Citrus CitERF6 Contributes to Citric Acid Degradation via Upregulation of CitAclα1, Encoding ATP-Citrate Lyase Subunit α

Comprehensive analyses of the citrus WRKY gene family involved in the metabolism of fruit sugars and organic acids

Sugars and organic acids are the main factors determining the flavor of citrus fruit. The WRKY transcription factor family plays a vital role in plant growth and development. However, there are still few studies about the regulation of citrus WRKY transcription factors (CsWRKYs) on sugars and organic acids in citrus fruit. In this work, a genome-wide analysis of CsWRKYs was carried out in the citrus genome, and a total of 81 CsWRKYs were identified, which contained conserved WRKY motifs. Cis-regulatory element analysis revealed that most of the CsWRKY promoters contained several kinds of hormone-responsive and abiotic-responsive cis-elements. Furthermore, gene expression analysis and fruit quality determination showed that multiple CsWRKYs were closely linked to fruit sugars and organic acids with the development of citrus fruit. Notably, transcriptome co-expression network analysis further indicated that three CsWRKYs, namely, CsWRKY3, CsWRKY47, and CsWRKY46, co-expressed with multiple genes involved in various pathways, such as Pyruvate metabolism and Citrate cycle. These CsWRKYs may participate in the metabolism of fruit sugars and organic acids by regulating carbohydrate metabolism genes in citrus fruit. These findings provide comprehensive knowledge of the CsWRKY family on the regulation of fruit quality.

Identification of CmACL genes in melon and analysis of their potential functions in fruit sugar and acid accumulation

Citric acid is the most important organic acid in melon and has a great influence on fruit flavor quality. ATP-citrate (pro-S) lyase (ACL) is a key regulator in the acetyl-CoA pathway and plays an important role in citric acid metabolism. In this study we analyzed the structure and phylogenetics of CmACL genes and their functions in sugar and acid accumulation in melon. A total of four CmACL genes were identified in the melon genome, and phylogenetic analysis assigned these genes into the α subfamily (CmACLα1 and CmACLα2) and the β subfamily (CmACLβ1 and CmACLβ2). Conserved motif and gene structure analyses showed that members of the same subfamily shared identical conserved motifs and gene structures, and probably have similar biological functions. Analysis of cis-acting elements revealed that CmACL promoter sequences contained regulatory elements related to light, stress, phytohormones, and growth and development, indicating that CmACL genes may be involved in melon growth and stress responses. The prediction of protein interaction network showed that CmACL proteins were closely related to the proteins belonging to tricarboxylic acid cycle, glyoxylic acid cycle and glycolytic pathway, suggesting that CmACL proteins may play an important role in sugar and acid metabolism. The expression of CmACLβ1 was significantly and positively correlated with sucrose content, and CmACLβ2 expression was significantly positively correlated with citric acid content, suggesting that CmACLβ1 and CmACLβ2 have important roles in sugar and acid accumulation in melon. Our results offer novel insights and avenues for the regulation of sugar and acid levels in melon and provide a theoretical foundation for breeding high-quality melon cultivars.

Weighted gene coexpression correlation network analysis reveals the potential molecular regulatory mechanism of citrate and anthocyanin accumulation between postharvest 'Bingtangcheng' and 'Tarocco' blood orange fruit

BACKGROUND

Organic acids and anthocyanins are the most important compounds for the flavor and nutritional quality of citrus fruit. However, there are few reports on the involvement of co-regulation of citrate and anthocyanin metabolism. Here, we performed a comparative transcriptome analysis to elucidate the genes and pathways involved in both citrate and anthocyanin accumulation in postharvest citrus fruit with 'Tarocco' blood orange (TBO; high accumulation) and 'Bingtangcheng' sweet orange (BTSO; low accumulation).

RESULTS

A robust core set of 825 DEGs were found to be temporally associated with citrate and anthocyanin accumulation throughout the storage period through transcriptome analysis. Further according to the results of weighted gene coexpression correlation network analysis (WGCNA), the turquoise and brown module was highly positively correlated with both of the content of citrate and anthocyanin, and p-type ATPase (PH8), phosphoenolpyruvate carboxylase kinase (PEPCK), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H) and glutathione S transferase (GST) were considered key structural genes. Moreover, MYB family transcription factor (PH4), Zinc finger PHD-type transcription factor (CHR4, HAC12), Zinc finger SWIM-type transcription factor (FAR1) and Zinc finger C3H1-type transcription factor (ATC3H64) were considered hub genes related to these structural genes. Further qRT-PCR analysis verified that these transcription factors were highly expressed in TBO fruit and their expression profiles were significantly positively correlated with the structural genes of citrate and anthocyanin metabolism as well as the content of citrate and anthocyanin content.

CONCLUSIONS

The findings suggest that the CHR4, FAR1, ATC3H64 and HAC12 may be the new transcription regulators participate in controlling the level of citrate and anthocyanin in postharvest TBO fruit in addition to PH4. These results may providing new insight into the regulation mechanism of citrate and anthocyanin accumulation in citrus fruit.

1H NMR-Based Metabolic Profiling to Follow Changes in Pomelo Cultivars during Postharvest Senescence

This study investigated metabolite changes in three pomelo cultivars during postharvest senescence using ¹H NMR-based metabolic profiling. Three pomelo cultivars, 'Hongroumiyou', 'Bairoumiyou' and 'Huangroumiyou', abbreviated as "R", "W" and "Y" according to the color of their juice sacs, were stored at 25 °C for 90 days, and NMR was applied to determine the metabolite changes in juice sacs during storage. Fifteen metabolites were identified, including organic acids, sugars, amino acids, fatty acids, phenols and naringin. Partial least squares discriminant analysis (PLS-DA) was used to screen the significant metabolites according to the variable importance for the projection (VIP) scores in three pomelo cultivars during 90 days of storage. Additionally, eight metabolites, naringin, alanine, asparagine, choline, citric acid, malic acid, phosphocholine and β-D-glucose, were screened to be the crucial biomarkers with VIP > 1. The undesirable flavor of "bitter and sour" during the 60 days of storage was mainly attributed to the naringin, citric acid and sugars. According to the correlation analysis, the citric acid content determined by NMR showed a significantly positive relationship with that analyzed by HPLC. These findings suggested that NMR technology was accurate and efficient for metabolomic analysis of pomelo fruit, and the ¹H NMR-based metabolic profiling can be efficient during quality evaluation and useful for improving the fruit flavor quality during postharvest storage.

FaGAPC2/FaPKc2.2 and FaPEPCK reveal differential citric acid metabolism regulation in late development of strawberry fruit

Citric acid is the primary organic acid that affects the taste of strawberry fruit. Glycolysis supplies key substrates for the tricarboxylic acid cycle (TCA cycle). However, little is known about the regulatory mechanisms of glycolytic genes on citric acid metabolism in strawberry fruits. In this study, the citric acid content of strawberry fruit displayed a trend of rising and decreasing from the initial red stage to the full red stage and then dark red stage. Thus, a difference in citric acid metabolic regulation was suspected during strawberry fruit development. In addition, overexpression of either cytoplasm glyceraldehyde-3-phosphate dehydrogenase (FxaC_14g13400, namely FaGAPC2) or pyruvate kinase (FxaC_15g00080, namely FaPKc2.2) inhibited strawberry fruit ripening and the accumulation of citric acid, leading to a range of maturity stages from partial red to full red stage. The combined transcriptome and metabolome analysis revealed that overexpression of FaGAPC2 and FaPKc2.2 significantly suppressed the expression of phosphoenolpyruvate carboxykinase (FxaC_1g21491, namely FaPEPCK) but enhanced the content of glutamine and aspartic acid. Meanwhile, the activities of PEPCK and glutamate decarboxylase (GAD) were inhibited, but the activities of glutamine synthase (GS) were increased in FaGAPC2/FaPKc2.2-overexpressed fruit. Further, functional verification demonstrated that overexpression of FaPEPCK can promote strawberry fruit ripening, resulting in a range of maturity stage from full red to dark red stage, while the citric acid synthase (CS) activities and citric acid content were significantly decreased. Overall, this study revealed that FaGAPC2/FaPKc2.2 and FaPEPCK perform an important role in reducing citric acid content in strawberry fruit, and FaGAPC2/FaPKc2.2 mainly by promoting the GS degradation pathway and FaPEPCK mainly by inhibiting the CS synthesis pathway.

Transcription Factor CitERF16 Is Involved in Citrus Fruit Sucrose Accumulation by Activating CitSWEET11d

Sugars are the primary products of photosynthesis and play an important role in plant growth and development. They contribute to sweetness and flavor of fleshy fruits and are pivotal to fruit quality, and their translocation and allocation are mainly dependent on sugar transporters. Genome-wide characterization of Satsuma mandarin identified eighteen SWEET family members that encode transporters which facilitate diffusion of sugar across cell membranes. Analysis of the expression profiles in tissues of mandarin fruit at different developmental stages showed that CitSWEET11d transcripts were significantly correlated with sucrose accumulation. Further studies indicated that overexpression of CitSWEET11d in citrus callus and tomato fruit showed a higher sucrose level compared to wild-type, suggesting that CitSWEET11d could enhance sucrose accumulation. In addition, we identified an ERF transcription factor CitERF16 by yeast one-hybrid screening assay which could directly bind to the DRE cis-element on the promoter of CitSWEET11d. Overexpression of CitERF16 in citrus callus significantly induced CitSWEET11d expression and elevated sucrose content, suggesting that CitERF16 acts as a positive regulator to promote sucrose accumulation via trans-activation of CitSWEET11d expression.

The effect of NH4+ on phosphoenolpyruvate carboxykinase gene expression, metabolic flux and citrate content of citrus juice sacs

Citrate is one of the most important metabolites determining the flavour of citrus fruit. It has been reported that nitrogen supply may have an impact on acid level of fruit. Here, the relationship between nitrogen metabolism and citrate catabolism was studied in pumelo juice sacs. Differences in metabolites, gene expression and flux distributions were analyzed in juice sacs incubated in medium with and without NH₄⁺. Compared with those incubated with NH₄⁺, juice sacs under nitrogen deficiency exhibited enhanced flux through phosphoenolpyruvate carboxykinase (PEPCK) and accelerated consumption of citrate, while the other two TCA cycle efflux points, through malic enzyme (ME) and glutamate dehydrogenase (GDH), were both repressed. Consistent with the estimated fluxes, the expression of PEPCK1 was upregulated under nitrogen deficiency, while that of GDH1, GDH2, NAD-ME1 and NADP-ME2 were all repressed. Thus, we propose that PEPCK1 contributes to citrate degradation under nitrogen limitation.

Citric Acid-Mediated Abiotic Stress Tolerance in Plants

Several recent studies have shown that citric acid/citrate (CA) can confer abiotic stress tolerance to plants. Exogenous CA application leads to improved growth and yield in crop plants under various abiotic stress conditions. Improved physiological outcomes are associated with higher photosynthetic rates, reduced reactive oxygen species, and better osmoregulation. Application of CA also induces antioxidant defense systems, promotes increased chlorophyll content, and affects secondary metabolism to limit plant growth restrictions under stress. In particular, CA has a major impact on relieving heavy metal stress by promoting precipitation, chelation, and sequestration of metal ions. This review summarizes the mechanisms that mediate CA-regulated changes in plants, primarily CA’s involvement in the control of physiological and molecular processes in plants under abiotic stress conditions. We also review genetic engineering strategies for CA-mediated abiotic stress tolerance. Finally, we propose a model to explain how CA’s position in complex metabolic networks involving the biosynthesis of phytohormones, amino acids, signaling molecules, and other secondary metabolites could explain some of its abiotic stress-ameliorating properties. This review summarizes our current understanding of CA-mediated abiotic stress tolerance and highlights areas where additional research is needed.