit softening, and in recent years ABA has been established as a core regulator of strawberry fruit ripening, whereas ethylene plays this role in climacteric fruits. Despite this major difference, several components of the interacting genetic regulatory network in strawberry, such as MADS-box and NAC transcription factors, are similar to those that operate in climacteric fruit. In this review, we summarize recent advances in understanding the role of ABA biosynthesis and signaling and the regulatory network of transcription factors and other phytohormones in strawberry fruit ripening. In addition to providing an update on its ripening, we discuss how strawberry research has helped generate a broader and more comprehensive understanding of the mechanism of non-climacteric fruit ripening and focus attention on the use of strawberry as a model platform for ripening studies.

Molecular cues of sugar signaling in plants

Sugars, the chemically bound form of energy, are formed by the absorption of photosynthetically active radiation and fixation in plants. During evolution, plants availed the sugar molecules as a resource, balancing molecule, and signaling molecule. The multifaceted role of sugar molecules in response to environmental stimuli makes it the central coordinator required for growth, survival, and continuity. During the course of evolution, the molecular networks have become complex to adapt or acclimate to the changing environment. Sugar molecules are sensed both intra and extracellularly by their specific sensors. The signal is transmitted by a signaling loop that involves various downstream signaling molecules, transcriptional factors and, most pertinent, the sensors TOR and SnRK1. In this review, the focus has been retained on the significance of the sugar sensors during signaling and induced modules to regulate plant growth, development, biotic and abiotic stress. It is interesting to visualize the sugar molecule as a signaling unit and not only a nutrient. Complete information on the downstream components of sugar signaling will open the gates for improving the qualitative and quantitative elements of crop plants.

Roles of abscisic acid in regulating ripening and quality of strawberry, a model non-climacteric fruit

Abscisic acid (ABA) is a dominant regulator of ripening and quality in non-climacteric fruits. Strawberry is regarded as a model non-climacteric fruit due to its extensive genetic studies and proven suitability for transgenic approaches to understanding gene function. Strawberry research has contributed to studies on color, flavor development, and fruit softening, and in recent years ABA has been established as a core regulator of strawberry fruit ripening, whereas ethylene plays this role in climacteric fruits. Despite this major difference, several components of the interacting genetic regulatory network in strawberry, such as MADS-box and NAC transcription factors, are similar to those that operate in climacteric fruit. In this review, we summarize recent advances in understanding the role of ABA biosynthesis and signaling and the regulatory network of transcription factors and other phytohormones in strawberry fruit ripening. In addition to providing an update on its ripening, we discuss how strawberry research has helped generate a broader and more comprehensive understanding of the mechanism of non-climacteric fruit ripening and focus attention on the use of strawberry as a model platform for ripening studies.

Plasmodesmata and their role in the regulation of phloem unloading during fruit development

Fruit consumption is fundamental to a balanced diet. The contemporary challenge of maintaining a steady food supply to meet the demands of a growing population is driving the development of strategies to improve the production and nutritional quality of fruit. Plasmodesmata, the structures that mediate symplasmic transport between plant cells, play an important role in phloem unloading and distribution of sugars and signalling molecules into developing organs. Targeted modifications to the structures and functioning of plasmodesmata have the potential to improve fruit development; however, knowledge on the mechanisms underpinning plasmodesmata regulation in this context is scarce. In this review, we have compiled current knowledge on plasmodesmata and their structural characterisation during the development of fruit organs. We discuss key questions on phloem unloading, including the pathway shift from symplasmic to apoplastic that takes place during the onset of ripening as potential targets for improving fruit quality.

Combined Profiling of Transcriptome and DNA Methylome Reveal Genes Involved in Accumulation of Soluble Sugars and Organic Acid in Apple Fruits

Organic acids and soluble sugars are the major determinants of fruit organoleptic quality. Additionally, DNA methylation has crucial regulatory effects on various processes. However, the epigenetic modifications in the regulation of organic acid and soluble sugar accumulation in apple fruits remain uncharacterized. In this study, DNA methylation and the transcriptome were compared between ‘Honeycrisp’ and ‘Qinguan’ mature fruits, which differ significantly regarding soluble sugar and organic acid contents. In both ‘Honeycrisp’ and ‘Qinguan’ mature fruits, the CG context had the highest level of DNA methylation, and then CHG and CHH contexts. The number and distribution of differentially methylated regions (DMRs) varied among genic regions and transposable elements. The DNA methylation levels in all three contexts in the DMRs were significantly higher in ‘Honeycrisp’ mature fruits than in ‘Qinguan’ mature fruits. A combined methylation and transcriptome analysis revealed a negative correlation between methylation levels and gene expression in DMRs in promoters and gene bodies in the CG and CHG contexts and in gene bodies in the CHH context. Two candidate genes (MdTSTa and MdMa11), which encode tonoplast-localized proteins, potentially associated with fruit soluble sugar contents and acidity were identified based on expression and DNA methylation levels. Overexpression of MdTSTa in tomato increased the fruit soluble sugar content. Moreover, transient expression of MdMa11 in tobacco leaves significantly decreased the pH value. Our results reflect the diversity in epigenetic modifications influencing gene expression and will facilitate further elucidating the complex mechanism underlying fruit soluble sugar and organic acid accumulation.

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.