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.

Comparative Anatomical and Transcriptomics Reveal the Larger Cell Size as a Major Contributor to Larger Fruit Size in Apricot.

Fruit size is one of the essential quality traits and influences the economic value of apricots. To explore the underlying mechanisms of the formation of differences in fruit size in apricots, we performed a comparative analysis of anatomical and transcriptomics dynamics during fruit growth and development in two apricot cultivars with contrasting fruit sizes (large-fruit Prunus armeniaca 'Sungold' and small-fruit P. sibirica 'F43'). Our analysis identified that the difference in fruit size was mainly caused by the difference in cell size between the two apricot cultivars. Compared with 'F43', the transcriptional programs exhibited significant differences in 'Sungold', mainly in the cell expansion period. After analysis, key differentially expressed genes (DEGs) most likely to influence cell size were screened out, including genes involved in auxin signal transduction and cell wall loosening mechanisms. Furthermore, weighted gene co-expression network analysis (WGCNA) revealed that PRE6/bHLH was identified as a hub gene, which interacted with 1 TIR1, 3 AUX/IAAs, 4 SAURs, 3 EXPs, and 1 CEL. Hence, a total of 13 key candidate genes were identified as positive regulators of fruit size in apricots. The results provide new insights into the molecular basis of fruit size control and lay a foundation for future breeding and cultivation of larger fruits in apricot.

15-cis-Phytoene Desaturase and 15-cis-Phytoene Synthase Can Catalyze the Synthesis of ß-Carotene and Influence the Color of Apricot Pulp.

Fruit color affects its commercial value. ß-carotene is the pigment that provides color for many fruits and vegetables. However, the molecular mechanism of ß-carotene metabolism during apricot ripening is largely unknown. Here, we investigated whether ß-carotene content affects apricot fruit color. First, the differences in ß-carotene content between orange apricot 'JTY' and white apricot 'X15' during nine developmental stages (S1-S9) were compared. ß-carotene contents highly significantly differed between 'JTY' and 'X15' from S5 (color transition stage) onwards. Whole-transcriptome analysis showed that the ß-carotene synthesis genes 15-cis-phytoene desaturase (PaPDS) and 15-cis-phytoene synthase (PaPSY) significantly differed between the two cultivars during the color transition stage. There was a 5 bp deletion in exon 11 of PaPDS in 'X15', which led to early termination of amino acid translation. Gene overexpression and virus-induced silencing analysis showed that truncated PaPDS disrupted the ß-carotene biosynthesis pathway in apricot pulp, resulting in decreased ß-carotene content and a white phenotype. Furthermore, virus-induced silencing analysis showed that PaPSY was also a key gene in ß-carotene biosynthesis. These findings provide new insights into the molecular regulation of apricot carotenoids and provide a theoretical reference for breeding new cultivars of apricot.

AprGPD: the apricot genomic and phenotypic database.

BACKGROUND: Apricot is cultivated worldwide because of its high nutritive content and strong adaptability. Its flesh is delicious and has a unique and pleasant aroma. Apricot kernel is also consumed as nuts. The genome of apricot has been sequenced, and the transcriptome, resequencing, and phenotype data have been increasely generated. However, with the emergence of new information, the data are expected to integrate, and disseminate. RESULTS: To better manage the continuous addition of new data and increase convenience, we constructed the apricot genomic and phenotypic database (AprGPD, http://apricotgpd.com ). At present, AprGPD contains three reference genomes, 1692 germplasms, 306 genome resequencing data, 90 RNA sequencing data. A set of user-friendly query, analysis, and visualization tools have been implemented in AprGPD. We have also performed a detailed analysis of 59 transcription factor families for the three genomes of apricot. CONCLUSION: Six modules are displayed in AprGPD, including species, germplasm, genome, variation, product, tools. The data integrated by AprGPD will be helpful for the molecular breeding of apricot.

Insights Into the Molecular Mechanisms of Late Flowering in Prunus sibirica by Whole-Genome and Transcriptome Analyses.

Freezing during the flowering of Prunus sibirica is detrimental to fruit production. The late flowering (LF) type, which is delayed by 7-15 days compared with the normal flowering (NF) type, avoids damages at low temperature, but the molecular mechanism of LF remains unclear. Therefore, this study was conducted to comprehensively characterize floral bud differentiation. A histological analysis showed that initial floral bud differentiation was delayed in the LF type compared to the NF type. Genome-wide associated studies (GWAS) showed that a candidate gene (PaF106G0600023738.01) was significantly associated with LF type. It was identified as trehalose-6-phosphate phosphatase (PsTPPF), which is involved in trehalose-6-phosphate (Tre6P) signaling pathway and acts on floral transition. A whole-transcriptome RNA sequencing analysis was conducted, and a total of 6,110 differential expression (DE) mRNAs, 1,351 DE lncRNAs, and 148 DE miRNAs were identified. In addition, 24 DE mRNAs related with floral transition were predicted, and these involved the following: three interactions between DE lncRNAs and DE mRNAs of photoperiod pathway with two mRNAs (COP1, PaF106G0400018289.01 and CO3, MXLOC_025744) and three lncRNAs (CCLR, LTCONS_00031803, COCLR1, LTCONS_00046726, and COCLR2, LTCONS_00046731); one interaction between DE miRNAs and DE mRNAs with one mRNA, encoding trehalose-6-phosphate synthase (PsTPS1, PaF106G0100001132.01), and one miRNA (miRNA167h). Combined with the expression profiles and Tre6P levels, functions of PsTPPF and PsTPS1 in Tre6P regulation were considered to be associated with flowering time. A new network of ceRNAs correlated with LF was constructed, and it consisted of one mRNA (PsTPS1), one lncRNA (TCLR, LTCONS_00034157), and one miRNA (miR167h). This study provided insight into the molecular regulatory mechanism of LF in Prunus sibirica.