Introduction

The grapevine (Vitis vinifera L.), as a prominent fruit crop, is cultivated extensively around the world, with a cultivation history extending over 11,000 years (Dong et al. 1987; Conde et al. 2007; Fontes et al. 2011; Candar 2023). The vascular system interconnects these parts, ensuring the transport of the compounds that allow fruit development (Zhang et al. 2009). Sucrose, the main photoassimilate synthesized in the leaves, is translocated to the berries, forming the backbone for the synthesis of sugars and acids. The metabolism of sugars and organic acids undergo dramatic shifts at the véraison stage (Brady 1987; Giovannoni 2001, 2004; Maria et al. 2011; Giovannoni et al. 2017; Liu et al. 2023).

Before véraison, the berry engages in cell division and growth, accumulating organic acids, primarily malic acid, while sugar concentration remains at a low level (Conde et al. 2007; Dai et al. 2013; Etienne et al. 2013; Batista-Silva et al. 2018). At this stage, Sucrose is actively unloaded to berries and subsequently hydrolyzed by cell wall invertases (CWINV) into glucose and fructose (Maria et al. 2011; Kuhn et al. 2014,). After uptake by the flesh cells, glucose is further metabolized to phosphoenolpyruvate (PEP) by glycolysis. PEP lies at a critical crossroad leading to two separate pathways towards malate synthesis (Sweetman et al. 2009). PEP carboxylase (PEPC) catalyzes the conversion of PEP to oxaloacetate (OAA), which is then reduced to malate by NAD-dependent malate dehydrogenase (NAD-MDH) in the cytoplasm (Givan 1999). Alternatively, PEP may be converted by pyruvate kinase (PK) to form pyruvate, which can be further reduced to malate by NADP-dependent malic enzyme (NADP-ME) (Farineau and Lavalmartin 1977; Taureilles-Saurel et al. 1995; Sweetman et al. 2009; Martínez-Esteso et al. 2011). Then the malate can be transported into the mitochondrial matrix by malate transporter embedded in the inner mitochondrial membrane. Once inside, a mitochondrial NAD-dependent malate dehydrogenase converts malate to OAA and NADH, or a NAD-dependent malic enzyme converts it to pyruvate, CO2, and NADH (Sweetman et al. 2009). These intermediates feed the tricarboxylic acid (TCA) cycle, with the potential for malate regeneration depending on the metabolic flux within the mitochondria (Beriashvili and Beriashvili 1996; Ollat and Gaudillère 1997; Hanning et al. 1999). Excess malate is ultimately transported into the vacuoles, a process critical for maintaining the cytosolic pH balance and regulating the acid taste of the berry (Martínez-Esteso et al. 2011).

Grape berries exhibit a remarkable ability to synthesize and accumulate malate at pre-véraison stage, not only through the import of photosynthetically fixed carbon from the leaves, but also through the photosynthetic activity of exocarp cells (Sweetman et al. 2009; Garrido et al. 2023). Despite the limited presence of stomata in the berry skin, respiratory CO2 contributes to the synthesis of malate in flesh cells. Respiratory CO2 is converted to bicarbonate ion (HCO3 −) by carbonic anhydrase within the cytoplasm (Blanke and Lenz 1989; Garrido et al. 2023,). Phosphoenolpyruvate carboxylase (PEPC) then catalyzes the formation of oxaloacetate (OAA) from HCO3 − and the formation of phosphoenolpyruvate (PEP) in an irreversible β-carboxylation reaction (Beriashvili and Beriashvili 1996; Sweetman et al. 2009). The OAA is subsequently reduced by NAD-MDH to form malate. The malate is not a metabolic end point; it can be shuttled into chloroplasts where it undergoes decarboxylation by NADP-ME (Maria et al. 2011; Garrido et al. 2023). This reaction releases CO2 which can be re-assimilated by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the Calvin-Benson-Bassham (CBB) cycle (Conde et al. 2007). The pyruvate resulting from this decarboxylation can be converted back to PEP by pyruvate, phosphate dikinase (PPDK), resulting in a regenerative loop within carbon metabolism (Ruffner 1982; Sweetman et al. 2009; Etienne et al. 2013; Garrido et al. 2023). The interconversion of pyruvate and malate provides connectivity to other essential metabolic pathways (Garrido et al. 2021). Both pyruvate and malate can feed the tricarboxylic acid (TCA) cycle, supporting cellular respiration and biosynthetic reactions (Fig. 3) (Etienne et al. 2013). Alternatively, malate can accumulate in the vacuole, contributing to the grape’s acidity, or it can serve as a substrate for gluconeogenesis, influencing sugar concentrations (Dai et al. 2013; Etienne et al. 2013; Reshef et al. 2022). Moreover, potassium influences the pH and acidity of grape must, with higher potassium levels often associated with lower acidity due to the interaction with malate in the berries (Rogiers et al. 2017).

Fig. 2
figure 2

Sugar accumulation and sugar metabolism in the grape cells. PEP, phosphoenolpyruvate; OAA, oxaloacetic acid; CWINV, cell wall invertase; NINV, neutral invertase; VINV, vacuolar invertase; PEPC, phosphoenolpyruvate carboxylase; PK, pyruvate kinase; NAD-MDH, NAD-linked malic enzyme; NADP-ME, NADP-linked malic enzyme; FK, fructokinase; SS, sucrose synthase; SPS, sucrose phosphate synthase; SPP, sucrose phosphate phosphatase; HK, hexokinase; PFK, phosphofructokinase. CBB, Calvin-Benson-Bassham; TCA, tricarboxylic acid cycle

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Post-véraison, there is an onset of hexose (glucose and fructose) accumulation and a concomitant decline in malate content (Davies and Robinson 1996). Sucrose metabolism is a central aspect of the biochemistry governing grape berry hexose accumulation (Ollat et al. 2002; Gambetta et al. 2010; Ruan 2014; Zhu et al. 2022). There is an overview of sugar metabolism in post-véraison berries (Fig. 2). At arrival in the berries, the sucrose transported by the phloem can be either hydrolyzed into glucose and fructose by invertases (INVs) or converted to UDPG and fructose by sucrose synthase (SS) (Li et al. 2012; Verma et al. 2011) (Fig. 2). Three types of invertases differ by their localization, cytosolic for the neutral invertase (NINV), vacuolar for the vacuolar invertase (VINV) and cell wall for the cell wall invertase (CWINV) (Ruan et al. 2010; Wang et al. 2014) (Fig. 2). The three types of invertase collectively ensure that hexose is available. SS provides an alternative route for sucrose degradation, generating fructose and UDP-glucose, which is particularly important for sustaining sucrose levels within cells (Verma et al. 2011). Hexokinase (HK) and fructokinase (FK) phosphorylate glucose and fructose to glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P), respectively (Jang et al. 1997; Granot et al. 2013) (Fig. 2). Phosphofructokinase (PFK) then acts on F6P converting it to fructose-1,6-bisphosphate (F1,6BP), channeling it into glycolysis and subsequently into the TCA cycle, a key energy-producing pathway in respiration (Ronimus and Morgan 2001) (Fig. 2). Sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (SPP) cooperate in the resynthesis of sucrose, reutilizing the products of SS activity to regenerate sucrose from UDP-glucose and F6P (Huber and Huber 1996; Tian et al. 2001; Bai et al. 2016). Sugarcane SUT5 and SUT6 are highly expressed in source leaves, aiding phloem loading (Zhang et al. 2016), SUT1 does not participate in phloem unloading but is involved in recycling sucrose leaked into the apoplast back to the vascular parenchyma cells (Glassop et al. 2017). Maize SUC4 is localized to the tonoplast and can export sucrose from vacuoles (Carpaneto et al. 2010; Schneider et al. 2012). Furthermore, AtSUC5 can also transport biotin (Ludwig et al. 2000), and AtSUC9 is able to transport a wide range of glucosides (Sivitz et al. 2007).

VvSUTs (VvSUC2, VvSUC11, VvSUC12, and VvSUC27) in different Vitis varieties focus on the expression, localization, function and regulation. VvSUC2 exhibits low expression levels or not detected across various tissues and organs. VvSUC27 is ubiquitously expressed in vegetative organs while is weakly expressed in berries (Afoufa-Bastien et al. 2010). The expression of VvSUC11 and VvSUC12 are relatively low in berries but stays stable during the ripening stages (Afoufa-Bastien et al. 2010). VvSUC12 and VvSUC27 were also expressed in seeds but at a lower level (Afoufa-Bastien et al. 2010). VvSUC11 and VvSUC12 with high-affinity/low-capacity to sucrose, control sugar distribution. VvSUC11, VvSUC12, and VvSUC27 can form homodimers and heterooligomers to guide the rapid transport of sucrose in SE (Cai et al. 2021). VvSUC27 is localized on the plasma membrane. Overexpressing VvSUCs (VvSUC11 or VvSUC12 or VvSUC27) in tobacco and Arabidopsis showed that the plants grew faster, had increased yield, and enhanced stress resistance (Cai et al. 2017, 2020). Similarly, SUTs in grape “Zuoshan-1” responded to various stresses, promoting sucrose metabolism and hormone synthesis (Cai et al. 2019). However, the research of VvSUTs function is still predominantly conducted in heterologous systems, such as Arabidopsis, tobacco. In fact, a direct assessment of their roles in sugar accumulation in grape berries is limited or almost non-existent. This gap highlights the need for more research in grape berries to fully understand the contributions of VvSUTs in sugar accumulation and ripening.

SWEETs are a novel transporter family in plants involved in cellular sugar efflux (Chen et al. 2010), primarily transporting substrates such as glucose, fructose, and sucrose (Chardon et al. 2013; Klemens et al. 2013; Eom et al. 2015). In angiosperms, there are an average of 20 SWEET family members, which are differentially expressed across diverse tissues and organs. In Arabidopsis, SWEET members are phylogenetically divided into four clades, with Clade I (SWEET1-2), Clade II (SWEET3-8), and Clade IV (SWEET16-17) mainly transporting monosaccharides, whereas Clade III (SWEET9-15) mainly transports sucrose (Chen et al. 2010, 2015). SWEET transporters can be localized in various subcellular compartments: SWEET1, 8, 9, 11, 12, and 15 are primarily localized to the plasma membrane (Seo et al. 2011; Kryvoruchko et al. 2016), SWEET2, 16, and 17 to the tonoplast (Chardon et al. 2013; Klemens et al. 2013; Guo et al. 2014; Chen et al. 2015), and SWEET9 to the Golgi membrane (Lin et al. 2014; Chen et al. 2015). SWEET proteins are involved in various functions including plant carbon partitioning, pollen nutrition supply, seed development, organ senescence, hormone transport and interactions between plants and pathogens (Chen et al. 2015; Hutin et al. 2015; Ho et al. 2019; Ni et al. 2020; Braun 2022; Xue et al. 2022; Radchuk et al. 2023). As research continues to deepen, the regulatory networks of SWEET proteins and their potential in improving crop yield and stress resistance are expected to be more comprehensively assessed and utilized.

In grapevine, there are 17 SWEET homologues, among which among which VvSWEETs (VvSWEET1, VvSWEET2a, VvSWEET2b, VvSWEET4, VvSWEET7, VvSWEET10, VvSWEET15 and VvSWEET17a) have been identified as being expressed during grape berries development. Among them, VvSWEET1, VvSWEET2a, VvSWEET2b, VvSWEET10, VvSWEET15, and VvSWEET17a displayed higher expression in Chardonnay berries than those in other organs (Zhang et al. 2019). VvSWEET10 is highly expressed in véraison (Zhang et al. 2019). Specifically, VvSWEET15 is strongly expressed in both véraison and post-véraison in Chardonnay berries and the expression level is much higher than that of VvSWEETs (VvSWEET1, VvSWEET2a, VvSWEET2b, VvSWEET10, VvSWEET15 and VvSWEET17a) (Zhang et al. 2019). VvSWEET10, a plasma membrane transporter, was found to significantly increase glucose, fructose, and total sugar content when overexpressed in grape callus and tomato (Zhang et al. 2019). VvSWEET15 was highly expressed in the three grape varieties and was positively correlated with the hexose content during ripening (Ren et al. 2001; Bai et al. 2016). The OsDOF11 transcription factor binds to the promoter regions of OsSUT1, OsSWEET11, and OsSWEET14 enhancing the expression of these genes, thereby affecting sugar transport in rice. The mutant Osdof11 exhibits dwarfed stature, reduced tillering, insensitivity to sucrose-mediated root growth inhibition, reduced sugar accumulation in leaves, and diminished phloem sucrose flow. The ABA-responsive transcription factor OsbZIP72 can bind to the promoter regions of OsSWEET13 and OsSWEET15, activating their expression in response to drought stress (Mathan et al. 2021). In cotton, the transcription factor GhMYB212 binds to the GhSWEET12 promoter, promoting its expression to regulate the carbon supply required for cotton fiber elongation (Sun et al. 2019). Within pear fruit, PuWRKY31 directly binds to the PuSWEET15 promoter, upregulating its expression and enhancing high sucrose accumulation in the fruit of high-sugar bud sports (Li et al. 2020). The lily transcription factor LoABF2 (an AREB/ABF binding factor) can bind to the LoSWEET14 promoter, inducing LoSWEET14 expression and participating in the ABA signaling pathway to promote soluble sugar accumulation in response to various abiotic stresses (Zeng et al. 2022). The VvMYB15 transcription factor is implicated activating the expression of VvSWET15 (Li et al. 2022). In apple (Malus × domestica) variety “Gala”, MdWRKY9 which bound to the MdSWEET9b promoter interacted with MdbZIP23 (basic leucine zipper) and MdbZIP46, and upregulated MdSWEET9b expression, thereby influenced apple fruit sugar accumulation (Zhang et al. 2023).

Post-translational research on sugar transporters primarily focuses on control by kinases and phosphatases. For example, the expression of monosaccharide transporters (VvHT3, VvHT4, VvHT5, and VvHT6) in grapevine is regulated by protein kinases (VvSK1), modulating sugar intake and accumulation (Lecourieux et al. 2010). Glucose can inhibit the transcription of VvHT1 via a process dependent on hexokinase (HXK) and can reduce the abundance of VvHT1 protein in the plasma membrane through HXK-mediated post-translational modifications (Conde et al. 2006). In Arabidopsis, the wall-associated kinase AtWAKL8 acts as a positive regulator of AtSUC2, capable of phosphorylating AtSUC2 thereby enhancing its sucrose-binding capacity (Xu et al. 2020). The ethylene-responsive transcription factor MaRAP2-4 activates the expression of the Arabidopsis SWEET10, modulating sugar accumulation to increase waterlogging tolerance and enhance the drought and salt tolerance of the Lamiaceae species (Mentha arvensis) (Phukan et al. 2018). Additionally, the transport activity of sugar transporters can be regulated through interaction with binding proteins. In potato, the interaction between StSP6A and StSWEET11 prevents the leakage of sucrose into the apoplastic space during tuber development and leads to reduced transport activity of StSWEET11 when bound to StSP6A in protoplasts and yeast (Abelenda et al. 2019). Rice copper transporters (OsCOPT1 and OsCOPT5) interact with OsSWEET11 to modulate copper distribution during infection with Xoo, although it is not yet clear if this interaction affects the sugar transport of OsSWEET11 (Yuan et al. 2010).

The transcriptional and post-translational regulation of sugar transporters uncover a complex network dictating the functional state of these proteins. Transcription factors orchestrate the transcriptional response to developmental cues and environmental stimuli, while kinases and phosphatases finely tune transporter activity to adapt to cellular needs. As research progresses, elucidating the precise dynamic regulatory mechanisms will be crucial for a more comprehensive understanding of sugar transport in plants, especially in grapevine, with implications for agricultural productivity and stress resilience.

Environmental factors influencing sugar accumulation

Temperature poses significant threats to viticulture in current and future global climate change scenarios (Venios et al. 2020). Temperature significantly influences grapevine metabolism and consequently sugar accumulation in grapes. Warmer temperatures accelerate the rate of sugar accumulation (measured in Brix) by enhancing photosynthetic activity in leaves, which leads to increased sugar production and transport to the berries (Stanfield et al. 2024). However, the highest quality wine is produced when the berries simultaneously achieve optimal sugar-to-acid ratios and maximum levels of pigments, aromas, and flavors (Gladstones 2011). High temperatures accelerate sugar accumulation in grape berries, leading growers to harvest early to avoid producing overly sweet, flat-tasting wines with high alcohol content, although the berries have not yet reached optimal flavor development (Delrot et al. 2020). This creates a challenge for winemakers because the sugars and flavors contents develop at different rates. To address this issue, growers select grape cultivars from hotter wine regions that possess traits enhancing hydraulic resistance. This adaptation helps improve wine quality by slowing the rate of sugar accumulation (Stanfield et al. 2024).

Sunlight exposure plays a pivotal role in sha** the quality of grape bunches and berries, significantly affecting the physiological and metabolic pathways of grapevines and ultimately influencing sugar accumulation in grapes (Friedel et al. 2015). Increased sunlight exposure boosts photosynthesis rates, potentially enhancing sugar availability for berry development. Berries that are fully exposed to sunlight tend to have smaller diameters and higher total soluble solids (up to 22.4 Brix) with lower acidity and juice pH compared to those in partial or complete shade (Somkuwar et al. 2023). This exposure also increases levels of hydroxybenzoic acid, gallic acid, ellagic acid, and anthocyanins, while decreasing flavan-3-ols and amino acids compared to shaded berries (Downey et al. 2004; Somkuwar et al. 2023). In contrast, shaded bunches show higher proline concentrations, underlining the profound impact of sunlight on the biochemical composition and quality of grape berries (Moukarzel et al. 2023). Additionally, the temperature of berry skins, elevated by direct sunlight, affects enzymatic activities crucial for sugar metabolism. Sunlight also influences the expression of genes involved in sugar transport and metabolism, further impacting sugar accumulation (Moukarzel et al. 2023). However, excessive sunlight or heat can cause detrimental effects like berry sunburn and reduced photosynthetic efficiency, potentially diminishing sugar content of berries (Gambetta et al. 2021). Therefore, achieving optimal sunlight exposure through proper vineyard management practices such as leaf removal, shoot positioning, and vine spacing is essential for maximizing sugar content and enhancing grape quality, which are vital for the final quality of wine (Smart 1985; Palliotti et al. 2011; Reynolds 2022).

Genetic diversity of sugar accumulation in grape berries

Within the Vitis genus, there is considerable genetic variability in both sugar composition and concentration. the total sugar concentration, commonly quantified as total soluble solids (TSS), ranges from 18.7 to 27 Brix at maturity across 78 cultivars of Vitis vinifera, which includes both table grape and wine grape varieties (Kliewer 1967a). Kliewer found a broader variation among 26 Vitis species from North America and the Middle East, with TSS at maturity spanning from 13.7 Brix in V. champinii to 31.5 Brix in V. riparia from Wyoming (Kliewer 1967b). Furthermore, among 18 Eurasian grape species in ** has enabled more effective Whole Genome Amplification (WGA) studies, especially in species with sparse genetic data (Lijavetzky et al. 2007; Pindo et al. 2008; **a et al. 2009; Lam et al. 2010; Dong et al. 2023). This has led to a significant increase in the identification of genetic variations like SNPs and InDels, which are crucial for understanding genetic diversity and relationships across different accessions (Myles et al. 2011). A recent study analyzed the genetic diversity of grapevine by resequencing genomic DNA from 27 V. vinifera and wild Vitis species, producing 46.9 Gb of DNA sequences (**n et al. 2013). Despite a low alignment rate with the reference genome, possibly due to its incompleteness or the substantial genetic variation between the samples and the reference, the researchers identified thousands of SNPs and InDels that suggest significant genetic diversity and divergence due to domestication (**n et al. 2013). They discovered genes involved in sugar metabolism that exhibited considerable differences in SNPs/InDels between wild and cultivated grapes, underscoring the role of these genes in grape berry development and sugar accumulation (**n et al. 2013). This genetic exploration not only enhances our understanding of influence of artificial selection on grapevine genetics biological mechanisms underlying sugar accumulation but also provided insights into the evolutionary dynamics that continue to shape this species.

Conclusion

The journey from flowering to the harvest of sweet ripe grape berries results depends on the supply of sugars, on a complex interplay between acid and sugar metabolism, the efficiency of sugar transport systems, and regulatory factors orchestrating these processes (Lucas et al. 2013; Griesser et al. 2024). While considerable progress has unraveled various sugar metabolism pathways and function of the enzymes, the roles and regulation of sugar transport proteins (SUC, HT, TMT, SWEET) in diverse fruit crops, their cellular localization, and the exact operational dynamics of these proteins within fruit tissues largely remain elusive (Lecourieux et al. 2014; Li et al. 2021; Ren et al. 2023). Enhanced knowledge on these fronts bears the promise of paving the way for advancing grapevine cultivation, enology, and viticultural practices.