Isolation and Characterization of DkPK Genes Associated with Natural Deastringency in C-PCNA Persimmon

AP3 promotes the synthesis of condensed tannin in fruit by positively regulating ANR expression

Condensed tannins are often found in fruits and nuts and have an astringent flavor. The synthesis pathway of condensed tannins is already clear, but few related regulatory factors have been explored. Previous studies about MADS-box transcription factors have mainly focused on the regulation of floral organ development. Recent studies have shown that MADS-box are also involved in fruit development, maturation, and quality. The fruit of Quercus fabri is rich in starch and nutrients in its kernel but is difficult to eat directly because of its high condensed tannin content. This study identified and functionally characterized the MADS-box transcription factor QfAP3 in Q. fabri. Functional analysis based on overexpression in Micro-Tom showed that QfAP3 promoted condensed tannin synthesis. By analyzing the expression trend of key genes in the condensed tannin synthesis pathway in Micro-Tom plants, we found that the expression trend of ANR was consistent with that of QfAP3, and QfAP3 could bind to the promoter of ANR and positively regulate it. This study has discovered new functions of MADS-box transcription factors in fruit quality formation, developed new regulatory factors for the synthesis pathway of condensed tannin, and provided a biotechnological method that can effectively reduce astringency in fruit.

Comparative transcriptome analysis reveals regulatory network and regulators associated with proanthocyanidin accumulation in persimmon

BACKGROUND

Proanthocyanidins (PAs) are important plant secondary metabolites that confer flavor, nutritional value, and resistance to pathogens. Persimmon is one of the PA richest crops. Mature fruits can be inedible because of the astringency caused by high PA levels and need to go through a de-astringency treatment before consumption. The molecular basis for PA accumulation is poorly known, particularly transcriptional regulators. We characterised three genotypes ('Luotiantianshi' (LT), 'Mopanshi' (MP), and 'Youhou' (YH)) with different PA accumulation patterns using an approach that combined PacBio full-length sequencing and Illumina-based RNA sequencing to build high-quality full-length transcriptomes. Additionally, we analysed transcriptome dynamics of the three genotypes (LT, MP, and YH) at four key fruit developmental stages.

RESULTS

A total of 96,463 transcripts were obtained. We identified 80,075 protein-coding sequences (CDSs), 71,137 simple sequence repeats (SSRs), and 27,845 long noncoding RNAs (lncRNAs). Pearson correlation coefficient (PCC), principal component analysis (PCA), and differentially expressed transcripts (DETs) analyses indicated that the four different developmental stages within a genotype exhibited similar transcriptome activities. A total of 2,164 transcripts specific to each fruit developmental stage were detected. The transcripts specific to early stages were attributed to phenylpropanoid and flavonoid biosynthesis. Co-expression network analyses revealed MEbrown and MEblue modules were strongly associated to PA accumulation. From these two modules, 20 hub TFs are potential regulators for PA accumulation. Among them, Cluster_78388 (SBP protein), Cluster_63454 (bZIP protein), and Cluster_66595 (MYB protein) appear to involve in the PA biosynthesis in Chinese genotypes.

CONCLUSIONS

This is the first high-quality reference transcriptome for commercial persimmon. Our work provides insights into the molecular pathways underlying PA accumulation and enhances our global understanding of transcriptome dynamics throughout fruit development.

DkMYB14 is a bifunctional transcription factor that regulates the accumulation of proanthocyanidin in persimmon fruit

Proanthocyanidins (PAs) are phenolic secondary metabolites that contribute to the protection of plant and human health. Persimmon (Diospyros kaki Thunb.) can accumulate abundant PAs in fruit, which cause a strong sensation of astringency. Proanthocyanidins can be classified into soluble and insoluble PAs; the former cause astringency but the latter do not. Soluble PAs can be converted into insoluble PAs upon interacting with acetaldehydes. We demonstrate here that DkMYB14, which regulates the accumulation of PA in persimmon fruit flesh, is a bifunctional transcription factor that acts as a repressor in PA biosynthesis but becomes an activator when involved in acetaldehyde biosynthesis. Interestingly, both functions contribute to the elimination of astringency by decreasing PA biosynthesis and promoting its insolubilization. We show that the amino acid Gly39 in the R2 domain and the ethylene response factor-associated amphiphilic repression-like motif in the C-terminal of DkMYB14 are essential for the regulation of both PA and acetaldehyde synthesis. The repressive function of DkMYB14 was lost after the mutation of either motif, and all activities of DkMYB14 were eliminated following the mutation of both motifs. Our results demonstrate that DkMYB14 functions as both a transcriptional activator and a repressor, directly repressing biosynthesis of PA and promoting its insolubilization, resulting in non-astringency in persimmon.

Transcriptome Analysis Revealed the Roles of Carbohydrate Metabolism on Differential Acetaldehyde Production Capacity in Persimmon Fruit in Response to High-CO2 Treatment

Persimmon (Diospyros kaki Thunb.) fruit is unique due to the continuous accumulation of soluble tannins during fruit development in most cultivars, which causes undesired astringency. High-CO₂ treatment was the most effective widely used method for astringency removal. However, differential effects of high-CO₂ treatment between cultivars were observed and the molecular basis remained inclusive. Previously, one cultivar ("Luoyangfangtianshengshi," LYFTSS) showed rapid deastringency, while two cultivars ("Shijiazhuanglianhuashi," SJZLHS; "Laopige," LPG) showed slow deastringency in response to high-CO₂ (95% CO₂) treatment. In this study, the metabolites (acetaldehyde and ethanol) related to deastringency were further analyzed and both acetaldehyde and ethanol were higher in SJZLHS and LYFTSS than that in LPG, where acetaldehyde was undetectable. Based on the RNA-seq data, the weighted gene coexpression network analysis (WGCNA) revealed that one module, comprised of 1773 unigenes, significantly correlated with the contents of acetaldehyde and ethanol (P < 0.001). Further analysis based on the acetaldehyde metabolism pathway indicated that the differentially expressed structural genes, including previously characterized DkADH and DkPDC and also their upstream members (e.g., PFK, phosphofructokinase), showed positive correlations with acetaldehyde production. Quantitative analysis of the precursor substances indicated that sucrose, glucose, and fructose exhibited limited differences between cultivar except for malic acid. However, the content of malic acid is much less than the total soluble sugar content. To verify the correlations between these genes and acetaldehyde production, the fruit from 14 more cultivars were collected and treated with high CO₂. After the treatment, acetaldehyde contents in different cultivars ranked in 30.4-255.5 μg/g FW. Real-time polymerase chain reaction (PCR) and correlation analysis indicated that the EVM0002315 (PFK) gene, belonging to carbohydrate metabolism, was significantly correlated with acetaldehyde content in fruit. Thus, it could be proposed that the differentially expressed carbohydrate metabolism related genes (especially PFK) are the basis for the variance of acetaldehyde production among different persimmon cultivars.

MiR858b Inhibits Proanthocyanidin Accumulation by the Repression of DkMYB19 and DkMYB20 in Persimmon

Persimmon proanthocyanidin (PA) biosynthesis is controlled by structural genes and regulated by transcription factors (TFs). MicroRNAs are a key factor involved in regulating gene expression at the posttranscriptional level whose functions in persimmon PA biosynthesis are poorly understood. Here, we identified a microRNA, miR858b, that putatively targets two R2R3-MYB TFs, DkMYB19 and DkMYB20. DkMYB19, DkMYB20, and miR858b showed divergent expression patterns during fruit development, and the interaction between miR858b and DkMYB19 or DkMYB20 was experimentally validated by 5' RNA ligase-mediated RACE, LUC enzyme activity analysis, and GFP signal detection. The DkMYB19 localized to the nucleus as well as the cytoplasm and DkMYB20 localized to the nucleus. The overexpression of miR858b led to the downregulation of DkMYB19 and DkMYB20, which reduced the content of PA, whereas a reduction in miR858b activity upregulated DkMYB19 and DkMYB20, resulting in a high content of PA in leaves transiently expressing a small tandem target mimic construct for blocking miR858 (STTM858b) in vivo. The transient transformation of miR858b in fruit discs in vitro also reduced the content of PA, while the content of PA increased under the transient transformation of fruit discs with STTM858b, DkMYB19, or DkMYB20. A similar phenomenon was observed upon the overexpression of miR858b in wild-type (WT) Arabidopsis and DkMYB19 or DkMYB20 in persimmon leaf calli. These findings suggested that miR858b repressed the expression of DkMYB19 and DkMYB20, which contributed to the PA accumulation in persimmon.

DkWRKY interacts with pyruvate kinase gene DkPK1 and promotes natural deastringency in C-PCNA persimmon

PAs, also known as condensed tannins, cause the astringency sensation in the persimmon fruit. The astringency of Chinese pollination-constant non-astringent (C-PCNA) persimmon (Diospyros kaki Thunb.) can be naturally removed on the tree, but the regulatory mechanisms of deastringency remain to be elucidated. In our previous research, DkPK1 was shown to be involved in the natural loss of astringency of C-PCNA persimmon fruit. In the present study, yeast one-hybrid (Y1H) library screening using the DkPK1 promoter as baits identified two DkWRKY transcription factor genes (DkWRKY3 and -15). The transcript levels of both DkWRKY3 and -15 exhibited a positive correlation with the decrease in soluble proanthocyanidin (PA) content during the last developmental stage in C-PCNA persimmon. Multiple sequence analysis and subcellular localization confirmed that DkWRKY3 and -15 belonging to the group II and I families, respectively, were both located in the nucleus. Dual-luciferase and Y1H assays demonstrated that DkWRKY3 and -15 can transactivate the DkPK1 promoters. The combination of DkWRKY3 and -15 most likely produced an additive activation effect compared to a single activator on DkPK1, although the two transcriptional activators were not capable of interacting. Notably, DkWRKY3 and -15 showed ubiquitous expression in various organs and abundant upregulation in seeds. Furthermore, transient overexpression of both DkWRKY3 and -15 in persimmon leaves led to a significant decrease in the content of soluble PAs but a significant increase in the expression levels of the acetaldehyde metabolism-related DkPK, DkPDC and DkADH genes. Thus, we suggest that DkWRKY3 and -15 are the upstream regulators of DkPK1 and positively regulate the natural deastringency in C-PCNA persimmon.

An integrated analysis based on transcriptome and proteome reveals deastringency-related genes in CPCNA persimmon

Persimmon fruits accumulate a large amount of proanthocyanidins (PAs) during development. PAs cause a dry or puckering sensation due to its astringency. Pollination constant and non-astringent (PCNA) persimmon fruits can lose astringency during fruit ripening. However, little is known about the mechanism of natural de-astringency of Chinese PCNA (CPCNA). To gain insight into the molecular events of CPCNA natural de-astringency, we used mRNA-seq and iTRAQ-based quantitative proteomic analysis to measure changes in genes and proteins expression at two key stages of natural astringency removal (i.e. 10 and 20 weeks after bloom) and water-treated (i.e. 40 °C·12 h) de-astringency fruits. Our analyses show that the three predominantly process in CPCNA de-astringency: (1) water treatment strongly up-regulates glycolysis/acetaldehyde metabolism, (2) expression of genes/proteins involved in PA biosynthetic pathway was remarkably reduced in natural and water-treated de-astringency, (3) sugar metabolism and ethylene related pathway were quite abundant in natural de-astringency. We also found ethylene-related TFs were quite abundant in natural de-astringency, followed by WRKY and NAC transcription factors. These results provide an initial understanding of the predominantly biological processes underlying the natural de-astringency and "coagulation effect" in CPCNA.

DkPK Genes Promote Natural Deastringency in C-PCNA Persimmon by Up-regulating DkPDC and DkADH Expression

The astringency of Chinese pollination-constant non-astringent (C-PCNA) persimmon (Diospyros kaki Thunb.) can be naturally removed on the tree. This process is controlled by a single locus and is dominant against other types of persimmons; therefore, this variant is an important candidate for commercial cultivation and the breeding of PCNA cultivars. In our previous study, six full-length coding sequences (CDS) for pyruvate kinase genes (DkPK1-6) were isolated, and DkPK1 is thought to be involved in the natural deastringency of C-PCNA persimmon fruit. Here, we characterize the eight other DkPK genes (DkPK7-14) from C-PCNA persimmon fruit based on transcriptome data. The transcript changes in DkPK7-14 genes and correlations with the proanthocyanidin (PA) content were investigated during different fruit development stages in C-PCNA, J-PCNA, and non-PCNA persimmon; DkPK7 and DkPK8 exhibited up-regulation patterns during the last developmental stage in C-PCNA persimmon that was negatively correlated with the decrease in soluble PAs. Phylogenetic analysis and subcellular localization analysis revealed that DkPK7 and DkPK8 are cytosolic proteins. Notably, DkPK7 and DkPK8 were ubiquitously expressed in various persimmon organs and abundantly up-regulated in seeds. Furthermore, transient over-expression of DkPK7 and DkPK8 in persimmon leaves led to a significant decrease in the content of soluble PAs but a significant increase in the expression levels of the pyruvate decarboxylase (DkPDC) and alcohol dehydrogenase genes (DkADH), which are closely related to acetaldehyde metabolism. The accumulated acetaldehyde that results from the up-regulation of the DkPDC and DkADH genes can combine with soluble PAs to form insoluble PAs, resulting in the removal of astringency from persimmon fruit. Thus, we suggest that both DkPK7 and DkPK8 are likely to be involved in natural deastringency via the up-regulation of DkPDC and DkADH expression during the last developmental stage in C-PCNA persimmon.