What controls fleshy fruit acidity? A review of malate and citrate accumulation in fruit cells

ClaPEPCK4: target gene for breeding innovative watermelon germplasm with low malic acid and high sweetness

Malic acid markedly affects watermelon flavor. Reducing the malic acid content can significantly increase the sweetness of watermelon. An effective solution strategy is to reduce watermelon malic acid content through molecular breeding technology. In this study, we measured the TSS and pH of six watermelon varieties at four growth nodes. The TSS content was very low at 10 DAP and accumulated rapidly at 18, 26, and 34 DAP. Three phosphoenolpyruvate carboxykinase (PEPCK) genes of watermelon were identified and analyzed. The ClaPEPCK4 expression was inversely proportional to malate content variations in fruits. In transgenic watermelon plants, overexpressing the ClaPEPCK4 gene, malic acid content markedly decreased. In the knockout transgenic watermelon plants, two SNP mutations and one base deletion occurred in the ClaPEPCK4 gene, with the malic acid content in the leaves increasing considerably and the PEPCK enzyme activity reduced to half of the wild-type. It is interesting that the ClaPEPCK4 gene triggered the closure of leaf stomata under dark conditions in the knockout transgenic plants, which indicated its involvement in stomatal movement. In conclusion, this study provides a gene target ClaPEPCK4 for creating innovative new high-sweetness watermelon varieties.

Transcriptome profiling of fruits at different development stages in the Field Muskmelon (Cucumis melo L. var. agrestis Naud.) "Mapao 1"

The Field Muskmelons, showing typically feral morphologies, are essential gene pools for the genetic improvement of cultivated melons. However, the potential gene networks of fruit growth and development in the Field Muskmelon are still poorly understood. Hence, a variety of Field Muskmelon "Mapao 1" showing typical features of the Field Muskmelon and having unique sweet and sour tastes was used in this study. The dynamic changes of the fruit weight, size, and shape of "Mapao 1" and the sugar, dry matter, and total acid contents in fruits of "Mapao 1" were checked with fruit samples collected at 7, 14, 21, 28, and 35 DAP. Given the significant differences in fruit size/shape and sucrose and acid contents at 7 and 28 DAP "Mapao1" fruits, transcriptomics was conducted to reveal their transcriptomic reprogramming between the 7 and 28 DAP fruit samples. A total of 3266 DEGs were up-regulated in the 7 DAP fruits, while there were 1329 up-regulated DEGs in the 28 DAP fruits. Furthermore, the DEGs potentially related to the fruit development and the sweetness and sourness were also discussed in the present work. The potential functions of DEG MELO3C005560 (CmINV1) encoding acid invertase, a key enzyme of sugar metabolism, were preliminarily explored from gene expression, phylogenic analysis, and subcellular localization, in which CmINV1 showed the highest expression level in fruits at the 7 DAP and the lowest level at the 14 DAP; CmINVI had the highest identity with that of cucumber (Cucumis sativus XP 004147588.1); the GFP-CmINV1 fusion protein was located on the plasma membrane. Results of this work provided valuable information about the key genes related to not only fruit growth and development, but also the sourness and sweetness formation in the Field Muskmelon.

Transcriptomic Profiling Uncovers Molecular Basis for Sugar and Acid Metabolism in Two Pomegranate (Punica granatum) Varieties

Soluble sugars and organic acids constitute the primary flavor determinants in fruits and elucidating their metabolic mechanisms provides crucial theoretical foundations for fruit breeding practices and food industry development. Through integrated physiological and transcriptomic analysis of pomegranate varieties 'Sharp Velvet' with high acid content and 'Azadi' with low acid content, this study demonstrated that the differences in flavor between the two varieties were mainly caused by differences in citric acid content rather than in soluble sugar content. Transcriptome profiling identified 11 candidate genes involved in sugar and acid metabolism, including three genes associated with soluble sugar metabolism (FBA1, SS, and SWEET16) and eight genes linked to organic acid metabolism (ADH1, GABP1, GABP2, GABP3, GABP4, ICL, ME1, and PDC4). These data indicated that differences in citric acid content between the two varieties mainly stemmed from differences in the regulation of the citric acid degradation pathway, which relies mainly on the γ-aminobutyric acid (GABA) branch rather than the isocitric acid lyase (ICL) pathway. Citric acid accumulation in pomegranate fruit was driven by metabolic fluxes rather than vesicular storage capacity. Weighted gene co-expression network analysis (WGCNA) uncovered a significant citric acid content associated module (r = -0.72) and predicted six core transcriptional regulators (bHLH42, ERF4, ERF062, WRKY6, WRKY23, and WRKY28) within this network. Notably, bHLH42, ERF4, and WRKY28 showed significant positive correlations with citric acid content, whereas ERF062, WRKY6, and WRKY23 demonstrated significant negative correlations. Our findings provide comprehensive insights into the genetic architecture governing soluble sugars and organic acids homeostasis in pomegranate, offering both a novel mechanistic understanding of fruit acidity regulation and valuable molecular targets for precision breeding of fruit quality traits.

Potassium Promotes Citric Acid Accumulation by Regulating Its Synthesis and Vacuole Storage in Newhall Navel Orange (Citrus sinensis)

Potassium (K) is recognized as a crucial element affecting fruit flavor quality and influences the accumulation of citric acid (CA). To gain deeper insights into how K regulates CA accumulation, we investigated CA synthesis in mitochondria, decomposition pathways, and vacuolar storage under both pot and field culture in Newhall navel orange (Citrus sinensis). Our findings demonstrated that an appropriate level of K enhanced CA concentrations primarily by modulating CA synthesis, as evidenced by the increased activities and gene expressions of citrate synthase (CS) and phosphoenolpyruvate carboxylase (PEPC) in the early stages of fruit development. Integration of mitochondrial proteomic and targeted metabolomic revealed that K promoted CA synthesis by accelerating the TCA cycle, which were supported by the upregulation of TCA cycle-associated proteins, including malate dehydrogenase, 2-oxoglutarate dehydrogenase E2 component, pyruvate dehydrogenase E2 component, and dihydrolipoamide dehydrogenase, along with elevated levels of TCA cycle-related metabolites such as CA, isocitrate, 2-oxoglutarate, and succinate. Additionally, K enhanced CA concentration in vacuoles by promoting vacuolar acidification, as evidenced by the upregulation of P-type ATPase (CsPH8) expression. In summary, our findings provide novel insights into K facilitating CA accumulation by modulating its synthesis and vacuole storage in Citrus.

Transcription factor MdNAC18.1 regulates malic acid accumulation in apple fruits

Malic acid, the most important organic acid component in the ripe apple fruit, is of great importance for the development of the fruit flavor and regulation of the metabolism. Previous studies have demonstrated that the P3A-ATPase MdMa11 plays a role in determining fruit acidity, and a total of 85 positive clones were identified using yeast one-hybrid screening based on the fragment in MdMa11 promoter. Among these positive clones, the NAM domain protein was designated as MdNAC18.1. The analysis of transgenic apple calli, fruits and tomatoes indicated that MdNAC18.1 induced the organic acids accumulation to regulate fruit acidity. Luciferase (LUC) and glucuronidase (GUS) activation assays showed that MdNAC18.1 binds to the G-box motif (5'-ACGT-3') located 5227 bp upstream of transcription initiation site of the MdMa11, thereby promoting its expression. Meanwhile, the expression of MdWRKY126, MdMDH5, MdtDT, MdMYB1, and MdVHP1 was found to be significantly increased in transgenic apple calli overexpressing MdNAC18.1 and decreased in MdNAC18.1-silenced transgenic apple calli. The G-box was identified in all these five genes. However, the GUS and LUC activation assays exhibited that MdNAC18.1 activated MdWRKY126, MdMDH5, MdtDT, and MdMYB1 expression. Our findings contribute valuable insights into the complex mechanism regulating the accumulation of malate in apple fruits.

Rapid degradation of ACLA, a subunit of ATP citrate lyase, via autophagy and 26S proteasome pathways to promote pepper growth-to-tolerance transition under heat stress

Citric acid in plant cells is crucial for growth as it serves as a precursor to multiple essential compounds. It also helps plants tolerate high temperatures. However, the mechanisms remain unclear regarding how citric acid balances its role in promoting growth and protecting against stress. We identified an ACLA protein, a subunit of ATP citrate lyase (ACL) in pepper (Capsicum annuum), that converts cytosolic citric acid into acetyl-CoA. Silencing ACLA reduced citric acid metabolites, leading to stunted growth and decreased heat tolerance. Conversely, ACLA-2 overexpression increased acetyl-CoA metabolites but reduced citric acid levels, which also led to reduced heat tolerance. However, applying exogenous citrate significantly improved the heat tolerance of ACLA-overexpressing plants compared with wild-types. This suggests that citric acid plays a dual role in the synthesis of structural components and in enhancing heat stress resistance. When plants are subjected to heat stress, ACL is rapidly degraded within 1 min. Treatments with E64d and MG132 demonstrated that autophagy and the 26S proteasome pathway contribute to this degradation. This dynamic degradation precisely regulates the dual role of ACL in growth and stress responses, indicating a novel mechanism by which plant cells rapidly adapt to environmental changes through the degradation of key enzymes.

Low-concentration NaCl foliar spraying enhances photosynthesis, mineral concentration, and fruit quality of strawberry during greenhouse high-temperature periods

BACKGROUND

Root-applied low-concentration NaCl (e.g., 40 mM) has been shown to maintain the yield and enhance both nutritional and functional quality in salt-sensitive strawberry cultivars. However, the potential benefits of foliar low-concentration NaCl application on strawberry plants have rarely been investigated to avoid secondary soil salinization through long-term root application, especially the effects on fruit quality during greenhouse high-temperature periods.

METHODS

Strawberry (Fragaria × ananassa Duch. cv. Benihoppe) plants were foliar sprayed with 0 (CK), 5, 10, 15, and 20 mM NaCl solutions once a day from fruit setting to ripening under high-temperature period in a solar greenhouse where quality deterioration was observed. The physiological traits of strawberry leaves and the quality of fruits were measured to explore the beneficial effects of low-concentration NaCl solutions.

RESULTS

Compared with the control (CK), foliar spraying with low-concentration NaCl solutions significantly increased the photosynthetic efficiency and mineral element content of strawberry leaves; enhanced the color of strawberry fruits; and increased the weight, size, color, soluble sugar content (e.g., glucose, fructose, and sucrose), and secondary metabolite production (e.g., vitamin C, phenolics, and flavonoids) of strawberry fruits. Additionally, foliar spraying with low-concentration NaCl solutions significantly decreased the organic acid content (e.g., malic and citric acids) in strawberry fruits. According to redundancy analysis, foliar spraying with NaCl induced the accumulation of Na in strawberry leaves and Cl in strawberry fruits, which may have contributed to the increase in physiological activity of leaves and the improvement in fruit quality, respectively.

CONCLUSION

The foliar spraying of 10-15 mM NaCl, an economical and beneficial method, improves photosynthetic efficiency, thereby promoting formation and accumulation of strawberry fruit nutrients under high-temperature period in greenhouses.

Transcriptome Analysis Reveals Distinct Differences in Organic Acid Metabolism Between the Pericarp and the Pulp of Cerasus humilis During Fruit Maturation

Organic acids are key components that determine the taste and flavor of fruits, playing a crucial role in maintaining fruit quality and nutritional value. To investigate the metabolic differences of organic acids between the fruit pericarp and the pulp during the developmental maturation of the Cerasus humilis, this experiment utilized Cerasus humilis cultivated in Inner Mongolia, China, as the experimental material. By measuring the malic acid and citric acid content, as well as the activities of the related metabolic enzymes in the fruit pericarp and the pulp at five developmental stages, this study investigated the characteristics of organic acid accumulation, changes in enzyme activities, and the expression trends of corresponding genes. Transcriptomic data were integrated to support the analysis. This study specifically analyzed the reasons for the differences in acidity between the pericarp and the pulp, and performed a correlation analysis of various indicators. The results indicated that, during development, the organic acid composition in both the pericarp and the pulp was primarily malic acid, with citric acid as a secondary component. The malic acid and citric acid content in the pericarp were significantly higher than in the pulp, resulting in greater overall acidity in the pericarp. The combined action of PEPC, NAD-MDH, and NADP-ME was identified as the primary reason for the differences in malic acid content between the pericarp and the pulp of Cerasus humilis. CS and ACO were identified as the key enzymes responsible for the lower citric acid content in the pulp compared to the pericarp. Furthermore, the expression levels of ChMDH2, ChME, ChCS2, ChCS3, ChACO1, and ChACO2 differed significantly between the fruit pericarp and the pulp, suggesting their regulatory roles in organic acid accumulation.

AP2 transcription factor CsAIL6 negatively regulates citric acid accumulation in citrus fruits by interacting with a WD40 protein CsAN11

Citric acid accumulation is an essential process in citrus fruits that determines fruit flavor and marketability. The MBW complex transcription factor genes, CsAN11, CsAN1, and CsPH4 play key roles in regulating citric acid accumulation. Although how to regulate CsAN1 or CsPH4 was widely investigated, studies on the regulation of CsAN11 are scarce. In this study, we characterized the AP2/ERF (APETALA2/ethylene response factor) transcription factor gene CsAIL6, which is lowly expressed in high-acid citrus varieties and highly expressed in low-acid citrus varieties. Overexpressing CsAIL6 obviously decreased the citric acid content in citrus fruits, calli, or tomatoes, whereas silencing CsAIL6 significantly increased the fruit citric acid content. Additionally, transcript levels of CsAN11, CsAN1, and CsPH4 were significantly increased by silencing CsAIL6; only the CsAN11 transcript level was significantly decreased by overexpressing CsAIL6. Similarly, only the tomato AN11 (SIAN11) transcript level in CsAIL6 stably overexpressing fruits was markedly lower than that in wild-type (WT) fruits. Further experiments revealed that overexpressing CsAN11 significantly increased the organic acid content but had no obvious influence on the CsAIL6 transcript level; in addition, CsAIL6 only interacts with CsAN11, rather than with CsAN1 and CsPH4 of the MBW complex. Taken together, our findings verified that CsAIL6 negatively regulates citric acid accumulation through directly interacting with the WD40 protein CsAN11, which provides a new mechanism for citric acid accumulation in fruits through the regulation of the MBW complex.

Visualizing the spatial distribution of metabolites in tomato fruit at different maturity stages by matrix-assisted laser desorption/ionization mass spectrometry imaging

Tomato is one of the highest-value fruit and vegetable crop worldwide, serving as an important source of micro-nutrients in the human diet. Understanding the spatial distribution changes of critical metabolites during fruit maturation is essential for investigating the physiological roles, nutritional value, and potential functional values of phytochemicals in tomato fruit. However, information on their spatial distribution remains limited. This study aimed to visualize the distribution differences of endogenous metabolites in tomatoes across four maturity stages (from green to red) using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Relative quantification results showed that as the fruit ripened, levels of soluble sugars, amino acids and volatile organic compounds (VOCs) increased significantly at the red ripening stage, while L-hydroxysuccinic acid exhibited an opposite trend, and citric acid initially decreased, then increased. Mass spectrometry imaging revealed that soluble sugars, organic acids, and amino acids were evenly distributed throughout the fruit across all maturity stages. During maturation, nine VOCs transitioned from a widespread distribution in the flesh tissue to concentrating near the peel, suggesting that aromatic compounds predominantly localize in the fruit's outer regions at full maturity. Additionally, a colocalization phylogenetic tree was constructed based on the spatial distribution imaging of each metabolite. These findings provide a deeper understanding of the changes and distribution of phytochemicals during tomato fruit development, offering a scientific basis for breeding, utilization, and production strategies.

Poly(lactic acid)-based materials with enhanced gas permeability for modified atmosphere packaging of Chinese bayberry

Biodegradable plastics are increasingly utilized in packaging, driven by green chemistry and environmental responsibility. Among them, poly(L-lactic acid) (PLLA) stands out due to its biodegradability and biocompatibility. However, its limited gas permeability and selectivity hinder its application in produce preservation. To address this, polyethylene glycol (PEG), which enhances CO2 selectivity, and polydimethylsiloxane (PDMS), known for its large free volume and high gas diffusion coefficients, were incorporated into PLLA to synthesize two triblock copolymers, PL-E-LA and PL-D-LA. Films with varying block ratios were prepared via solution casting. The results showed that elongation at break for PL-E-LA and PL-D-LA increased by 2.5-fold and 8.7-fold, respectively, while their crystallization temperatures (Tcc) decreased to 79.4 °C and 108.3 °C. Scanning electron microscopy (SEM) revealed spherical phase separation in PL-D-LA and finger-like structures in PL-E-LA. By blending these copolymers, gas permeability and the CO2/O2 permeability ratio were optimized. At 5 °C, the CO2/O2 permeability ratio of PL(D25/E75)LA films reached 9.1, meeting the ideal range (8-10:1) for fresh produce packaging. Atomic force microscopy (AFM) confirmed the PL(D25/E75)LA film exhibited the lowest surface height (52 nm) and roughness (Ra = 4.051), with a fine, uniform phase separation that facilitated gas diffusion and optimized permeability. The application of PL(D25/E75)LA to Chinese bayberry preservation effectively reduced weight loss, delayed firmness degradation, and maintained quality attributes such as color, sugar, and acid content, demonstrating its superior preservation performance. This study highlights the potential of PLLA-based packaging materials with optimized phase separation for produce storage and transportation.

Changes in fruit quality parameters and volatile compounds in four wampee varieties at different ripening stages

Wampee is a nutritious fruit with an attractive taste, and its quality and flavor vary significantly at different ripening stages. Here, we comprehensively characterized the flavor quality of four wampee varieties at five ripening stages. The TSS, TSS/TA, soluble sugars, and most amino acids increased in all varieties, while TA and organic acids consistently decreased with fruit ripening. Furthermore, 57 volatiles were identified by HS-SPME-GC-MS, most of which first increased and then decreased, and fifteen aroma-active markers were screened. Overall, CIE a* value was significantly correlated with fruit sweet and sour taste and can serve as an indicator of wampee maturity. Wampee fruits at stage-4 with CIE a* of 4-8 are suitable for fresh consumption for better balance between taste and aroma, and those at stage-3 with CIE a* of 0-4 are suitable for processing. These findings provide important insights into the optimal harvest timing and quality improvement of wampee.

Mutagenesis of AcSQBP9 in kiwifruit results in reduction of malate via alteration of the expression of a plastidial malate dehydrogenase

Organic acids are major contributors to the flavor of fleshy fruits. In kiwifruit, the Al-ACTIVATED MALATE TRANSPORTER gene (AcALMT1) is key to the accumulation of citrate, while factors driving malate metabolism remain largely unknown. During kiwifruit (Actinidia chinensis cv "Hongyang") development, a rapid decline of malate content was observed between 6 and 12 weeks after full bloom (WAFB), which was studied using RNA-seq analysis. Co-expression network analysis indicated that expression of the chloroplast localized AcPNAD-MDH1 (Plastid-Localized NAD-Dependent Malate Dehydrogenase) negatively correlated with malate content. Overexpression of AcPNAD-MDH1 in kiwifruit resulted lower malate and citrate content in leaves. Among 15 transcription factors that are highly correlated with the expression of AcPNAD-MDH1, AcSQBP9 (SQUAMOSA PROMOTER-BINDING PROTEIN) was shown to directly bind the promoter of AcPNAD-MDH1 to repress transcriptional activity. Moreover, targeted CRISPR-Cas9-induced mutagenesis of AcSQBP9 in kiwifruit produced a significant decrease in malate and citrate, accompanied by an increase in AcPNAD-MDH1 expression. Both PNAD-MDH and SQBP have not been widely studied in fruit metabolism, so the present omics-oriented study provides insights for both kiwifruit and general plant organic acid metabolism.

Effects of Irrigation Water Amount and Humic Acid on β-Glucan Synthesis in Post-Anthesis Grains of Naked Oats

Naked oats offer substantial nutritional and health benefits, primarily due to their main dietary fiber component, soluble β-(1,3)(1,4)-D-glucan (β-glucan). In a pool experiment, humic acid (HA) was applied once during both the booting and anthesis stages at varying irrigation amounts (60 mm, 120 mm, and 180 mm) to assess changes in β-glucan content in grains post-anthesis. Results indicated that at 5 days post-anthesis (DPA), the β-glucan content (3.14% W/W) in grains increased by 16%with the application of HA, compared to the control treatment of spraying an equal volume of water (p < 0.01). The β-glucan content (4.13%, 4.51%) at 15 and 25 DPA reflects increases of 9% and 5% compared to the control. Overall, the application of HA enhanced the β-glucan content in grains, with levels gradually increasing at 5, 15, and 25 DPA; however, the amplitude of the increase gradually declined over time. The β-glucan content in grains at 5 and 15 DPA, along with glucose content in panicles at 20 DPA, directly influenced the β-glucan content in grains at 25 DPA. At 10 DPA, the distribution of sucrose in the leaves and panicles influences the soluble sugar content, subsequently regulating the β-glucan content in the grains at 15 DPA. Specifically, the sucrose content in the leaves exerts a positive regulatory effect, whereas in the panicles exerts a negative regulatory effect.

CitGATA7 interact with histone acetyltransferase CitHAG28 to promote citric acid degradation by regulating the glutamine synthetase pathway in citrus

Organic acid is a crucial indicator of fruit quality traits. Citric acid, the predominant organic acid in citrus fruit, directly influences its edible quality and economic value. While the transcriptional regulatory mechanisms of citric acid metabolism have been extensively studied, the understanding about the transcriptional and epigenetic co-regulation mechanisms is limited. This study characterized a transcription factor, CitGATA7, which directly binds to and activates the expression of genes associated with the glutamine synthetase pathway regulating citric acid degradation. These genes include the aconitase encoding gene CitACO3, the isocitrate dehydrogenase encoding gene CitIDH1, and the glutamine synthetase encoding gene CitGS1. Furthermore, CitGATA7 physically interacts with the histone acetyltransferase CitHAG28 to enhance histone 3 acetylation levels near the transcription start site of CitACO3, CitIDH1, and CitGS1, thereby increasing their transcription and promoting citric acid degradation. The findings demonstrate that the CitGATA7-CitHAG28 protein complex transcriptionally regulate the expression of the GS pathway genes, i.e., CitACO3, CitIDH1, and CitGS1, via histone acetylation, thus promoting citric acid catabolism. This study establishes a direct link between transcriptional regulation and histone acetylation regarding citric acid metabolism, providing insights for strategies to manipulate organic acid accumulation in fruit.

Genetic and QTL analyses of sugarand acid content in sweet cherry (Prunus avium L.)

Sweet cherry is very appreciated by consumers because of its attractive appearance and taste, which is determined by the balanced sweet-sour flavor. In this work, the genetics of soluble solid content (SSC), titratable acidity (TA), sugars, and organic acids was investigated in sweet cherry to facilitate breeding improvement for fruit quality. The fruits of five sweet cherry populations (N = 372), three F1 and two F2, were sampled over two years to evaluate SSC, TA, and the content of individual sugars (glucose, fructose, sorbitol, and sucrose) and organic acids (malic, quinic, oxalic, citric, and shikimic) by ultra-performance liquid chromatography. Glucose, followed by fructose, was the most abundant sugar, while malic acid was the predominant acid. Sorbitol and malic acid were the most stable compounds between years, and had the highest heritability, being also the best correlated to SSC and TA, respectively, revealing their relevance for breeding. Significantly positive correlations were observed among sugars and SSC, and acids and TA, but high interannual variability between years was observed for all traits. Quantitative trait loci (QTL) mapping for SSC, sugars, TA, and organic acids was performed using a multi-family approach with FlexQTL™. Twenty QTLs were detected consistently during the two phenotyped years, and several relevant regions with overlapping QTLs for sugars and acids were also identified. The results confirmed major stable SSC and TA QTLs on the linkage groups 4 and 6, respectively. Within the main LG4 SSC QTL region, where maturity and fruit development time QTLs have been previously detected, three stable sugar (glucose, sorbitol, and sucrose) and two acid (quinic, shikimic) QTLs were also identified, suggesting a pleiotropic effect of ripening date on the content of these compounds. The major malic acid QTL overlapped with TA QTL on LG6; thus, TA QTL mapping on LG6 may correspond to malic acid QTLs. Haplotype analyses of major SSC and sugars QTL on LG4, and TA and malic acid on LG6 revealed haplotypes of breeding interest. Several candidate genes previously identified in other Prunus fruit species, like peach, were found to collocate with the QTLs detected herein. This work reports QTLs regions and haplotypes of sugar and acid content in a Prunus nonclimacteric stone fruit for the first time.

Unraveling the genetic architecture of blueberry fruit quality traits: major loci control organic acid content while more complex genetic mechanisms control texture and sugar content

BACKGROUND

Fruit quality traits, including taste, flavor, texture, and shelf-life, have emerged as important breeding priorities in blueberry (Vaccinium corymbosum). Organic acids and sugars play crucial roles in the perception of blueberry taste/flavor, where low and high consumer liking are correlated with high organic acids and high sugars, respectively. Blueberry texture and appearance are also critical for shelf-life quality and consumers' willingness-to-pay. As the genetic mechanisms that determine these fruit quality traits remain largely unknown, in this study, an F1 mapping population was used to perform quantitative trait loci (QTL) mapping for pH, titratable acidity (TA), organic acids, total soluble solids (TSS), sugars, fruit size, and texture at harvest and/or post-storage and weight loss.

RESULTS

Twenty-eight QTLs were detected for acidity-related parameters (pH, TA, and organic acid content). Six QTLs for pH, TA, and citric acid, two for quinic acid, and two for shikimic acid with major effects were consistently detected across two years on the same genomic regions on chromosomes 3, 4, and 5, respectively. Putative candidate genes for these QTLs were also identified using comparative transcriptomic analysis. No QTL was detected for malic acid content, TSS, or individual sugar content. A total of 146 QTLs with minor effects were identified for texture- and size-related parameters. With a few exceptions, these QTLs were generally inconsistent over years and post-storage, indicating a highly quantitative nature.

CONCLUSIONS

Our findings enhance the understanding of the genetic basis underlying fruit quality traits in blueberry and guide future work to exploit DNA-informed selection strategies in blueberry breeding programs. The major-effect QTLs identified for acidity-related fruit characteristics could be potential targets to develop DNA markers for marker-assisted selection (MAS). On the other hand, genomic selection may be a more suitable approach than MAS when targeting fruit texture, sugars, or size.

The MADS-Box Transcription Factor EjAGL18 Negatively Regulates Malic Acid Content in Loquat by Repressing EjtDT1

Malic acid is the major organic acid in loquat fruit, contributing to the sourness of fruit and affecting fruit flavor. However, the transcriptional regulation of malic acid in loquat is not well understood. Here, we discovered a MADS-box transcription factor (TF), EjAGL18, that regulated malic acid accumulation in loquat. EjAGL18 is a nucleus-localized TF without transcriptional activity. The expression of EjAGL18 increased during fruit ripening, opposite to the accumulation pattern of malic acid in loquat. The transient overexpression of EjAGL18 in loquat fruit downregulated malic acid accumulation and the transcriptional level of the tonoplast dicarboxylate transporter EjtDT1. Conversely, silencing EjAGL18 in loquat fruit upregulated the malic acid content and EjtDT1 expression level. Dual-luciferase assays and yeast one-hybrid experiments further confirmed that EjAGL18 could bind to the promoter of EjtDT1 and repress its transcriptions. Furthermore, the transient overexpression of EjtDT1 in loquat fruit increased the malic acid content. These results revealed that EjAGL18 negatively regulates malic acid content by repressing EjtDT1 in loquat. This study broadens the understanding of the MADS-box TF's regulatory mechanisms in malic acid and provides new insights into fruit flavor improvement in loquat.

Graph pangenome reveals the regulation of malate content in blood-fleshed peach by NAC transcription factors

BACKGROUND

Fruit acidity and color are important quality attributes in peaches. Although there are some exceptions, blood-fleshed peaches typically have a sour taste. However, little is known about the genetic variations linking organic acid and color regulation in peaches.

RESULTS

Here, we report a peach graph-based pangenome constructed from sixteen individual genome assemblies, capturing abundant structural variations and 82.3 Mb of sequences absent in the reference genome. Pangenome analysis reveals a long terminal repeat retrotransposon insertion in the promoter of the NAC transcription factor (TF) PpBL in blood-fleshed peaches, which enhances PpBL expression. Genome-wide association study identifies a significant association between PpBL and malate content. Silencing PpBL in peach fruit and ectopic overexpression of PpBL in tomatoes confirm that PpBL is a positive regulator of malate accumulation. Furthermore, we demonstrate that PpBL works synergistically with another NAC TF, PpNAC1, to activate the transcription of the aluminum-activated malate transporter PpALMT4, leading to increased malate content.

CONCLUSIONS

These findings, along with previous research showing that PpBL and PpNAC1 also regulate anthocyanin accumulation, explain the red coloration and sour taste in blood-fleshed peach fruits.

ATP Citrate Lyase ClACLB-1 Facilitates Citrate Cleavage in Lemon

Citric acid is an important organic acid with wide applications and diverse biological functionality. As the predominant organic acid in lemons, citric acid plays a crucial role in determining the flavor of citrus, especially in lemons. ATP citrate lyase (ACL, EC4.1.3.8) is the keg gene in citric acid metabolism. Several research studies on ACL only focused on high-sugar- and low-acid-content citrus varieties; however, the ACL mechanism in lemons with high acid and low sugar levels remains undetermined. In this study, a key candidate gene, ClACLB-1, for citrate cleavage was identified from the genome data of 'Xiangshui' lemon [Citrus limon (L.) Burm f.]. The putative protein coded by the gene ClACLB-1 is localized in the nuclear and cell membranes. The ClACLB-1 gene was expressed in all tissues, with the highest expression in male flowers and the lowest expression in mature fruits; the expression decreased during lemon fruit development. The overexpression of ClACLB-1 in transgenic tomatoes significantly increases the activity of citrate lyase, which subsequently reduces citric acid content. This study clarified the function of the ClACLB-1 gene in cleaving citric acid, provided new insights into the citric acid metabolism of citrus, and offered a theoretical reference for reducing acid and increasing sugar in citrus to improve fruit quality. It also helped to enhance the understanding of the metabolism and role of citrate in plants.

A linker histone acts as a transcription factor to orchestrate malic acid accumulation in apple in response to sorbitol

High carbohydrate availability promotes malic acid accumulation in fleshy fruits, but the underlying mechanism is not known. Here, we show that antisense repression of ALDOSE-6-PHOSPHATE REDUCTASE in apple (Malus domestica) decreases the concentrations of sorbitol and malate and the transcript levels of several genes involved in vacuolar malate transport, including the aluminum-activated malate transporter (ALMT) gene MdALMT9 (Ma1), the P-ATPase gene MdPH5, the MYB transcription factor gene MdMYB73, and the cold-induced basic helix-loop-helix transcription factor gene MdCIbHLH1, in fruit and leaves. We identified a linker histone H1 variant, MdH1.1, which complements the Arabidopsis (Arabidopsis thaliana) H1 deficient mutant and functions as a transcription factor. MdH1.1 activates MdMYB73, MdCIbHLH1, and MdPH5 expression by directly binding to their promoters. MdMYB73, in return, binds to the promoter of MdH1.1 to enhance its transcription. This MdH1.1-MdMYB73 feedback loop responds to sorbitol, regulating Ma1 expression. Antisense suppression of either MdH1.1 or MdMYB73 expression significantly decreases whereas overexpression increases Ma1 expression and malate accumulation. These findings demonstrate that MdH1.1, in addition to being an architectural protein for chromatin structure, operates as a transcription factor orchestrating malic acid accumulation in response to sorbitol, revealing how sugar signaling modulates vacuolar malate transport via a linker histone in plants.

Low-Molecular-Weight Organic Acid as an Alternative to Promote the Rooting of Persimmon Rootstock Shoot Cuttings

Organic acids are naturally present in plants and exert a positive influence on plant development, which justifies surveying their potential effect on adventitious root (AR) formation. In this study, 0.0298 mol/L (4000 mg/L) of malic acid and 0.0267 mol/L (4000 mg/L) of tartaric acid were used to explore the effects of low-molecular-weight organic acid on the rooting of persimmon rootstock Diospyros lotus L. during cutting propagation. After organic acid treatment, the rooting percentage and the survival rate significantly increased, accompanied by a greater development of lateral roots. Anatomical analysis revealed that Diospyros lotus L. exhibits characteristics that induce root primordia, and organic acid treatment can enhance the differentiation of root primordia. Furthermore, treatment with organic acid led to a substantial decrease in soluble sugar and starch contents, along with a slight increase in soluble protein content during early cutting stages. Additionally, the indole-3-acetic acid (IAA) content peaked in the early stages of AR formation and was significantly higher than that of the control, while abscisic acid (ABA) levels exhibited the opposite trend. Comparatively, gibberellic acid (GA3) remained at extremely low levels throughout the rooting process in the organic acid groups compared to the control. In conclusion, the current study uncovers the anatomical structure over time during AR formation, revealing the dynamic changes in the related main nutrients and hormones and providing new ideas and a new practical approach for improving root regeneration in persimmon rootstock cuttings.

Virtual Cold Chain Method with Comprehensive Evaluation to Reveal the Effects of Temperature Abuse on Blueberry Quality

Blueberry is one of the most perishable fruits, and the postharvest supply chain environment temperature has great effects on fruit quality. In order to determine the critical steps referring to non-optimal conditions and the key quality indexes in response to temperature along the cold chain, 15 time-temperature scenarios were conducted simultaneously for the storage of blueberry fruits and 17 quality attributes were determined. The results indicated that different cold chain steps under abusing temperatures significantly affected blueberry qualities. Based on the comprehensive evaluation analysis, storage in farm at 8 °C and a 10 h delay in precooling were found to be the critical steps that had strong impacts on the qualities of blueberries, affecting 33 and 31 total qualities during shelf life, respectively. Furthermore, seven effective indexes, including the sensory scores, weight loss, decay rate, content of titratable acid, malondialdehyde, respiratory rate and antioxidant activities, were confirmed to be the key quality attributes influenced by the cold chain temperature. It is necessary to circulate postharvest blueberry fruits under relatively isothermal and optimum temperatures throughout the cold chain to maintain the preferred quality, especially at the initial stage of the supply chain.

Multi-Omics Analysis Uncovers the Mechanism for Enhanced Organic Acid Accumulation in Peach (Prunus persica L.) Fruit from High-Altitude Areas

The early-ripening peach industry has undergone rapid development in the Panxi region of the Sichuan Basin in recent years. However, after the introduction of some new peach varieties to the high-altitude peach-producing areas in Panxi, the titratable acid content in peach fruit has significantly increased. This study compared the fruit quality indicators of early-ripening peach varieties cultivated in Xide County (a high-altitude peach-producing area) and Longquanyi District (a low-altitude peach-producing area) in Sichuan Province and analyzed the differences in organic acid metabolism by combining primary metabolomic and transcriptomic approaches. The results showed that the 'Zhongtaohongyu' fruit from the high-altitude peach-producing area had a much higher accumulation of malic acid and, accordingly, a significantly higher organic acid content than the other samples. The lower annual average temperature and stronger ultraviolet radiation in high-altitude peach-producing areas may lead to the increased expression of genes (PpNAD-ME1, PpNADP-ME3, and PpPEPC1) in the organic acid synthesis pathway and the decreased expression of genes (PpACO2, PpNAD-MDH2/3/4/5, and PpPEPCK2) in the organic acid degradation pathway in peach fruit, ultimately resulting in the accumulation of more organic acids. Among them, the downregulation of the key genes PpNAD-MDH3/4/5 involved in malic acid metabolism may be the main reason for the higher malic acid accumulation in peach fruit from high-altitude peach-producing areas. Overall, this study elucidates the mechanism by which environmental factors enhance the accumulation of organic acids in peach fruit from high-altitude peach-producing areas from a multi-omics perspective, as well as providing a theoretical basis for screening key genes involved in organic acid metabolism in peach fruit.

Identification of Primary Metabolite Profiles Reveals Quality Characteristics of Citrus maxima 'Shatian Yu' from Different Origins

Citrus maxima 'Shatian Yu' displays varying nutritional profiles influenced by its production area. This study evaluated pomelo fruits from four primary and one developing 'Shatian Yu' production area. Notably, 'Shatian Yu' from the Guilin area exhibited higher sugar and alcohol content, suggesting enhanced taste. Principal component analysis and OPLS-DA revealed significant metabolite differences among production areas. In Guilin, variations were observed in a few substances, including sugars, alcohols, and phenolic acids. When compared with Rong City, Guilin showed a decrease in four phenolic acids and an increase in three organic acids, eighteen amino acids, eighteen lipids, and one vitamin. This comprehensive analysis provides valuable insights for the development of 'Shatian Yu' cultivation, highlighting the impact of production areas on fruit quality.

The Telomere-to-Telomere Genome of Jaboticaba Reveals the Genetic Basis of Fruit Color and Citric Acid Content

Jaboticaba is a typical tropical plant that blossoms and bears fruit on the tree trunks and branches. The fruits resemble grapes in appearance and texture and are also known as "treegrapes". Currently, research on the genomics of jaboticaba is lacking. In this study, we constructed an integrated, telomere-to-telomere (T2T) gap-free reference genome and two nearly complete haploid genomes, thereby providing a high-quality genomic resource. Furthermore, we unveiled the evolutionary history of several species within the Myrtaceae family, highlighting significant expansions in metabolic pathways such as the citric acid cycle, glycolysis/gluconeogenesis, and phenylpropanoid biosynthesis throughout their evolutionary process. Transcriptome analysis of jaboticaba fruits of different colors revealed that the development of fruit skin color in jaboticaba is associated with the phenylpropanoid and flavonoid biosynthesis pathways, with the flavanone 3-hydroxylase (F3H) gene potentially regulating fruit skin color. Additionally, by constructing the regulatory pathway of the citric acid cycle, we found that low citric acid content is correlated with high expression levels of genes such as thiamin diphosphate (ThDP) and low expression of phosphoenolpyruvate carboxykinase (PEPCK), indicating that PEPCK positively regulates citric acid content. These T2T genomic resources will accelerate jaboticaba pepper genetic improvement and help to understand jaboticaba genome evolution.

Distinctive acidity in citrus fruit is linked to loss of proanthocyanidin biosynthesis

The distinctive acidity of citrus fruit is determined by a regulatory complex of MYB and bHLH transcription factors together with a WDR protein (MBW complex) which operates in the unique juice vesicles of the fruit. We describe a mutation affecting the MYB protein, named Nicole, in sweet orange and identify its target genes that determine hyperacidification, specifically. We propose that the acidity, typical of citrus fruits, was the result of a loss of the ability of Nicole to activate the gene encoding anthocyanidin reductase, an enzyme essential for the synthesis of proanthocyanidins, which are absent in citrus fruit.

Analysis of Main Components of Five Mulberry Varieties in Tropics

Mulberries (Morus alba L.) contain rich and beneficial nutrients for human health. However, as a temperate adaptive species, high-temperature and high-humidity climate conditions may alter the main nutritional value of mulberries after their intended arrival in tropical regions, which has not yet been reported on. In this study, we analyzed the differences in morphology, sugars, organic acids, free amino acids, and aroma contents of five mulberry varieties in the tropics between two harvesting periods. The results show that the full-ripe fruits of M. laevigata W (TLM) have the longest fruit length (83.67 mm) and highest brix (25.90); meanwhile, full-ripe fruits of M. atropurpurea R (D10M) have the longest fruit transverse stem (20.00 ± 0.577 mm) and single-fruit weight (9.63 ± 0.033 g). Fructose, glucose, and sucrose were the main sugars, and oxalic acid, quinic acid, malic acid, and citric acid were the main organic acids in all varieties; in addition, the sucrose content in mature fruits of M. laevigata W. (BLM) and M. alba L. BZZ (BZM) was significantly higher than other sugars. Twenty free amino acids were detected in all five varieties and asparagine was the main free amino acid. A total of 100 volatile compounds were identified, including 31 esters, 20 aldehydes, 14 hydrocarbons, 11 alcohols, 10 acids, 6 ketones, and 8 others. Although the main components of five mulberry full-ripe fruits were significantly higher than the green-ripe fruits, gamma-amino butyric acid and a few other components were otherwise. The research results show that the tropical climate conditions could increase the main nutritional components of mulberries, but the specific molecular regulatory mechanisms need to be further analyzed.

The art of tartness: the genetics of organic acid content in fresh fruits

Organic acids are major determinants of fruit flavor and a primary focus of fruit crop breeding. The accumulation of organic acids is determined by their synthesis, degradation, and transport, all of which are manipulated by sophisticated genetic mechanisms. Constant exploration of the genetic basis of organic acid accumulation, especially through linkage analysis, association analysis, and evolutionary analysis, have identified numerous loci in recent decades. In this review, the genetic loci and genes responsible for malate and citrate contents in fruits are discussed from the genetic perspective. Technologies such as gene transformation and genome editing as well as efficient breeding using marker-assisted selection (MAS) and genomic selection (GS) are expected to break the bottleneck of traditional fruit crop breeding and promote fruit quality improvement.

Combining transcriptomics and HPLC to uncover variations in quality formation between 'Benihoppe' and 'Fenyu No.1' strawberries

'Benihoppe' and 'Fenyu No.1' are representative varieties of red and pink strawberries in China, possess distinct hue and flavor profiles. This study analyzed the underlying biochemical and molecular differences of two varieties utilizing transcriptomics and high-performance liquid chromatography (HPLC). Ripening 'Benihoppe' fruits accumulated more sucrose and pelargonin-3-glucoside (P3G) with a little cyanidin and higher firmness. Whereas ripening 'Fenyu No.1' fruits contained more fructose, glucose, malic acid and ascorbic acid (AsA), but less P3G and citric acid. Moreover, genotype significantly influenced phenolic compounds contents in strawberries. Transcriptome analysis revealed that pectin degradation (PL, PG, PE), sucrose synthesis (CWINV, SUS, TPS) and citric acid metabolism (α-OGDH, ICDH, GAD, GS, GDH, PEPCK, AST) were weakened in 'Benihoppe' fruit. In contrast, the synthesis of sucrose (CWINH, SPS), citric acid (CS, PEPC), anthocyanin (F3H, F3'H, F3'5'H, DFR, UFGT and ANS), and citric acid transport (V-ATPase) was enhanced. In 'Fenyu No.1' fruit, the degradation of sucrose, citric acid, and pectin was enhanced, along with the synthesis of malic acid (ME) and ascorbic acid (PMM, MDHAR and GaLUR). However, anthocyanins synthesis, glucose metabolism (HK, G6PI, PFK, G6PDH, PGK, PGM, ENO, PK), fructose metabolism (FK), citric acid synthesis and transport, and AsA degradation (AO, APX) were relatively weak. RT-qPCR results corroborated the transcriptome data. In conclusion, this study revealed the distinctions and characteristics of strawberries with different fruit colors regarding texture, flavor and color formation processes. These findings offer valuable insights for regulating metabolic pathways and identifying key candidate genes to improve strawberry quality.

Dynamic Analysis of the Fruit Sugar-Acid Profile in a Fresh-Sweet Mutant and Wild Type in 'Shatangju' (Citrus reticulata cv.)

Citrate is a major determinant of fruit flavor quality. Currently, citrus species and/or varieties with significant alterations in citrate level have greatly advanced the molecular basis of citrate accumulation in fruit. However, in-depth dissections of the molecular mechanism specific to citrate accumulation are still limited due to the lack of mutants, especially within one single variety. In this study, a fresh-sweet 'Shatangju' mutant (Citrus reticulata cv.) was obtained during a survey of citrus resources in Guangdong, China, and the phenotype, fruit morphology, and primary flavor profiles were comparatively analyzed. Unlike the wild-type 'Shatangju' (WT), the mutant (MT) material exhibited a dwarfed and multi-branched tree shape, delayed flowering and fruit ripening at maturity, a prolonged fruit tree-retention time, and a decreased single fruit weight at maturity. Dynamic measurement of the metabolite levels further suggested that the contents and fluctuation patterns of vitamin C, malate, quinate, and oxalate showed no obvious difference between MT and MT fruits, while the citrate level in MT fruits significantly decreased over various developmental stages, ranging from 0.356 to 1.91 mg g-1 FW. In addition, the accumulation patterns of the major soluble sugars (sucrose, fructose, and glucose), as well as the sugar/acid ratio, were also altered in MT fruits during development. Taken together, this study provides a novel acid-free 'Shatangju' mutant, which can serve as a powerful tool for the research of fruit flavor quality, especially for the comprehensive understanding of the molecular mechanism of citrate accumulation in fruits.

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.

Whole-genome resequencing identifies candidate genes and allelic variation in the MdNADP-ME promoter that regulate fruit malate and fructose contents in apple

Soluble sugar and organic acids are key determinants of fruit organoleptic quality and directly affect the commodity value and economic returns of fruit crops. We performed whole-genome sequencing of the apple varieties Gala and Xiahongrou, along with their F1 hybrids, to construct a high-density bin map. Our quantitative genetic analysis pinpointed 53 quantitative trait loci (QTLs) related to 11 sugar and acid traits. We identified a candidate gene, MdNADP-ME, responsible for malate degradation, in a stable QTL on linkage group 15. Sequence analysis revealed an A/C SNP in the promoter region (MEp-799) that influences binding of the MdMYB2 transcription factor, thereby affecting MdNADP-ME expression. In our study of various apple genotypes, this SNP has been demonstrated to be linked to malate and fructose levels. We also developed a dCAPS marker associated with fruit fructose content. These results substantiate the role of MdNADP-ME in maintaining the equilibrium between sugar and acid contents in apple fruits.

An insertion in the promoter of a malate dehydrogenase gene regulates malic acid content in apple fruit

Malic acid is an important flavor determinant in apple (Malus × domestica Borkh.) fruit. One known variation controlling malic acid is the A/G single nucleotide polymorphism in an aluminum-activated malate transporter gene (MdMa1). Nevertheless, there are still differences in malic acid content in apple varieties with the same Ma1 genotype (Ma1/Ma1 homozygous), such as 'Honeycrisp' (high malic acid content) and 'Qinguan' (low malic acid content), indicating that other loci may influence malic acid and fruit acidity. Here, the F1 (Filial 1) hybrid generation of 'Honeycrisp' × 'Qinguan' was used to analyze quantitative trait loci for malic acid content. A major locus (Ma7) was identified on chromosome 13. Within this locus, a malate dehydrogenase gene, MDH1 (MdMa7), was the best candidate for further study. Subcellular localization suggested that MdMa7 encodes a cytosolic protein. Overexpression and RNA interference of MdMa7 in apple fruit increased and decreased malic acid content, respectively. An insertion/deletion (indel) in the MdMa7 promoter was found to affect MdMa7 expression and malic acid content in both hybrids and other cultivated varieties. The insertion and deletion genotypes were designated as MA7 and ma7, respectively. The transcription factor MdbHLH74 was found to stimulate MdMa7 expression in the MA7 genotype but not in the ma7 genotype. Transient transformation of fruit showed that MdbHLH74 affected MdMa7 expression and malic acid content in 'Gala' (MA7/MA7) but not in 'Fuji' (ma7/ma7). Our results indicated that genetic variation in the MdMa7 (MDH1) promoter alters the binding ability of the transcription factor MdbHLH74, which alters MdMa7 (MDH1) transcription and the malic acid content in apple fruit, especially in Ma1/Ma1 homozygous accessions.

Transcriptional factor MdESE3 controls fruit acidity by activating genes regulating malic acid content in apple

Acidity is a key factor controlling fruit flavor and quality. In a previous study, combined transcriptome and methylation analyses identified a P3A-type ATPase from apple (Malus domestica), MdMa11, which regulates vacuolar pH when expressed in Nicotiana benthamiana leaves. In this study, the role of MdMa11 in controlling fruit acidity was verified in apple calli, fruits, and plantlets. In addition, we isolated an APETALA2 domain-containing transcription factor, designated MdESE3, based on yeast one-hybrid (Y1H) screening using the MdMa11 promoter as bait. A subcellular localization assay indicated that MdESE3 localized to the nucleus. Analyses of transgenic apple calli, fruits, and plantlets, as well as tomatoes, demonstrated that MdESE3 enhances fruit acidity and organic acid accumulation. Meanwhile, chromatin immunoprecipitation quantitative PCR, luciferase (LUC) transactivation assays, and GUS reporter assays indicated that MdESE3 could bind to the ethylene-responsive element (ERE; 5'-TTTAAAAT-3') upstream of the MdMa11 transcription start site, thereby activating its expression. Furthermore, MdtDT, MdDTC2, and MdMDH12 expression increased in apple fruits and plantlets overexpressing MdESE3 and decreased in apple fruits and plantlets where MdESE3 was silenced. The ERE was found in MdtDT and MdMDH12 promoters, but not in the MdDTC2 promoter. The Y1H, LUC transactivation assays, and GUS reporter assays indicated that MdESE3 could bind to the MdtDT and MdMDH12 promoters and activate their expression. Our findings provide valuable functional validation of MdESE3 and its role in the transcriptional regulation of MdMa11, MdtDT, and MdMDH12 and malic acid accumulation in apple.

The Role of Low-Molecular-Weight Organic Acids in Metal Homeostasis in Plants

Low-molecular-weight organic acids (LMWOAs) are essential O-containing metal-binding ligands involved in maintaining metal homeostasis, various metabolic processes, and plant responses to biotic and abiotic stress. Malate, citrate, and oxalate play a crucial role in metal detoxification and transport throughout the plant. This review provides a comparative analysis of the accumulation of LMWOAs in excluders, which store metals mainly in roots, and hyperaccumulators, which accumulate metals mainly in shoots. Modern concepts of the mechanisms of LMWOA secretion by the roots of excluders and hyperaccumulators are summarized, and the formation of various metal complexes with LMWOAs in the vacuole and conducting tissues, playing an important role in the mechanisms of metal detoxification and transport, is discussed. Molecular mechanisms of transport of LMWOAs and their complexes with metals across cell membranes are reviewed. It is discussed whether different endogenous levels of LMWOAs in plants determine their metal tolerance. While playing an important role in maintaining metal homeostasis, LMWOAs apparently make a minor contribution to the mechanisms of metal hyperaccumulation, which is associated mainly with root exudates increasing metal bioavailability and enhanced xylem loading of LMWOAs. The studies of metal-binding compounds may also contribute to the development of approaches used in biofortification, phytoremediation, and phytomining.

Identification of the effects of alkalinity exposure on the gills of oriental river prawns, Macrobrachium nipponense

Macrobrachium nipponense is an important commercial freshwater species in China. However, the ability of alkali tolerance of M. nipponense is insufficient to culture in the major saline-alkali water source in China. Thus, it is urgently needed to perform the genetic improvement of alkali tolerance in this species. In the present study, we aimed to analyse the effects of alkali treatment on gills in this species after 96 h alkalinity exposure under the alkali concentrations of 0 mmol/L, 4 mmol/L, 8 mmol/L, and 12 mmol/L through performing the histological observations, measurement of antioxidant enzymes, metabolic profiling analysis, and transcriptome profiling analysis. The results of the present study revealed that alkali treatment stimulated the contents of malondialdehyde, glutathione, glutathione peroxidase in gills, indicating these antioxidant enzymes plays essential roles in the protection of body from the damage, caused by the alkali treatment. In addition, high concentration of alkali treatment (> 8 mmol/L) resulted in the damage of gill membrane and haemolymph vessel, affecting the normal respiratory function of gill. Metabolic profiling analysis revealed that Metabolic pathways, Biosynthesis of secondary metabolites, Biosynthesis of plant secondary metabolites, Microbial metabolism in diverse environments, Biosynthesis of amino acids were identified as the main enriched metabolic pathways of differentially expressed metabolites, which are consistent with the previous publications, treated by the various environmental factors. Transcriptome profiling analyses revealed that the alkali concentration of 12 mmol/L has more regulatory effects on the changes of gene expression than the other alkali concentrations. KEGG analysis revealed that Phagosome, Lysosome, Glycolysis/Gluconeogenesis, Purine Metabolism, Amino sugar and nucleotide sugar metabolism, and Endocytosis were identified as the main enriched metabolic pathways in the present study, predicting these metabolic pathways may be involved in the adaption of alkali treatment in M. nipponense. Phagosome, Lysosome, Purine Metabolism, and Endocytosis are immune-related metabolic pathways, while Glycolysis/Gluconeogenesis, and Amino sugar and nucleotide sugar metabolism are energy metabolism-related metabolic pathways. Quantitative PCR analyses of differentially expressed genes (DEGs) verified the accuracy of the RNA-Seq. Alkali treatment significantly stimulated the expressions of DEGs from the metabolic pathways of Phagosome and Lysosome, suggesting Phagosome and Lysosome play essential roles in the regulation of alkali tolerance in this species, as well as the genes from these metabolic pathways. The present study identified the effects of alkali treatment on gills, providing valuable evidences for the genetic improvement of alkali tolerance in M. nipponense.

An alginate-based edible coating containing lactic acid bacteria extends the shelf life of fresh strawberry (Fragaria × ananassa Duch.)

Edible coatings, formulated with sodium alginate and various strains of lactic acid bacteria, were evaluated for their effectiveness in extending the shelf life and mitigating microbial risks associated with strawberries. This study specifically employed strains of Lacticaseibacillus paracasei, Lacticaseibacillus rhamnosus, and Lacticaseibacillus plantarum as antimicrobial agents. Through physicochemical property analysis, the alginate-based antimicrobial coating proved most effective in reducing the strawberry weight loss rate, decay index, and ascorbic acid degradation. Over time, all treatments exhibited increased fungal growth. However, strawberries treated with alginate and lactic acid bacteria recorded lower final colony formation counts-6.82 log CFU/g for SA + LPC, 6.04 log CFU/g for SA + LGG, and 6.26 log CFU/g for SA + LP-compared to 8.73 log CFU/g in the control group. In terms of bacterial resistance under gastrointestinal conditions, L. paracasei demonstrated the highest survival rate post-simulated gastric fluid exposure, while L. plantarum showed the greatest resilience post-simulated intestinal fluid exposure. These findings underscore the efficacy of alginate-based antimicrobial coatings in not only enhancing the storage quality of strawberries but also ensuring microbial safety and potential benefits for gut health.

Identification of key sensory and chemical factors determining flavor quality of Xinyu mandarin during ripening and storage

Xinyu mandarin is popular for its good flavor, but its flavor deteriorates during postharvest storage. To better understand the underlying basis of this change, the dynamics of the sensory profiles were investigated throughout fruit ripening and storage. Sweetness and sourness, determined especially by sucrose and citric acid content, were identified as the key sensory factors in flavor establishment during ripening, but not in flavor deterioration during storage. Postharvest flavor deterioration is mainly attributed to the reduction of retronasal aroma and the development of off-flavor. Furthermore, sugars, acids and volatile compounds were analyzed. Among the 101 detected volatile compounds, 10 changed significantly during the ripening process. The concentrations of 15 volatile components decreased during late postharvest storage, among which α-pinene and d-limonene were likely to play key roles in the reduction of aroma. Three volatile compounds were found to increase during storage, associated with off-flavor development.

From acidity to sweetness: a comprehensive review of carbon accumulation in grape berries

Most of the carbon found in fruits at harvest is imported by the phloem. Imported carbon provide the material needed for the accumulation of sugars, organic acids, secondary compounds, in addition to the material needed for the synthesis of cell walls. The accumulation of sugars during fruit development influences not only sweetness but also various parameters controlling fruit composition (fruit "quality"). The accumulation of organic acids and sugar in grape berry flesh cells is a key process for berry development and ripening. The present review presents an update of the research on grape berry development, anatomical structure, sugar and acid metabolism, sugar transporters, and regulatory factors.

Alternative Splicing Underpins the ALMT9 Transporter Function for Vacuolar Malic Acid Accumulation in Apple

Vacuolar malic acid accumulation largely determines fruit acidity, a key trait for the taste and flavor of apple and other fleshy fruits. Aluminum-activated malate transporter 9 (ALMT9/Ma1) underlies a major genetic locus, Ma, for fruit acidity in apple, but how the protein transports malate across the tonoplast is unclear. Here, it is shown that overexpression of the coding sequence of Ma1 (Ma1α) drastically decreases fruit acidity in "Royal Gala" apple, leading to uncovering alternative splicing underpins Ma1's function. Alternative splicing generates two isoforms: Ma1β is 68 amino acids shorter with much lower expression than the full-length protein Ma1α. Ma1β does not transport malate itself but interacts with the functional Ma1α to form heterodimers, creating synergy with Ma1α for malate transport in a threshold manner (When Ma1β/Ma1α ≥ 1/8). Overexpression of Ma1α triggers feedback inhibition on the native Ma1 expression via transcription factor MYB73, decreasing the Ma1β level well below the threshold that leads to significant reductions in Ma1 function and malic acid accumulation in fruit. Overexpression of Ma1α and Ma1β or genomic Ma1 increases both isoforms proportionally and enhances fruit malic acid accumulation. These findings reveal an essential role of alternative splicing in ALMT9-mediated malate transport underlying apple fruit acidity.

Composite Coating of Oleaster Gum Containing Cuminal Keeps Postharvest Quality of Cherry Tomatoes by Reducing Respiration and Potentiating Antioxidant System

Exploring the green and affordable protection of perishable cherry tomato fruits during storage, herein, the protective efficacy, and its underpinning mechanisms, of a coating of oleaster gum, alone or incorporated with cuminal, on cherry tomatoes stored at ambient temperature was investigated. The composite coating of oleaster gum with 0.1% cuminal reduced the decay, respiration rate, weight loss, and softening of the fruits and decelerated the decreases in their total soluble solid, titratable acidity, and soluble protein levels, and therefore maintained their marketability. Furthermore, it reduced the accumulation of O2·- and H2O2 in the fruits and mitigated cell membrane lipid oxidation and permeabilization, thereby retarding their senescence. Instrumentally, it elevated the activities of superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase and the levels of ascorbic acid and glutathione. This potentiation of the fruits' antioxidant system makes this composite coating a promising approach to keeping the postharvest quality of perishable fruits.

Effect of indoor dehydration on the chemical composition of Marselan grapes in Heyang of China

Marselan is a red wine grape variety with great brewing prospects. The aim of this study was to investigate the effect of postharvest indoor dehydration on the quality of Marselan grapes. For two consecutive years, the harvested grapes were dehydrated naturally indoors (24-28°C). Fresh grapes were used as a control, and dehydrated samples were taken every 7 days during the period of dehydration until ending at day 28. Dehydration treatment increased degrees Brix, reducing sugars, glycerol, and malic acid. On day 7, there was an increase in protocatechuic acid, p-coumaric acid, and total tannin of 26.00%-27.73%, 11.43%-52.52%, and 39.74%-70.45%, respectively. With increasing dehydration time, total phenols, total flavonoids and total flavanols in the skins were decreased by 17.05%-38.13%, 24.32%-57.38%, and 17.05%-59.48%, respectively, with an increase in pH, citric acid, and ascorbic acid contents of grape juice by 7.66%-21.43%, 100%-137.50%, and 61.29%-258.82%, respectively. On day 21, the esters were increased by 1.10-1.75 factors. Partial least square-discriminant analysis result of volatile compounds showed that ethyl acetate, 1-propanol, 1-propanol, 2-methyl-, 1-hexanol, and 1-butanol, 3-methyl- were the predominant characteristic flavor compounds during dehydration of Marselan grapes. The effect of indoor dehydration on Marselan grape quality offered application value for China's later dehydration wine production.

Genome-wide variants and optimal allelic combinations for citric acid in tomato

Citric acid (CA) plays a crucial role as a fruit flavor enhancer and serves as a mediator in multiple metabolic pathways in tomato fruit development. Understanding factors influencing CA metabolism is essential for enhancing fruit flavor and CA-mediated biological processes. The accumulation of CA, however, is influenced by a complex interplay of genetic and environmental factors, leading to challenges in accurately predicting and regulating its levels. In this study, we conducted a genome-wide association study (GWAS) on CA, employing six landmark models based on genome-wide variations including structural variants, insertions and deletions, and single nucleotide polymorphisms. The identification of 11 high-confidence candidate genes was further facilitated by leveraging linkage disequilibrium and causal variants associated with CA. The transcriptome data from candidate genes were examined, revealing higher correlations between the expression of certain candidate genes and changes in CA metabolism. Three CA-associated genes exerted a positive regulatory effect on CA accumulation, while the remaining genes exhibited negative impacts based on gene cluster and correlation analyses. The CA content of tomatoes is primarily influenced by improvement sweeps with minimal influence from domestication sweeps in the long-term breeding history, as evidenced by population differentiation and variants distribution. The presence of various causal variants within candidate genes is implicated in the heterogeneity of CA content observed among the tomato accessions. This observation suggests a potential correlation between the number of alternative alleles and CA content. This study offers significant function-based markers that can be utilized in marker-assisted breeding, thereby enhancing their value and applicability.

Tracking the Variations in Trace Elements, Some Nutrients, Phenolics, and Anthocyanins in Grewia asiatica L. (Phalsa) at Different Fruit Development Stages

Grewia asiatica L. (phalsa) is a very prevalent berry in Pakistan and is consumed extensively as raw or in the form of juice. Here, for the first time, we assessed phalsa from Pakistan in terms of variations in macro and micro minerals, nutrients, and bio-active phyto-constituents including total phenolic and anthocyanin contents at different fruit developmental stages. It was found that the sugars in phalsa increased from D1 (small at the initial fruit setting stage) to D6 development stage (fully ripened fruit) where sugars at D5 (near to fully ripe) and D6 stages were many times greater than at D1, D2 (unripe close to full-size completion), D3 (close to semi ripe), and D4 stage (semi ripened and full-size attainment). Total acidity of was declined in all developmental stages, where the D1 stage displayed maximum and D6 with the lowest acidity. Ascorbic acid was decreased from D1 to D2 and then increased gradually from D3 to D5 stages. At the D6 stage, again a steep decline in ascorbic acid was observed. The total phenolics (mg gallic acid equivalents/100g) at stage D6 were higher (136.02 ± 1.17), whereas D1 being the lowermost in total phenolic content (79.89 ± 1.72). For anthocyanins (mg/100g), an increasing pattern of changes was observed in all stages of phalsa fruit where the D1 stage showed lower (13.97 ± 4.84) anthocyanin contents which then increased gradually at stage D2 (67.79 ± 6.73), but increased sharply at D3 (199.66 ± 4.90), D4 (211.02 ± 18.85), D5 (328.41 ±14.96) and D6 (532.30 ± 8.51) stages. A total of four anthocyanins such as cyanidin, delphidine-3-glucoside, pelargonidin, and malvidin in phalsa were identified using HPLC procedures, and a significant > 90 % DPPH inhibition in phalsa was observed at the D5 and D6 development stages. The macro and micro minerals including Ni, Zn, Fe, Ca, Cu, Mg, Na, P, and K contents were decreased from initial (D1) stage to the final (D6) development stage, while only Fe displayed an increasing trend from the initial to final fruit development stages (D1-D6). Conclusively, these findings could be of great interest for patients who are intended to consume phalsa as adjuvant therapy against diabetes and metabolic syndromes and other diseases involving reactive oxygen species with minimum metal toxicity.

Effect of 1% malic acid spray on diabetes mellitus-induced xerostomia: A randomized clinical trial

OBJECTIVE

To evaluate the effectiveness of topical sialogogue spray containing 1% malic acid in the treatment of xerostomia in patients with type 2 diabetes mellitus.

MATERIAL AND METHODS

A randomized double-blind controlled clinical trial was conducted on 52 patients with type 2 diabetes mellitus suffering from xerostomia. Patients were assigned equally to test group who received a topical sialogogue spray containing 1% malic acid and control group who received a placebo spray. Both groups were instructed to use the spray on demand for 2 weeks. The Summated Xerostomia Inventory-Dutch Version questionnaire (SXI-D) and the unstimulated salivary flow rate were evaluated for all patients at baseline, 2 and 4 weeks after malic acid/placebo application.

RESULTS

At 2 week's follow-up, the unstimulated salivary flow rate increased significantly from 0.059 ± 0.024 to 0.191 ± 0.064 and from 0.055 ± 0.026 to 0.078 ± 0.032 for test and control groups, respectively, with a statistically significant difference favoring the test group. SXI-D scores showed a significant decrease from 10.73 ± 2.22 to 8.38 ± 2.28 in the test group (p < 0.05), while in the control group it decreased from 10.62 ± 1.75 to 10.23 ± 1.48 (p > 0.05).

CONCLUSION

A sialogogue spray containing 1% malic acid increased the unstimulated salivary flow rate in patients with type 2 diabetes mellitus suffering from xerostomia.

Evaluation of morphological traits, biochemical parameters and seeding availability pattern among Citrus limon 'Assam lemon' accessions across Assam

The Assam lemon is a highly valued Citrus cultivar known for its unique aroma, flavor, and appearance. This study aimed to investigate the morphological, seeding pattern and biochemical variations within 132 populations of Assam lemon from across 22 districts of Assam along with the control samples, with the objective to offer comprehensive understanding that could facilitate the improvement of breeding programs and further improvement of this important cultivar. Clustering based on UPGMA algorithm for morphological and seeding pattern data were analysed at population level, revealed two major clusters, where all the populations of Upper Assam districts were in the same cluster with the original stock (control population). The populations from Tinsukia and Dhemaji districts displayed more close similarities with the control population in comparison to populations of Upper Assam districts. Another interesting observation was regarding flowering patterns, while populations from Upper Assam districts excluding Golaghat district displayed both bisexual and unisexual flowers with less concentration of unisexual flowers, other remaining districts had bisexual and unisexual flowers of almost equal concentration. Unisexual flowers contained only the male reproductive organs with 40 anthers, while bisexual flowers had 36 anthers. Seeding patterns were examined across the districts, and it was found that populations from Tinsukia, Dhemaji, Lakhimpur, Dibrugarh, Jorhat, and the control population exhibited seedless characteristic while populations from other selected districts displayed a combination of seedless and seeded traits. Interestingly, Golaghat district appears as the linking district and showed availability of both seeded and seedless Assam lemon fruit, connecting the regions of Barak valley, Central, Lower, North and Upper Assam. Biochemical analysis showed significant variations across districts, however, the populations from Dhemaji, Tinsukia, Lakhimpur, Dibrugarh, and Jorhat districts displayed similarity with the control population. The study also investigated variability in soil nutrient content revealing substantial variation among the populations studied. This comprehensive investigation provides valuable insights into the morphological, seeding pattern, and biochemical diversity within the Assam lemon cultivar. These findings can be instrumental in breeding programs to enhance the cultivar, particularly in producing high-quality seedless fruits to meet consumer demands.

Enrichment of grape berries and tomato fruit with health-promoting tartaric acid by expression of the Vitis vinifera transketolase VvTK2 gene

Tartaric acid (TA) is a major non-fermentable plant soluble acid that abundantly occur in grapes and wines, imparting low pH and tart flavour to berries thereby regulating numerous quality attributes of wine, such as flavour, microbial stability, and aging potential. Evaluation of acidity in mature fruits of 21 wine grape (Vitis vinifera) varieties revealed significant variation between 'Beichun' and 'Gewürztraminer', which was correlated with TA content. RNA-seq analysis of fruits from the two cultivars at different developmental stages revealed that a transketolase gene, VvTK2, was significantly dominantly expressed in the high TA phenotype 'Beichun' variety. Subcellular localization assay showed that VvTK2 protein was located in the chloroplast. Virus-induced VvTK2 gene silencing significantly decreased the expression of 2-keto-L-gulonic acid reductase (Vv2-KGR) as well as L-idonate dehydrogenase (VvL-IdnDH3) and inhibited TA accumulation, while its transient over-expression in grape showed the opposite results. Heterologous VvTK2 over-expression in tomato demonstrated its obvious capacity to induce TA synthesis. Overall, these results highlights a novel role of VvTK2 in modulating TA biosynthesis, which could be an excellent strategy for future genetic improvement of grape flavour.

Formal Assessment of Agreement and Similarity between an Open-Source and a Reference Industrial Device with an Application to a Low-Cost pH Logger

Open-source devices are nowadays used in a vast number of research fields like medicine, education, agriculture, and sports, among others. In this work, an open-source, portable, low-cost pH logger, appropriate for in situ measurements, was designed and developed to assist in experiments on agricultural produce manufacturing. Τhe device was calibrated manually using pH buffers for values of 4.01 and 7.01. Then, it was tested by manually measuring the pH from the juice of citrus fruits. A waterproof temperature sensor was added to the device for temperature compensation when measuring the pH. A formal method comparison process between the open-source device and a Hanna HI9024 Waterproof pH Meter was designed to assess their agreement. We derived indices of agreement and graphical assessment tools using mixed-effects models. The advantages and disadvantages of interpreting agreement through the proposed procedure are discussed. In our illustration, the indices reported mediocre agreement and the subsequent similarity analysis revealed a fixed bias of 0.22 pH units. After recalibration, agreement between the devices improved to excellent levels. The process can be followed in general to avoid misleading or over-simplistic results of studies reporting solely correlation coefficients for formal comparison purposes.

Worldwide productivity and research trend on fruit quality: a bibliometric study

Introduction

As one of the important sources of food for human beings, fruits have been extensively studied. To better guide basic and applied research, it is urgent to conduct a systematic analysis of these studies based on extensive literature collection.

Methods

Based on the Web of Science Core Collection database, this study uses R language and CiteSpace to conduct bibliometric analysis and data mining on the literatures related to fruit quality from January 2013 to June 2023.

Results

The results indicated that among various fruits, tomatoes have been most frequently studied with special interests in photosynthesis, fruit development, and molecular breeding. The research direction primarily focused on fruit resistance and storage characteristics. Among the indicators related to fruit quality, antioxidant activity has the highest co-occurrence with other indicators of fruit quality, especially with nutrients such as anthocyanins, phenolic substances, sugars, and fruit firmness.

Discussion

Currently, adaptation to stress and antioxidant activity are recognized as prominent research focal points in this field. Fruit morphology, particularly fruit size, irrigation methods, application of molecular technology, and infection prevention, represent potential areas of interests in future research on fruit quality.