A Tonoplast Sugar Transporter Underlies a Sugar Accumulation QTL in Watermelon

Functions of membrane proteins in regulating fruit ripening and stress responses of horticultural crops

Fruit ripening is accompanied by the development of fruit quality traits; however, this process also increases the fruit's susceptibility to various environmental stresses, including pathogen attacks and other stress factors. Therefore, modulating the fruit ripening process and defense responses is crucial for maintaining fruit quality and extending shelf life. Membrane proteins play intricate roles in mediating signal transduction, ion transport, and many other important biological processes, thus attracting extensive research interest. This review mainly focuses on the functions of membrane proteins in regulating fruit ripening and defense responses against biotic and abiotic factors, addresses their potential as targets for improving fruit quality and resistance to environmental challenges, and further highlights some open questions to be addressed.

Virus-Induced galactinol-sucrose galactosyltransferase 2 Silencing Delays Tomato Fruit Ripening

Tomato fruit ripening is an elaborate genetic trait correlating with significant changes at physiological and biochemical levels. Sugar metabolism plays an important role in this highly orchestrated process and ultimately determines the quality and nutritional value of fruit. However, the mode of molecular regulation is not well understood. Galactinoal-sucrose galactosyltransferase (GSGT), a key enzyme in the biosynthesis of raffinose family oligosaccharides (RFOs), can transfer the galactose unit from 1-α-D-galactosyl-myo-inositol to sucrose and yield raffinose, or catalyze the reverse reaction. In the present study, the expression of SlGSGT2 was decreased by Potato Virus X (PVX)-mediated gene silencing, which led to an unripe phenotype in tomato fruit. The physiological and biochemical changes induced by SlGSGT2 silencing suggested that the process of fruit ripening was delayed as well. SlGSGT2 silencing also led to significant changes in gene expression levels associated with ethylene production, pigment accumulation, and ripening-associated transcription factors (TFs). In addition, the interaction between SlGSGT2 and SlSPL-CNR indicated a possible regulatory mechanism via ripening-related TFs. These findings would contribute to illustrating the biological functions of GSGT2 in tomato fruit ripening and quality forming.

Comprehensive identification and expression analyses of sugar transporter genes reveal the role of GmSTP22 in salt stress resistance in soybean

The transport, compartmentation and allocation of sugar are critical for plant growth and development, as well as for stress resistance, but sugar transporter genes have not been comprehensively characterized in soybean. Here, we performed a genome-wide identification and expression analyses of sugar transporter genes in soybean in order to reveal their putative functions. A total of 122 genes encoding sucrose transporters (SUTs) and monosaccharide transporters (MSTs) were identified in soybean. They were classified into 8 subfamilies according to their phylogenetic relationships and their conserved motifs. Comparative genomics analysis indicated that whole genome duplication/segmental duplication and tandem duplication contributed to the expansion of sugar transporter genes in soybean. Expression analysis by retrieving transcriptome datasets suggested that most of these sugar transporter genes were expressed in various tissues, and a number of genes exhibited tissue-specific expression patterns. Several genes including GmSTP21, GmSFP8, and GmPLT5/6/7/8/9 were predominantly expressed in nodules, and GmPLT8 was significantly induced by rhizobia inoculation in root hairs. Transcript profiling and qRT-PCR analyses suggested that half of these sugar transporter genes were significantly induced or repressed under stresses like salt, drought, and cold. In addition, GmSTP22 was found to be localized in the plasma membrane, and its overexpression promoted plant growth and salt tolerance in transgenic Arabidopsis under the supplement with glucose or sucrose. This study provides insights into the evolutionary expansion, expression pattern and functional divergence of sugar transporter gene family, and will enable further understanding of their biological functions in the regulation of growth, yield formation and stress resistance of soybean.

Multi-omic applications for understanding and enhancing tropical fruit flavour

Consumer trends towards nutrient-rich foods are contributing to global increasing demand for tropical fruit. However, commercial cultivars in the breeding pipeline that are tailored to meet market demand are at risk of possessing reduced fruit flavour qualities. This stems from recurrent prioritised selection for superior agronomic traits and not fruit flavour, which may in turn reduce consumer satisfaction. There is realisation that fruit quality traits, inclusive of flavour, must be equally selected for; but currently, there are limited tools and resources available to select for fruit flavour traits, particularly in tropical fruit species. Although sugars, acids, and volatile organic compounds are known to define fruit flavour, the specific combinations of these, that result in defined consumer preferences, remain unknown for many tropical fruit species. To define and include fruit flavour preferences in selective breeding, it is vital to determine the metabolites that underpin them. Then, objective quantitative analysis may be implemented instead of solely relying on human sensory panels. This may lead to the development of selective genetic markers through integrated omics approaches that target biosynthetic pathways of flavour active compounds. In this review, we explore progress in the development of tools to be able to strategically define and select for consumer-preferred flavour profiles in the breeding of new cultivars of tropical fruit species.

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.

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.

Promoter variations of ClERF1 gene determines flesh firmness in watermelon

BACKGROUND

Flesh firmness is a critical factor that influences fruit storability, shelf-life and consumer's preference as well. However, less is known about the key genetic factors that are associated with flesh firmness in fresh fruits like watermelon.

RESULTS

In this study, through bulk segregant analysis (BSA-seq), we identified a quantitative trait locus (QTL) that influenced variations in flesh firmness among recombinant inbred lines (RIL) developed from cross between the Citrullus mucosospermus accession ZJU152 with hard-flesh and Citrullus lanatus accession ZJU163 with soft-flesh. Fine mapping and sequence variations analyses revealed that ethylene-responsive factor 1 (ClERF1) was the most likely candidate gene for watermelon flesh firmness. Furthermore, several variations existed in the promoter region between ClERF1 of two parents, and significantly higher expressions of ClERF1 were found in hard-flesh ZJU152 compared with soft-flesh ZJU163 at key developmental stages. DUAL-LUC and GUS assays suggested much stronger promoter activity in ZJU152 over ZJU163. In addition, the kompetitive allele-specific PCR (KASP) genotyping datasets of RIL populations and germplasm accessions further supported ClERF1 as a possible candidate gene for fruit flesh firmness variability and the hard-flesh genotype might only exist in wild species C. mucosospermus. Through yeast one-hybrid (Y1H) and dual luciferase assay, we found that ClERF1 could directly bind to the promoters of auxin-responsive protein (ClAux/IAA) and exostosin family protein (ClEXT) and positively regulated their expressions influencing fruit ripening and cell wall biosynthesis.

CONCLUSIONS

Our results indicate that ClERF1 encoding an ethylene-responsive factor 1 is associated with flesh firmness in watermelon and provide mechanistic insight into the regulation of flesh firmness, and the ClERF1 gene is potentially applicable to the molecular improvement of fruit-flesh firmness by design breeding.

A natural mutation in the promoter of the aconitase gene ZjACO3 influences fruit citric acid content in jujube

Jujube (Ziziphus jujuba Mill.) is the most economically important fruit tree of the Rhamnaceae and was domesticated from wild or sour jujube (Z. jujuba Mill. var. spinosa Hu). During the process of domestication, there was a substantial reduction in the content of organic acids, particularly malate and citrate, which greatly influence the taste and nutritional value of the fruit. We previously demonstrated that ZjALMT4 is crucial for malate accumulation. However, the mechanism of citrate degradation in jujube remains poorly understood. In the present study, aconitase ZjACO3 was shown to participate in citric acid degradation in the cytoplasm through the GABA pathway. Interestingly, we discovered an E-box mutation in the ZjACO3 promoter (-484A > G; CAAGTG in sour jujube mutated to CAGGTG in cultivated jujube) that was strongly correlated with fruit citrate content; 'A' represented a high-citrate genotype and 'G' represented a low-citrate genotype. We developed and validated an ACO-based Kompetitive allele-specific PCR (KASP) marker for determining citric acid content. Yeast one-hybrid screening, transient dual-luciferase assays, and overexpression analyses showed that the transcription factor ZjbHLH113 protein directly binds to CAGGTG in the promoter of ZjACO3 in cultivated jujube plants, transcriptionally activating ZjACO3 expression, and enhancing citric acid degradation. Conversely, binding ability of the ZjbHLH113 protein to CAAGTG was weakened in sour jujube, thereby promoting citrate accumulation in the fruit. These findings will assist in elucidating the mechanism by which ZjACO3 modulates citrate accumulation in sour jujube and its cultivars.

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.

Vacuolar proteomic analysis reveals tonoplast transporters for accumulation of citric acid and sugar in citrus fruit

Vacuole largely dictates the fruit taste and flavor, as most of the sugars and organic acids are stored in the vacuoles of the fruit. However, difficulties associated with vacuole separation severely hinder identification and characterization of vacuolar proteins in fruit species. In this study, we established an effective approach for separating vacuoles and successfully purified vacuolar protein from six types of citrus fruit with varying patterns of sugar and organic acid contents. By using label-free LC-MS/MS proteomic analysis, 1443 core proteins were found to be associated with the essential functions of vacuole in citrus fruit. Correlation analysis of metabolite concentration with proteomic data revealed a transporter system for the accumulation of organic acid and soluble sugars in citrus. Furthermore, we characterized the physiological roles of selected key tonoplast transporters, ABCG15, Dict2.1, TMT2, and STP7 in the accumulation of citric acid and sugars. These findings provide a novel perspective and practical solution for investigating the transporters underlying the formation of citrus taste and flavor.

Two adjacent NAC transcription factors regulate fruit maturity date and flavor in peach

Although maturity date (MD) is an essential factor affecting fresh fruit marketing and has a pleiotropic effect on fruit taste qualities, the underlying mechanisms remain largely unclear. In this study, we functionally characterized two adjacent NAM-ATAF1/2-CUC2 (NAC) transcription factors (TFs), PpNAC1 and PpNAC5, both of which were associated with fruit MD in peach. PpNAC1 and PpNAC5 were found capable of activating transcription of genes associated with cell elongation, cell wall degradation and ethylene biosynthesis, suggesting their regulatory roles in fruit enlargement and ripening. Furthermore, PpNAC1 and PpNAC5 had pleiotropic effects on fruit taste due to their ability to activate transcription of genes for sugar accumulation and organic acid degradation. Interestingly, both PpNAC1 and PpNAC5 orthologues were found in fruit-producing angiosperms and adjacently arranged in all 91 tested dicots but absent in fruitless gymnosperms, suggesting their important roles in fruit development. Our results provide insight into the regulatory roles of NAC TFs in MD and fruit taste.

Transcriptome Analysis Reveals Fruit Quality Formation in Actinidia eriantha Benth

Actinidia chinensis Planch. is a fruit tree originating from China that is abundant in the wild. Actinidia eriantha Benth. is a type of A. chinensis that has emerged in recent years. The shape of A. eriantha is an elongated oval, and the skin is covered with dense, non-shedding milk-white hairs. The mature fruit has flesh that is bright green in colour, and the fruit has a strong flavour and a grass-like smell. It is appreciated for its rich nutrient content and unique flavour. Vitamin C, sugar, and organic acids are key factors in the quality and flavour composition of A. eriantha but have not yet been systematically analysed. Therefore, we sequenced the transcriptome of A. eriantha at three developmental stages and labelled them S1, S2, and S3, and comparisons of S1 vs. S2, S1 vs. S3, and S2 vs. S3 revealed 1218, 4019, and 3759 upregulated differentially expressed genes and 1823, 3415, and 2226 downregulated differentially expressed genes, respectively. Furthermore, the upregulated differentially expressed genes included 213 core genes, and Gene Ontology enrichment analysis showed that they were enriched in hormones, sugars, organic acids, and many organic metabolic pathways. The downregulated differentially expressed genes included 207 core genes, which were enriched in the light signalling pathway. We further constructed the metabolic pathways of sugars, organic acids, and vitamin C in A. eriantha and identified the genes involved in vitamin C, sugar, and organic acid synthesis in A. eriantha fruits at different stages. During fruit development, the vitamin C content decreased, the carbohydrate compound content increased, and the organic acid content decreased. The gene expression patterns were closely related to the accumulation patterns of vitamin C, sugars, and organic acids in A. eriantha. The above results lay the foundation for the accumulation of vitamin C, sugars, and organic acids in A. eriantha and for understanding flavour formation in A. eriantha.

Genome-wide association study of soluble solids content, flesh color, and fruit shape in citron watermelon

Fruit quality traits are crucial determinants of consumers' willingness to purchase watermelon produce, making them major goals for breeding programs. There is limited information on the genetic underpinnings of fruit quality traits in watermelon. A total of 125 citron watermelon (Citrullus amarus) accessions were genotyped using single nucleotide polymorphisms (SNPs) molecular markers generated via whole-genome resequencing. A total of 2,126,759 genome-wide SNP markers were used to uncover marker-trait associations using single and multi-locus GWAS models. High broad-sense heritability for fruit quality traits was detected. Correlation analysis among traits revealed positive relationships, with the exception of fruit diameter and fruit shape index (ratio of fruit length to fruit diameter), which was negative. A total of 37 significant SNP markers associated with soluble solids content, flesh color, fruit length, fruit diameter, and fruit shape index traits were uncovered. These peak SNPs accounted for 2.1%-23.4% of the phenotypic variation explained showing the quantitative inheritance nature of the evaluated traits. Candidate genes relevant to fruit quality traits were uncovered on chromosomes Ca01, Ca03, Ca06, and Ca07. These significant molecular markers and candidate genes will be useful in marker-assisted breeding of fruit quality traits in watermelon.

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.

Transcriptional repression of MdMa1 by MdMYB21 in Ma locus decreases malic acid content in apple fruit

Malic acid is a major organic acid component of apples and a crucial determinant of fruit organoleptic quality. A candidate gene for malic acid content, designated MdMa1, was previously identified in the Ma locus, which is a major quantitative trait locus (QTL) for apple fruit acidity located on the linkage group 16. Region-based association mapping to detect candidate genes in the Ma locus identified MdMa1 and an additional MdMYB21 gene putatively associated with malic acid. MdMYB21 was significantly associated with fruit malic acid content, accounting for ~7.48% of the observed phenotypic variation in the apple germplasm collection. Analyses of transgenic apple calli, fruits and tomatoes demonstrated that MdMYB21 negatively regulated malic acid accumulation. The apple fruit acidity-related MdMa1 and its tomato ortholog, SlALMT9, exhibited lower expression profiles in apple calli, mature fruits and tomatoes in which MdMYB21 was overexpressed, compared with their corresponding wild-type variety. MdMYB21 directly binds to the MdMa1 promoter and represses its expression. Interestingly, a 2-bp variation in the MdMYB21 promoter region altered its expression and regulation of its target gene, MdMa1, expression. Our findings not only demonstrate the efficiency of integrating QTL and association mapping in the identification of candidate genes controlling complex traits in apples, but also provide insights into the complex regulatory mechanism of fruit malic acid accumulation.

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.

Transcriptome and Metabolome Reveal Sugar and Organic Acid Accumulation in Rosa roxburghii Fruit

Sugars and organic acids significantly impact fruit sensory quality, but their accumulation patterns and regulatory mechanisms during the development of Rosa roxburghii fruit are still unclear. We utilized transcriptomics and metabolomics to investigate genes related to sugar and organic acid metabolism in Rosa roxburghii. Metabolomics data revealed that sucrose, glucose and fructose were the primary sugars, whereas citric acid and malic acid were the primary organic acids in Rosa roxburghii fruit. We constructed the metabolic pathways of major sugars and organic acids in Rosa roxburghii and identified five key genes involved in sugar and organic acid synthesis. In addition, we identified a module containing 132 transcription factors that was significantly associated with sucrose, citric acid and malic acid. Based on quantitative polymerase chain reaction (qPCR), we identified 13 transcription factors involved in sugar and organic acid metabolism, including the transcription factor RrANL2 and the sucrose synthase gene RrSUS3. Further yeast one-hybrid and dual luciferase assays showed that RrANL2 could bind to the promoter of RrSUS3 to increase its expression. These results provide new insights into the metabolism of sugars and organic acids in Rosa roxburghii fruit.

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.

Creation of a watermelon haploid inducer line via ClDMP3-mediated single fertilization of the central cell

The use of doubled haploids is one of the most efficient breeding methods in modern agriculture. Irradiation of pollen grains has been shown to induce haploids in cucurbit crops, possibly because it causes preferential fertilization of the central cell over the egg cell. Disruption of the DMP gene is known to induce single fertilization of the central cell, which can lead to the formation of haploids. In the present study, a detailed method of creating a watermelon haploid inducer line via ClDMP3 mutation is described. The cldmp3 mutant induced haploids in multiple watermelon genotypes at rates of up to 1.12%. These haploids were confirmed via fluorescent markers, flow cytometry, molecular markers, and immuno-staining. The haploid inducer created by this method has the potential to greatly advance watermelon breeding in the future.

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

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

Multi-omics integration to explore the molecular insight into the volatile organic compounds in watermelon

A range of volatile organic compounds played an important role in the formation of watermelon fruit aroma, while due to the low content and difficulty in detection, it is often neglected in watermelon breeding programs, resulting in a decline in fruit flavor. VOCs in the flesh of 194 watermelon accessions and seven cultivars at four developmental stages were determined by SPME-GC-MS. Ten metabolites with significant differences in the natural population and positive accumulation during fruit development are considered to be the key metabolite related to watermelon fruit aroma. And the link between metabolite and, flesh color and sugar content by correlation analysis was established. The results of the genome-wide association study showed that (5E)-6,10-dimethylundeca-5,9-dien-2-one, and 1-(4-methylphenyl) ethanone were colocalized with watermelon flesh color on chromosome 4, which may be regulated by LCYB and CCD. (E)-4-(2,6,6-trimethylcyclohexen-1-yl)but-3-en-2-one is the VOC produced by the cleavage of carotenoids, which has a positive correlation with the sugar content of the fruit, and the candidate gene Cla97C05G092490 on chromosome 5 may interact with PSY to influence the accumulation of this metabolite. In addition, Cla97C02G049790 (enol reductase), Cla97C03G051490 (omega-3 fatty acid desaturase gene), LOX, and ADH may play important roles in the synthesis of fatty acids and their derived VOCs. Taken together, our findings provide molecular insights into the accumulation and natural variation of VOCs in watermelon, and give data support for breeding watermelon cultivars with better flavor.

Potato tonoplast sugar transporter 1 controls tuber sugar accumulation during postharvest cold storage

Cold-induced sweetening (CIS), the undesirable sugar accumulation in cold-stored potato (Solanum tuberosum L.) tubers, is a severe postharvest issue in the potato processing industry. Although the process of sucrose hydrolysis by vacuolar invertase during potato CIS is well understood, there is limited knowledge about the transportation of sucrose from the cytosol to the vacuole during postharvest cold storage. Here, we report that among the three potato tonoplast sugar transporters (TSTs), StTST1 exhibits the highest expression in tubers during postharvest cold storage. Subcellular localization analysis demonstrates that StTST1 is a tonoplast-localized protein. StTST1 knockdown decreases reducing sugar accumulation in tubers during low-temperature storage. Compared to wild-type, potato chips produced from StTST1-silenced tubers displayed significantly lower acrylamide levels and lighter color after cold storage. Transcriptome analysis manifests that suppression of StTST1 promotes starch synthesis and inhibits starch degradation in cold-stored tubers. We further establish that the increased sucrose content in the StTST1-silenced tubers might cause a decrease in the ABA content, thereby inhibiting the ABA-signaling pathway. We demonstrate that the down-regulation of β-amylase StBAM1 in StTST1-silenced tubers might be directly controlled by ABA-responsive element-binding proteins (AREBs). Altogether, we have shown that StTST1 plays a critical role in sugar accumulation and starch metabolism regulation during postharvest cold storage. Thus, our findings provide a new strategy to improve the frying quality of cold-stored tubers and reduce the acrylamide content in potato chips.

Watermelon domestication was shaped by stepwise selection and regulation of the metabolome

Although crop domestication has greatly aided human civilization, the sequential domestication and regulation of most quality traits remain poorly understood. Here, we report the stepwise selection and regulation of major fruit quality traits that occurred during watermelon evolution. The levels of fruit cucurbitacins and flavonoids were negatively selected during speciation, whereas sugar and carotenoid contents were positively selected during domestication. Interestingly, fruit malic acid and citric acid showed the opposite selection trends during the improvement. We identified a novel gene cluster (CGC1, cucurbitacin gene cluster on chromosome 1) containing both regulatory and structural genes involved in cucurbitacin biosynthesis, which revealed a cascade of transcriptional regulation operating mechanisms. In the CGC1, an allele caused a single nucleotide change in ClERF1 binding sites (GCC-box) in the promoter of ClBh1, which resulted in reduced expression of ClBh1 and inhibition of cucurbitacin synthesis in cultivated watermelon. Functional analysis revealed that a rare insertion of 244 amino acids, which arose in C. amarus and became fixed in sweet watermelon, in ClOSC (oxidosqualene cyclase) was critical for the negative selection of cucurbitacins during watermelon evolution. This research provides an important resource for metabolomics-assisted breeding in watermelon and for exploring metabolic pathway regulation mechanisms.

Genetic mapping of a single nuclear locus determines the white flesh color in watermelon (Citrullus lanatus L.)

INTRODUCTION

Flesh color is an important trait in watermelon (Citrullus lanatus L.). Several flesh color genes have been identified in watermelon; however, the inheritance of and the molecular basis underlying the white flesh trait remain largely unknown.

METHODS

In this study, segregation populations were constructed by crossing the canary yellow flesh line HSH-F with the white flesh line Sanbai to fine-map the white flesh gene in watermelon.

RESULTS

Genetic analysis indicated that the white flesh trait is controlled by a single recessive locus, termed Clwf2. Map-based cloning delimited the Clwf2 locus to a 132.3-kb region on chromosome 6. The candidate region contains 13 putative genes, and four of them-Cla97C06G121860, Cla97C06G121880, Cla97C06G121890, and Cla97C06G121900-were significantly downregulated in the white flesh compared to the canary yellow flesh watermelon fruits. The Cla97C06G121890 gene, which encodes a tetratricopeptide repeat protein, showed almost no expression in the white flesh fruit before maturity, whereas it had a very high expression in the canary yellow flesh fruit at 18 days after pollination. Transmission electron microscopy revealed rounded and regularly shaped chromoplasts in both the canary yellow and white flesh fruits. Further quantitative real-time PCR analysis showed that the expression levels of several key plastid division genes and almost the entire carotenoid biosynthesis pathway genes were downregulated in the white flesh compared to the canary yellow flesh fruits.

DISCUSSION

This study suggests that the proliferation inhibition of chromoplasts and downregulation of the CBP genes block the accumulation of carotenoids in watermelon and lead to white flesh. These findings advance and extend the understanding of the molecular mechanisms underlying white flesh trait formation and carotenoid biosynthesis in watermelon.

Low-acidity ALUMINUM-DEPENDENT MALATE TRANSPORTER4 genotype determines malate content in cultivated jujube

Jujube (Ziziphus jujuba Mill.), the most economically important fruit tree in Rhamnaceae, was domesticated from sour jujube (Z. jujuba Mill. var. spinosa (Bunge) Hu ex H.F.Chow.). During domestication, fruit sweetness increased and acidity decreased. Reduction in organic acid content is crucial for the increase in sweetness of jujube fruit. In this study, the determination of malate content among 46 sour jujube and 35 cultivated jujube accessions revealed that malate content varied widely in sour jujube (0.90-13.31 mg g-1) but to a lesser extent in cultivated jujube (0.33-2.81 mg g-1). Transcriptome sequencing analysis showed that the expression level of Aluminum-Dependent Malate Transporter 4 (ZjALMT4) was substantially higher in sour jujube than in jujube. Correlation analysis of mRNA abundance and fruit malate content and transient gene overexpression showed that ZjALMT4 participates in malate accumulation. Further sequencing analyses revealed that three genotypes of the W-box in the promoter of ZjALMT4 in sour jujube associated with malate content were detected, and the genotype associated with low malate content was fixed in jujube. Yeast one-hybrid screening showed that ZjWRKY7 binds to the W-box region of the high-acidity genotype in sour jujube, whereas the binding ability was weakened in jujube. Transient dual-luciferase and overexpression analyses showed that ZjWRKY7 directly binds to the promoter of ZjALMT4, activating its transcription, and thereby promoting malate accumulation. These findings provide insights into the mechanism by which ZjALMT4 modulates malate accumulation in sour jujube and jujube. The results are of theoretical and practical importance for the exploitation and domestication of germplasm resources.

Two vacuolar invertase inhibitors PpINHa and PpINH3 display opposite effects on fruit sugar accumulation in peach

Soluble sugars are an important determinant of fruit taste, but their accumulation mechanisms remain elusive. In this study, we report two vacuolar invertase inhibitor genes involved in sugar accumulation in peach, PpINHa and PpINH3. Transient overexpression of PpINH3 in peach fruits resulted in an increase in sugar content, while the opposite trend was detected for PpINHa. Unexpectedly, PpINH3 and PpINHa both had no physical interaction with vacuolar invertase (VIN). Moreover, the PpVIN genes had no or extremely low expression in fruits at the ripening stage. These results suggested that the regulatory role of PpINHa and PpINH3 in sugar accumulation is unlikely due to their interaction with PpVINs. Additionally, overexpression of PpINHa and PpINH3 had an impact on transcription of genes related to fruit sugar metabolism and transport, which is likely responsible for their regulatory role in fruit sugar accumulation. Altogether, these results indicated an important role of PpINHs in fruit accumulation in peach. Our study provides new insights into molecular mechanisms underlying sugar accumulation, which could be useful for genetic improvement of fruit taste in breeding programs of peach and other fruit crops.

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.

Genome-wide association study provides genetic insights into natural variation in watermelon rind thickness and single fruit weight

Rind thickness and fruit weight are agronomic traits closely related to quality and yield, which have attracted much attention from consumers and breeders. However, the genetic mechanism of these two traits is still not well understood in natural populations. In this study, rind thickness and single fruit weight in 151 watermelon accessions were determined in 2019 and 2020, and genome-wide association analysis was performed by integrating phenotypic and genotype data. Abundant phenotypic variation was found in the test population, and the watermelon with thinner rind thickness tended to have smaller fruit weights. Five significant SNPs were closely associated with rind thickness on chromosome 2 by Genome-wide association study (GWAS), i.e., 32344170, 32321308, 32304738, 32328501, and 32311192. And there were 21 genes were annotated in the candidate interval, most notably, Cla97C02G044160 belonged to the MADS family, and part of the genes in this family played an important role in regulating organ size, further analysis of gene structure, gene expression level, and phylogenetic tree showed that Cla97C02G044160 was a candidate gene for regulating target traits. In conclusion, our study provides molecular insights into the natural variation of watermelon rind thickness and single fruit weight, meanwhile, providing data support for molecular marker-assisted breeding.

QTL mapping reveals candidate genes for main agronomic traits in Luffa based on a high-resolution genetic map

Luffa is an important medicinal and edible vegetable crop of Cucurbitaceae. Strong heterosis effects and strikingly complementary characteristics were found between the two domesticated Luffa cultivars, Luffa acutangula and Luffa cylindrica. To explore the genetic basis underlying their important agronomic traits, we constructed the first interspecific high-density genetic linkage map using a BC₁ population of 110 lines derived from a cross between S1174 (Luffa acutangula) and P93075 (Luffa cylindrica). The map spanned a total of 2246.74 cM with an average distance of 0.48 cM between adjacent markers. Thereafter, a large-scale field-based quantitative trait loci (QTLs) mapping was conducted for 25 important agronomic traits and 40 significant genetic loci distributed across 11 chromosomes were detected. Notably, a vital QTL (qID2) located on chromosome 9 with a minimum distance of 23 kb was identified to be responsible for the internode diameter and explained 11% of the phenotypic variation. Lac09g006860 (LacCRWN3), encoding a nuclear lamina protein involved in the control of nuclear morphology, was the only gene harbored in qID2. Sequence alignment showed completely different promoter sequences between the two parental alleles of LacCRWN3 except for some nonsynonymous single nucleotide polymorphisms (SNPs) in exons, and the expression level in thick-stem P93075 was distinctively higher than that in thin-stem S1174. According to the natural variation analysis of a population of 183 inbred lines, two main haplotypes were found for LacCRWN3: the P93075-like and S1174-like, with the former haplotype lines exhibiting significantly thicker internode diameters than those of the latter haplotype lines. It showed that LacCRWN3, as the only CRWN3 gene in Cucurbitaceae, was the most likely candidate gene regulating the internode diameter of Luffa. Our findings will be beneficial for deciphering the molecular mechanism of key phenotypic traits and promoting maker-assisted breeding in Luffa.

A telomere-to-telomere gap-free reference genome of watermelon and its mutation library provide important resources for gene discovery and breeding

Watermelon, Citrullus lanatus, is the world's third largest fruit crop. Reference genomes with gaps and a narrow genetic base hinder functional genomics and genetic improvement of watermelon. Here, we report the assembly of a telomere-to-telomere gap-free genome of the elite watermelon inbred line G42 by incorporating high-coverage and accurate long-read sequencing data with multiple assembly strategies. All 11 chromosomes have been assembled into single-contig pseudomolecules without gaps, representing the highest completeness and assembly quality to date. The G42 reference genome is 369 321 829 bp in length and contains 24 205 predicted protein-coding genes, with all 22 telomeres and 11 centromeres characterized. Furthermore, we established a pollen-EMS mutagenesis protocol and obtained over 200 000 M1 seeds from G42 . In a sampling pool, 48 monogenic phenotypic mutations, selected from 223 M1 and 78 M2 mutants with morphological changes, were confirmed. The average mutation density was 1 SNP/1.69 Mb and 1 indel/4.55 Mb per M1 plant and 1 SNP/1.08 Mb and 1 indel/6.25 Mb per M2 plant. Taking advantage of the gap-free G42 genome, 8039 mutations from 32 plants sampled from M1 and M2 families were identified with 100% accuracy, whereas only 25% of the randomly selected mutations identified using the 97103v2 reference genome could be confirmed. Using this library and the gap-free genome, two genes responsible for elongated fruit shape and male sterility (ClMS1) were identified, both caused by a single base change from G to A. The validated gap-free genome and its EMS mutation library provide invaluable resources for functional genomics and genetic improvement of watermelon.

Alkaline α-galactosidase 2 (CsAGA2) plays a pivotal role in mediating source-sink communication in cucumber

Sugars are necessary for plant growth and fruit development. Cucumber (Cucumis sativus L.) transports sugars, mainly raffinose family oligosaccharides (RFOs), in the vascular bundle. As the dominant sugars in cucumber fruit, glucose and fructose are derived from sucrose, which is the product of RFO hydrolysis by α-galactosidase (α-Gal). Here, we characterized the cucumber alkaline α-galactosidase 2 (CsAGA2) gene and found that CsAGA2 has undergone human selection during cucumber domestication. Further experiments showed that the expression of CsAGA2 increases gradually during fruit development, especially in fruit vasculature. In CsAGA2-RNA interference (RNAi) lines, fruit growth was delayed because of lower hexose production in the peduncle and fruit main vascular bundle (MVB). In contrast, CsAGA2-overexpressing (OE) plants displayed bigger fruits. Functional enrichment analysis of transcriptional data indicated that genes related to sugar metabolism, cell wall metabolism, and hormone signaling were significantly downregulated in the peduncle and fruit MVBs of CsAGA2-RNAi plants. Moreover, downregulation of CsAGA2 also caused negative feedback regulation on source leaves, which was shown by reduced photosynthetic efficiency, fewer plasmodesmata at the surface between mesophyll cell and intermediary cell (IC) or between IC and sieve element, and downregulated gene expression and enzyme activities related to phloem loading, as well as decreased sugar production and exportation from leaves and petioles. The opposite trend was observed in CsAGA2-OE lines. Overall, we conclude that CsAGA2 is essential for cucumber fruit set and development through mediation of sugar communication between sink strength and source activity.

Fine Mapping and Functional Analysis of Major Regulatory Genes of Soluble Solids Content in Wax Gourd (Benincasa hispida)

Soluble solids content (SSC) is an important quality trait of wax gourd, but reports about its regulatory genes are scarce. In this study, the SSC regulatory gene BhSSC2.1 in wax gourd was mined via quantitative trait locus (QTL) mapping based on high-density genetic mapping containing 12 linkage groups (LG) and bulked segregant analysis (BSA)-seq. QTL mapping and BSA-seq revealed for the first time that the SSC QTL (107.658–108.176 cM) of wax gourd was on Chr2 (LG2). The interpretable phenotypic variation rate and maximum LOD were 16.033% and 6.454, respectively. The QTL interval contained 13 genes. Real-time fluorescence quantitative expression analysis, functional annotation, and sequence analysis suggested that Bch02G016960, named BhSSC2.1, was a candidate regulatory gene of the SSC in wax gourd. Functional annotation of this gene showed that it codes for a NADP-dependent malic enzyme. According to BhSSC2.1 sequence variation, an InDel marker was developed for molecular marker-assisted breeding of wax gourd. This study will lay the foundation for future studies regarding breeding and understanding genetic mechanisms of wax gourd.

PpybZIP43 contributes to sucrose synthesis in pear fruits by activating PpySPS3 expression and interacts with PpySTOP1

Sucrose is an important factor affecting sweetness and flavor in pear fruits, but the molecular mechanism of sucrose synthesis regulation is relatively unknown. Here, we characterized a transcription factor gene from pear (Pyrus pyrifolia Nakai cv. "Hosui") fruits, PpybZIP43, and found that the transient overexpression of PpybZIP43 in pear fruits significantly increased the sucrose content and the relative expression level of sucrose phosphate synthase genes (PpySPS3 and PpySPS8). Subcellular localization analysis in tobacco leaves showed that PpybZIP43 was localized in the nucleus. Yeast one-hybrid, electrophoretic mobility shift assay (EMSA), and dual-luciferase reporter assays indicated that PpybZIP43 was able to activate the expression of PpySPS3 by binding specifically to the G-box (CACGTG) element in the promoter. The protein-protein interaction assays using yeast two-hybrid, bimolecular fluorescence complementation (BiFC), firefly luciferase complementation imaging (LCI), and glutathione S-transferase (GST) pull-down demonstrated that PpybZIP43 could directly interact with PpySTOP1 to form a transcription complex. This study is helpful for understanding the molecular basis of sucrose synthesis and accumulation in pear fruits and provides candidate genes for breeding.

Contributions of sugar transporters to crop yield and fruit quality

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

Quantitative Transcriptomic and Proteomic Analysis of Fruit Development and Ripening in Watermelon (Citrullus lanatus)

Fruit ripening is a highly complicated process, which is modulated by phytohormones, signal regulators and environmental factors playing in an intricate network that regulates ripening-related genes expression. Although transcriptomics is an effective tool to predict protein levels, protein abundances are also extensively affected by post-transcriptional and post-translational regulations. Here, we used RNA sequencing (RNA-seq) and tandem mass tag (TMT)-based quantitative proteomics to study the comprehensive mRNA and protein expression changes during fruit development and ripening in watermelon, a non-climacteric fruit. A total of 6,226 proteins were quantified, and the large number of quantitative proteins is comparable to proteomic studies in model organisms such as Oryza sativa L. and Arabidopsis. Base on our proteome methodology, integrative analysis of the transcriptome and proteome showed that the mRNA and protein levels were poorly correlated, and the correlation coefficients decreased during fruit ripening. Proteomic results showed that proteins involved in alternative splicing and the ubiquitin proteasome pathway were dynamically expressed during ripening. Furthermore, the spliceosome and proteasome were significantly enriched by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, suggesting that post-transcriptional and post-translational mechanisms might play important roles in regulation of fruit ripening-associated genes expression, which might account for the poor correlation between mRNAs and proteins during fruit ripening. Our comprehensive transcriptomic and proteomic data offer a valuable resource for watermelon research, and provide new insights into the molecular mechanisms underlying the complex regulatory networks of fruit ripening.

Genetic Analysis of Fruit Quality Traits in Sweet Watermelon (Citrullus lanatus var. lanatus): A Review

Fruit quality traits of sweet watermelon (Citrullus lanatus var. lanatus) are crucial for new product development and commercialization. Sweet watermelon fruit quality traits are affected by the compositions of phytochemical compounds, phytohormones, and fruit flesh firmness which are affected by genes, the growing environment and their interaction. These compositions determine fruit ripening, eating quality, and postharvest shelf-life. Knowledge of the genetic profile and analyses of quality traits in watermelon is vital to develop improved cultivars with enhanced nutritional compositions, consumer-preferred traits, and extended storage life. This review aims to present the opportunities and progress made on the genetic analysis of fruit quality traits in watermelon as a guide for quality breeding based on economic and end-user attributes. The first section of the review highlights the genetic mechanisms involved in the biosynthesis of phytochemical compounds (i.e., sugars, carotenoids, amino acids, organic acids, and volatile compounds), phytohormones (i.e., ethylene and abscisic acid) and fruit flesh structural components (i.e., cellulose, hemicellulose, and pectin) elicited during watermelon fruit development and ripening. The second section pinpoints the progress on the development of molecular markers and quantitative trait loci (QTL) analysis for phytochemical compounds, phytohormones and fruit quality attributes. The review presents gene-editing technology and innovations associated with fruit quality traits for selection and accelerated cultivar development. Finally, the paper discussed gene actions conditioning fruit ripening in citron watermelon (C. lanatus var. citroides [L. H. Bailey] Mansf. ex Greb.) as reference genetic resources to guide current and future breeding. Information presented in this review is useful for watermelon variety design, product profiling and development to serve the diverse value chains of the crop.

Integrative analysis of transcriptomic and metabolomic profiles reveals new insights into the molecular foundation of fruit quality formation in Citrullus lanatus (Thunb.) Matsum. & Nakai

In this study, an integrated transcriptome and metabolome analysis was used to explore the molecular foundation of fruit quality in two parent lines of Citrullus lanatus with distinct flesh characteristics, including ‘14-1’ (sweet, red, and soft) and ‘W600’ (bitter, light yellow, and firm), as well as the corresponding F1 population (bitter, light yellow, and firm). Numerous differentially expressed genes (DEGs) were identified in the fruit samples: 3,766 DEGs for ‘14-1’ vs. ‘W600’, 2,767 for ‘14-1’ vs. F1, and 1,178 for F1 vs. ‘W600’ at the transition stage; and 4,221 for ‘14-1’ vs. ‘W600’, 2,447 for ‘14-1’ vs. F1, and 446 for F1 vs. ‘W600’ at the maturity stage. Weighted gene co-expression network analysis (WGCNA) revealed that a gene module including 1,111 DEGs was closely associated with flesh taste and color, and another gene module including 1,575 DEGs contributed significantly to flesh texture. The metabolomic results showed that there were 447 differential metabolites (DMs) for ‘14-1’ vs. ‘W600’ fruits, 394 for ‘14-1’ vs. F1, and 298 for F1 vs. ‘W600’ at the maturity stage. Combining WGNCA and metabolomic results, several DEGs and DMs were further identified as hub players in fruit quality formation: six DEGs with four DMs for flesh sweetness; six DEGs with 13 DMs for bitterness; nine DEGs with 10 DMs for flesh color; and nine DEGs with four DMs for flesh texture. Altogether, these observations not only expand our knowledge of the molecular basis of fruit quality in watermelon, but also provide potential targets for future watermelon improvement.

Variation in the promoter of the sorbitol dehydrogenase gene MdSDH2 affects binding of the transcription factor MdABI3 and alters fructose content in apple fruit

Fructose (Fru) content is a key determinant of fruit sweetness and quality. An F1 hybrid population of the apple cultivars 'Honeycrisp' × 'Qinguan' was used to investigate the quantitative trait locus (QTL) regions and genes controlling Fru content in fruit. A stable QTL on linkage group (LG) 01 in 'Honeycrisp' was detected on the single nucleotide polymorphism (SNP) genetic linkage maps. In this region, a sorbitol dehydrogenase (SDH) gene, MdSDH2, was detected and showed promoter variations and differential expression patterns between 'Honeycrisp' and 'Qinguan' fruits as well as their hybrids. A SNP variant (A/G) in the MdSDH2 promoter region (SDH2p-491) affected the binding ability of the transcription factor MdABI3, which can affect the expression of MdSDH2. Promoter sequences with an A nucleotide at SDH2p-491 had stronger binding affinity for MdABI3 than those with a G. Among 27 domesticated apple cultivars and wild relatives, this SNP (A/G) was associated with Fru content. Our results indicate that MdSDH2 can alter Fru content as the major regulatory gene and that ABA signaling might be involved in Fru content accumulation in apple fruit.

Multi-omics approaches identify a key gene, PpTST1, for organic acid accumulation in peach

Organic acid content in fruit is an important determinant of peach organoleptic quality, which undergoes considerable variations during development and maturation. However, its molecular mechanism remains largely unclear. In this study, an integrative approach of genome-wide association studies and comparative transcriptome analysis were applied to identify candidate genes involved in organic acid accumulation in peach. A key gene PpTST1, encoding tonoplast sugar transporter, was identified and the genotype of PpTST1 with a single-base transversion (G1584T) in the third exon which leads to a single amino acid substitution (Q528H) was associated with low level of organic acid content in peach. Overexpression of PpTST1His resulted in reduced organic acid content along with increased sugar content both in peach and tomato fruits, suggesting its dual function in sugar accumulation and organic acid content reduction. Two V-type proton ATPases interact with PpTST1 in yeast two-hybridization assay. In addition, the G1584T transversion appeared and gradually accumulated during domestication and improvement, which indicated that PpTST1 was under selection. The identification and characterization of PpTST1 would facilitate the improvement of peach fruit quality.

Suppression of the tonoplast sugar transporter StTST3.2 improves quality of potato chips

Which sugar transporter regulates sugar accumulation in tubers is largely unknown. Accumulation of reducing sugar (RS) in potato (Solanum tuberosum L.) tubers negatively affects the quality of tubers undergoing the frying process. However, little is known about the genes involved in regulating RS content in tubers at harvest. Here, we have identified two tonoplast sugar transporter (TST) 3-type isoforms (StTST3.1 and StTST3.2) in potato. Quantitative real-time PCR results indicate that StTST3.1 and StTST3.2 possess distinct expression patterns in various potato tissues. StTST3.2 was found to be the expressed TST3-type isoform in tubers. Further subcellular localization analysis revealed that StTST3.2 was targeted to the tonoplast. Silencing of StTST3.2 in potato by stable transformation resulted in significantly lower RS content in tubers at harvest or after room temperature storage, suggesting StTST3.2 plays an important role in RS accumulation in tubers. Accordingly, compared with the unsilenced control, potato chips processed from StTST3.2-silenced tubers exhibited lighter color and dramatically decreased acrylamide production at harvest or after room temperature storage. In addition, we demonstrated that silencing of StTST3.2 has no significant effect on potato growth and development. Thus, suppression of StTST3.2 could be another effective approach for improving processing quality and decreasing acrylamide content in potato tubers.

Cucurbitaceae genome evolution, gene function and molecular breeding

The Cucurbitaceae is one of the most genetically diverse plant families in the world. Many of them are important vegetables or medicinal plants and are widely distributed worldwide. The rapid development of sequencing technologies and bioinformatic algorithms has enabled the generation of genome sequences of numerous important Cucurbitaceae species. This has greatly facilitated research on gene identification, genome evolution, genetic variation and molecular breeding of cucurbit crops. So far, genome sequences of 18 different cucurbit species belonging to tribes Benincaseae, Cucurbiteae, Sicyoeae, Momordiceae and Siraitieae have been deciphered. This review summarizes the genome sequence information, evolutionary relationship, and functional genes associated with important agronomic traits (e.g., fruit quality). The progress of molecular breeding in cucurbit crops and prospects for future applications of Cucurbitaceae genome information are also discussed.

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

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

The branchless gene Clbl in watermelon encoding a TERMINAL FLOWER 1 protein regulates the number of lateral branches

A SNP mutation in Clbl gene encoding TERMINAL FLOWER 1 protein is responsible for watermelon branchless. Lateral branching is one of the most important traits, which directly determines plant architecture and crop productivity. Commercial watermelon has the characteristics of multiple lateral branches, and it is time-consuming and labor-costing to manually remove the lateral branches in traditional watermelon cultivation. In our present study, a lateral branchless trait was identified in watermelon material WCZ, and genetic analysis revealed that it was controlled by a single recessive gene, which named as Clbl (Citrullus lanatus branchless). A bulked segregant sequencing (BSA-seq) and linkage analysis was conducted to primarily map Clbl on watermelon chromosome 4. Next-generation sequencing-aided marker discovery and a large mapping population consisting of 1406 F₂ plants were used to further map Clbl locus into a 9011-bp candidate region, which harbored only one candidate gene Cla018392 encoding a TERMINAL FLOWER 1 protein. Sequence comparison of Cla018392 between two parental lines revealed that there was a SNP detected from C to A in the coding region in the branchless inbred line WCZ, which resulted in a mutation from alanine (GCA) to glutamate (GAA) at the fourth exon. A dCAPS marker was developed from the SNP locus, which was co-segregated with the branchless phenotype in both BC₁ and F₂ population, and it was further validated in 152 natural watermelon accessions. qRT-PCR and in situ hybridization showed that the expression level of Cla018392 was significantly reduced in the axillary bud and apical bud in branchless line WCZ. Ectopic expression of ClTFL1 in Arabidopsis showed an increased number of lateral branches. The results of this study will be helpful for better understanding the molecular mechanism of lateral branch development in watermelon and for the development of marker-assisted selection (MAS) for new branchless watermelon cultivars.

Extreme sugar accumulation in late fig ripening is accompanied by global changes in sugar metabolism and transporter gene expression

Female fig (Ficus carica L.) fruit are characterized by a major increase in volume and sugar content during the final week of development. A detailed developmental analysis of water and dry matter accumulation during these final days indicated a temporal separation between the increase in volume due to increasing water content and a subsequent sharp increase in sugar content during a few days. The results present fig as an extreme example of sugar import and accumulation, with calculated import rates that are one order of magnitude higher than those of other sugar-accumulating sweet fruit species. To shed light on the metabolic changes occurring during this period, we followed the expression pattern of 80 genes encoding sugar metabolism enzymes and sugar transporter proteins identified in fig fruit. A parallel comparison with male fig fruits, which do not accumulate sugar during ripening, highlighted the genes specifically related to sugar accumulation. Tissue-specific analysis indicated that the expression of genes involved in sugar metabolism and transport undergoes a global transition.

Highly efficient, genotype-independent transformation and gene editing in watermelon (Citrullus lanatus) using a chimeric ClGRF4-GIF1 gene

Efficient genetic transformation has the potential to advance research and breeding in watermelon (Citrullus lanatus), but regeneration from tissue culture remains challenging. Previous work showed that expressing a fusion of two interacting transcription factors, GROWTH-REGULATING FACTOR4 (GRF4) and GRF-INTERACTING FACTOR1 (GIF1), improved regeneration in wheat (Triticum aestivum). By overexpressing a chimeric fusion of ClGRF4 and ClGIF1, we achieved highly efficient transformation in watermelon. Mutating the mi396 microRNA target site in ClGRF further boosted the transformation efficiency up to 67.27% in a genotype-independent manner. ClGRF4-GIF1 can also be combined with clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) genome editing tools to achieve highly efficient gene editing in watermelon, which we used to successfully create diploid seedless watermelon. This research thus puts forward a powerful transformation tool for future watermelon research and breeding.

Characterization of the molecular mechanism underlying the dwarfism of dsh mutant watermelon plants

Developing dwarf watermelon is a major objective among breeders. The dsh dwarf watermelon germplasm developed in our laboratory is genetically stable. We previously produced preliminary evidence that Cla010726, which encodes a gibberellin 20-oxidase-like protein, is the primary gene controlling dwarfism in watermelon. However, the underlying genetic mechanism was unknown. In this study, we characterized the spontaneous recessive mutant dsh, which is a gibberellin (GA)-deficient mutant. Many of the phenotypic traits of dsh plants are similar to those of known GA-deficient mutants. The dsh plants were sensitive to exogenous bioactive GAs, which increased seedling height. Moreover, a quantitative analysis of endogenous GA₃ proved that the bioactive GA₃ content was substantially lower than normal in dsh. Additionally, the T₅ClaGA20ox RNAi plants generally exhibited dwarfism, with short stems and internodes as well as small leaves and fruit. An examination of the transgenic plants carrying the ClaGA20ox1 promoter-GUS and mutant ClaGA20ox2 promoter-GUS constructs confirmed that two promoter sites are involved in the regulation of ClaGA20ox expression. Hence, mutations in the promoter of the GA20ox gene, which encodes a key enzyme involved in gibberellin biosynthesis, lead to the dwarfism of watermelon plants. The dsh mutant is a potentially useful germplasm resource for developing new watermelon varieties exhibiting dwarfism.