Sugar composition and transcriptome analysis in developing 'Fengtang' plum (Prunus salicina Lindl.) reveal candidate genes regulating sugar accumulation

A comprehensive study on the fruit quality of a late-ripening mutant variety of plum

The fruit ripening period is an important factor affecting fruit quality and commercial value.To investigate why WSQCL ripens about 15 days later than QCL and why WSQCL has a more astringent flavor. This study analyzed the dynamics of fruit development and quality of three plum varieties, including "Late-maturing Qiangcuili" (WSQCL), 'Qiangcuili' (QCL), and 'Cuihongli' (CHL), to explore the differences in plums quality formation and the ripening period between the late-maturing variant WSQCL and its parental cultivars. The results indicated that the order of the fruit growth rate was QCL > WSQCL > CHL. During fruit development of the three plum varieties, the changes in soluble sugar (SS), titratable acid (TA), starch, chlorophyll, carotenoids, and phenolic contents were consistent, while the accumulation of total phenols, SS, and TA differed significantly between WSQCL and QCL. WSQCL had higher expression of genes related to phenolic compound synthesis than QCL, and phenolic compound synthesis was closely associated with the expression of PAL3, 4CL, HCT1, and CHS. Principal component analysis revealed differences between WSQCL and the other two varieties during the middle and late stages of fruit development. This study provides a reference for quality formation and development of the potential value of WSQCL.

Identification of the SWEET gene family and functional characterization of PsSWEET1a and PsSWEET17b in the regulation of sugar accumulation in 'Fengtang' plum (Prunus salicina Lindl.)

BACKGROUND

'Fengtang' plum is a cultivar known for its significant sugar accumulation. Although various studies have been conducted on sugar metabolism, the specific molecular mechanisms underlying the high sugar accumulation in 'Fengtang' plum remain largely unexplored. Here, we present the role of the Sugars Will Eventually be Exported Transporters (SWEETs) family in regulating sugar accumulation in 'Fengtang' plum fruits.

RESULTS

In this study, 18 PsSWEET genes were identified based on homology with Arabidopsis genes and the Pfam database (ID: PF03083). Alignment of multiple sequences revealed that the seven alpha-helical transmembrane regions (7-TMs) are largely conserved in the PsSWEET family. Phylogenetic analysis demonstrated that the 18 SWEET family members could be categorized into four clades. Nine predicted motifs were identified within the PsSWEET genes of plum. The PsSWEET genes were unevenly distributed across five chromosomes, and synteny analysis revealed three pairs of fragmented duplication events. PsSWEET1a and PsSWEET17b are pivotal in 'Fengtang' plum fruit development. Subcellular localization analyses indicated that PsSWEET1a is localized to the nucleus and cytoplasm, while PsSWEET17b is associated with the vacuolar membrane. Gene function was further validated through transient silencing and overexpression of the PsSWEET1a and PsSWEET17b genes in plum fruits, which significantly impacted their soluble sugar content. Heterologous expression of PsSWEET1a and PsSWEET17b in tomato resulted in an increase in soluble sugar content due to the modulation of sugar accumulation-related genes and enzyme activities.

CONCLUSION

The genes PsSWEET1a and PsSWEET17b, which regulate the content of soluble sugar in plum fruit, were successfully identified. This study provides a comprehensive insight into the SWEET gene family of plum, offering novel perspectives on the regulation of sugar accumulation in fruit and laying a critical foundation for further enhancement of plum fruit quality.

Sucrose synthase 3 improves fruit quality in grape

Sucrose synthase (SS) is one of the key functional enzymes involved in sugar metabolism in plants, and its activity can directly affect sugar accumulation, thereby influencing fruit quality. Our previous research found an interaction between VvSS3 and VvSnRK1β. In this study, we discovered that SS enzyme activity was mainly oriented towards decomposition, with the highest activity occurring before the veraison stage, and the overall trend of SS enzyme activity changes was positively correlated with exogenous ABA concentration. Site-directed mutagenesis revealed that VvSS3S176 and VvSS3S381 were crucial sites for the interaction between VvSS3 and VvSnRK1β. Subcellular localization results showed that VvSS3S176 and VvSS3S381, as well as VvSS3, were located on the cell membrane. VvSS3 was sensitive to ABA, promoting the accumulation of soluble sugars and anthocyanins in transgenic callus, increasing endogenous ABA content, and reducing organic acid components. VvSS3S176 and VvSS3S381 only altered the activity of SS in the synthesis direction, while the transcription level of VvSnRK1β in transgenic callus significantly decreased. After exogenous ABA treatment, Ser176 and Ser381 reduced the inhibition of VvSS3 on VvSnRK1β expression. Mutation of the binding sites of VvSS3 prevented the formation of complexes VvSnRK1β-VvSS3, thus interfering with downstream metabolism. This suggests that VvSnRK1β may form a protein complex by interacting with VvSS3, participating in the accumulation of soluble sugars in grape fruits mediated by ABA response.

PsbHLH58 positively regulates sucrose accumulation by modulating Sucrose synthase 4 in 'Fengtang' plum (Prunus salicina Lindl.)

Sugar content is an important factor that largely determines fruit quality. 'Fengtang' plum (Prunus salicina Lindl.) is recognized for its high soluble sugar content, and the Sucrose synthase 4 (PsSUS4) functions as the controlling step in sucrose accumulation. Nevertheless, the transcriptional mechanism of PsSUS4 underlying sucrose production in this high-sugar plum remains unclear. In this work, a bHLH transcription factor PsbHLH58 was identified to be positively correlated with PsSUS4 expression and fruit sucrose content based on the transcriptome data and qRT-PCR validation. Further yeast one-hybrid and dual-luciferase assays confirmed that PsbHLH58 acts as a transcriptional activator of PsSUS4 via specifically binding to its promoter. Manipulation of the PsbHLH58 expression in fruits through transient overexpression and gene silencing resulted in significant corresponding changes in PsSUS4 expression and sucrose accumulation. In addition, it was also found that the expression levels of PsbHLH58 and PsSUS4 are positively regulated by ethylene, accompanied by an increased sugar accumulation in the fruits. Consequently, these results demonstrated a novel mechanism involving the PsbHLH58-PsSUS4 module that mediates the sugar accumulation in 'Fengtang' plum and provided a new insight into the future fruit quality improvement.

Soluble sugar, organic acid and phenolic composition and flavor evaluation of plum fruits

Plums (Prunus salicina and Prunus domestica) are prevalent in southwestern China, and have attracted interest owing to their delectable taste and exceptional nutritional properties. Therefore, this study aimed to investigate the nutritional and flavor properties of plum to improve its nutritional utilization. Specifically, we determined the soluble sugars, organic acids, and phenolic components in 86 accessions using high-performance liquid chromatography. Notably, glucose, fructose, malic, and quinic acids were the predominant sweetness and acidity in plums, with sucrose contributing more to the sweetness of the flesh than the peel. Moreover, The peel contains 5.5 fold more phenolics than flesh, epicatechin, gallic acid, and proanthocyanidins C1 and B2 were the primary sources of astringency. Correlation and principal component analyses showed eight core factors for plum flavor rating, and a specific rating criterion was established. Conclusively, these findings provide information on the integrated flavor evaluation criteria and for enhancing optimal breeding of plums.

Lysine acetylproteome analysis reveals the lysine acetylation in developing fruit and a key acetylated protein involved in sucrose accumulation in Rosa roxburghii Tratt

Lysine acetylation is a common post-translational modification of proteins in plants. Rosa roxburghii Tratt. is an economically important fruit tree known for its high nutritional value. However, the characteristics of acetylome-related proteins during fruit development in this crop remain unknown. This study aimed to explore the global acetylproteome of R. roxburghii fruit to identify key lysine-acetylated proteins associated with its quality traits. A total of 4280 acetylated proteins were identified, among them, 981 proteins exhibited differential acetylation (DA) while 19 proteins showed increased acetylation level consistently on individual sites. Functional classification revealed that these DA proteins were primarily associated with central metabolic pathways, carbohydrate metabolism, terpenoids and polyketides metabolism, lipid metabolism, and amino acid metabolism, highlighting the importance of lysine acetylation in fruit quality formation. Notably, the most significant up-regulated acetylation occurred in sucrose synthase (SuS1), a key enzyme in sucrose biosynthesis. Enzyme assays, RNA-seq and proteome analysis indicated that SuS activity, which was independent of its transcriptome and proteome level, may be enhanced by up-acetylation, ultimately increasing sucrose accumulation. Thus, these findings offer a better understanding of the global acetylproteome of R. roxburghii fruit, while also uncover a novel mechanism of acetylated SuS-mediated in sucrose metabolism in plant. SIGNIFICANCE: Rosa roxburghii Tratt. is an important horticultural crop whose commercial value is closely linked to its fruit quality. Acetylation modification is a post-translational mechanism observed in plants, which regulates the physiological functions and metabolic fluxes involved in various biological processes. The regulatory mechanism of lysine acetylation in the fruit quality formation in perennial woody plants has not been fully elucidated, while most of the research has primarily focused on annual crops. Therefore, this study, for the first time, uses Rosaceae fruits as the research material to elucidate the regulatory role of lysine-acetylated proteins in fruit development, identify key metabolic processes influencing fruit quality formation, and provide valuable insights for cultivation strategies.

Sorbitol metabolism plays a key role in the differential accumulation of sugar in two plum cultivars

Sugar is vital for plant growth and determines fruit quality via its content and composition. This study explores the differential sugar accumulation in two plum varieties, 'Fengtangli (FTL)' and 'Siyueli (SYL)'. The result showed that 'FTL' fruit displayed higher soluble solids and sugar content at various development stages. Metabolomic analysis indicated increased sorbitol in 'FTL', linked to elevated sorbitol-6-phosphate-dehydrogenase (S6PDH) activity. Transcriptome analysis identified a key gene for sorbitol synthesis, PsS6PDH4, which was significantly higher expressed in 'FTL' than in 'SYL'. The function of the PsS6PDH4 gene was verified in strawberry, apple, and plum fruits using transient overexpression and virus-induced gene silencing techniques. The results showed that overexpression of the PsS6PDH4 gene in strawberry, apple, and plum fruits promoted the accumulation of soluble solids content and sorbitol, while inhibition of the gene reduced soluble solids content and sorbitol content. Meanwhile, analysis of the relationship between PsS6PDH4 gene expression, sorbitol, and soluble solids content in four different plum varieties revealed a significant correlation between PsS6PDH4 gene expression and soluble solids content as well as sorbitol content. This research discovered PsS6PDH4 as a crucial regulator of sugar metabolism in plum, with potential applications in improving fruit sweetness and nutritional value in various fruit species. Understanding these molecular pathways can lead to innovative approaches for enhancing fruit quality, benefiting sustainable agriculture and consumer preferences in the global fruit industry.

Transcriptome and Metabolome Analyses Reveal Sugar and Acid Accumulation during Apricot Fruit Development

The apricot (Prunus armeniaca L.) is a fruit that belongs to the Rosaceae family; it has a unique flavor and is of important economic and nutritional value. The composition and content of soluble sugars and organic acids in fruit are key factors in determining the flavor quality. However, the molecular mechanism of sugar and acid accumulation in apricots remains unclear. We measured sucrose, fructose, glucose, sorbitol, starch, malate, citric acid, titratable acid, and pH, and investigated the transcriptome profiles of three apricots (the high-sugar cultivar 'Shushanggan', common-sugar cultivar 'Sungold', and low-sugar cultivar 'F43') at three distinct developmental phases. The findings indicated that 'Shushanggan' accumulates a greater amount of sucrose, glucose, fructose, and sorbitol, and less citric acid and titratable acid, resulting in a better flavor; 'Sungold' mainly accumulates more sucrose and less citric acid and starch for the second flavor; and 'F43' mainly accumulates more titratable acid, citric acid, and starch for a lesser degree of sweetness. We investigated the DEGs associated with the starch and sucrose metabolism pathways, citrate cycle pathway, glycolysis pathway, and a handful of sugar transporter proteins, which were considered to be important regulators of sugar and acid accumulation. Additionally, an analysis of the co-expression network of weighted genes unveiled a robust correlation between the brown module and sucrose, glucose, and fructose, with VIP being identified as a hub gene that interacted with four sugar transporter proteins (SLC35B3, SLC32A, SLC2A8, and SLC2A13), as well as three structural genes for sugar and acid metabolism (MUR3, E3.2.1.67, and CSLD). Furthermore, we found some lncRNAs and miRNAs that regulate these genes. Our findings provide clues to the functional genes related to sugar metabolism, and lay the foundation for the selection and cultivation of high-sugar apricots in the future.

Transcriptome Insights into Candidate Genes of the SWEET Family and Carotenoid Biosynthesis during Fruit Growth and Development in Prunus salicina 'Huangguan'

The Chinese plum (Prunus salicina L.) is a fruit tree belonging to the Rosaceae family, native to south-eastern China and widely cultivated throughout the world. Fruit sugar metabolism and color change is an important physiological behavior that directly determines flavor and aroma. Our study analyzed six stages of fruit growth and development using RNA-seq, yielding a total of 14,973 DEGs, and further evaluation of key DEGs revealed a focus on sugar metabolism, flavonoid biosynthesis, carotenoid biosynthesis, and photosynthesis. Using GO and KEGG to enrich differential genes in the pathway, we selected 107 differential genes and obtained 49 significant differential genes related to glucose metabolism. The results of the correlation analyses indicated that two genes of the SWEET family, evm.TU.Chr1.3663 (PsSWEET9) and evm.TU.Chr4.676 (PsSWEET2), could be closely related to the composition of soluble sugars, which was also confirmed in the ethylene treatment experiments. In addition, analysis of the TOP 20 pathways between different growth stages and the green stage, as well as transient overexpression in chili, suggested that capsanthin/capsorubin synthase (PsCCS) of the carotenoid biosynthetic pathway contributed to the color change of plum fruit. These findings provide an insight into the molecular mechanisms involved in the ripening and color change of plum fruit.