Multiomic Analysis Elucidates the Reasons Underlying the Differential Metabolite Accumulation in Citrus Mature Leaves and Fruit Juice Sacs

Analysis of Flavonoid Metabolites in Citrus reticulata 'Chachi' at Different Collection Stages Using UPLC-ESI-MS/MS

Flavonoids are essential substances with antioxidant properties and high medicinal value. Citrus reticulata 'Chachi' peel (CRCP) is rich in flavonoids and has numerous health benefits. The different maturity periods of CRCP can affect the flavonoid contents and pharmacological effects. In this study, we successfully performed UPLC-ESI-MS/MS-based metabolic analysis to compare the metabolites of CRCP at different harvesting periods (Jul, Aug, Sep, Oct, Nov, and Dec) using a systematic approach. The results revealed the identification of a total of 168 flavonoid metabolites, including 61 flavones, 54 flavonols, 14 flavone C-glycosides, 14 dihydroflavones, 9 flavanones, 8 isoflavones, 3 flavanols, 3 dihydroflavonols, and 2 chalcones. Clustering analysis and PCA were used to separate the CRCP samples collected at different stages. Furthermore, from July to December, the relative contents of isoflavones, dihydroflavones, and dihydroflavonols gradually increased and flavanols gradually decreased over time. The relative content of flavonoid C-glycosides showed an increasing and then decreasing trend, reaching the highest value in August. This study contributes to a better understanding of flavonoid metabolites in CRCP at different harvesting stages and informs their potential future utilization.

Integrated Transcriptome and Metabolome Analysis Reveals Phenylpropanoid Biosynthesis and Phytohormone Signaling Contribute to "Candidatus Liberibacter asiaticus" Accumulation in Citrus Fruit Piths (Fluffy Albedo)

"Candidatus Liberibacter asiaticus" (CLas) is a phloem-restricted α-proteobacterium that is associated with citrus huanglongbing (HLB), which is the most destructive disease that affects all varieties of citrus. Although midrib is usually used as a material for CLas detection, we recently found that the bacterium was enriched in fruits, especially in the fruit pith. However, no study has revealed the molecular basis of these two parts in responding to CLas infection. Therefore, we performed transcriptome and UHPLC-MS-based targeted and untargeted metabolomics analyses in order to organize the essential genes and metabolites that are involved. Transcriptome and metabolome characterized 4834 differentially expressed genes (DEGs) and 383 differentially accumulated metabolites (DAMs) between the two materials, wherein 179 DEGs and 44 DAMs were affected by HLB in both of the tissues, involving the pathways of phenylpropanoid biosynthesis, phytohormone signaling transduction, starch and sucrose metabolism, and photosynthesis. Notably, we discovered that the gene expression that is related to beta-glucosidase and endoglucanase was up-regulated in fruits. In addition, defense-related gene expression and metabolite accumulation were significantly down-regulated in infected fruits. Taken together, the decreased amount of jasmonic acid, coupled with the reduced accumulation of phenylpropanoid and the increased proliferation of indole-3-acetic acid, salicylic acid, and abscisic acid, compared to leaf midribs, may contribute largely to the enrichment of CLas in fruit piths, resulting in disorders of photosynthesis and starch and sucrose metabolism.

Herbaceous peony PlACLB2 positively regulates red petal formation by promoting anthocyanin accumulation

ATP-citrate lyase (ACL) gene catalyzes the formation of acetyl-CoA to provide intermediate precursors for many secondary metabolites, and also plays an important role in anthocyanin biosynthesis of plants. Herbaceous peony (Paeonia lactiflora Pall.) is an international cut flower known for its rich flower colors, however, the function of the ACL gene in flower color regulation is still unclear. Here, double-colored P. lactiflora 'Hebao Jinlian' were used to study the molecular mechanism of red petal, and acetyl-CoA and anthocyanin biosynthesis related PlACLB2, PlCHS, PlDFR, PlANS, and PlbHLH1 genes were initially found to highly expressed in the red outer-petals. The expression pattern of PlACLB2 was consistent with the spatial accumulation of anthocyanins. The correlation analysis of PlACLB2 expression pattern, acetyl-CoA content, and anthocyanin accumulation revealed that PlACLB2 was positively correlated with the acetyl-CoA and anthocyanin contents with correlation coefficients of 0.82 and 0.80. Moreover, multiple sequence alignment identified two typical conserved domains in PlACLB2, and phylogenetic analysis clustered PlACLB2 into the ACLB clade. PlACLB2 was localized in the nucleus and cytoplasm. On the one hand, silencing PlACLB2 in P. lactiflora red outer-petal resulted in lighter petal color and decreased acetyl-CoA accumulation, and quantitative analysis detected that PlACLB2-silenced petals lost more anthocyanins than the control groups with a decrease of 31.0%, and the main pigment component cyanidin-3,5-O-diglucoside was reduced by 31.9%. On the other hand, overexpression of PlACLB2 significantly promoted red coloration, acetyl-CoA content, and anthocyanin accumulation in tobacco flowers. These results demonstrated that PlACLB2 promoted anthocyanin accumulation by increasing the abundance of its precursor substrate acetyl-CoA, thereby regulating the formation of the red petals in P. lactiflora.

Impact of Various Microbial-Fermented Methods on the Chemical Profile of Dark Tea Using a Single Raw Tea Material

In the present study, we produced Pu-erh, Liubao, Qingzhuan, and Fuzhuan teas using a single raw tea material and applied widely targeted metabolomics to study the impact of various microbial-fermented methods on the chemical profile of dark tea. The contents of catechins and free amino acids decreased drastically, whereas the contents of gallic acid and theabrownins increased significantly during microbial fermentation. Pu-erh tea had the highest content of theabrownins (11.82 ± 0.49%). Moreover, MS-based metabolomics analysis revealed that the different types of dark teas were significantly different from their raw material. A total of 85 differential metabolites were screened among 569 metabolites identified referring to self-compiled database. Glycosylated, hydroxylated, methylated, and condensed and oxidated products originating from microbial bioconversion of their corresponding primitive forms were significantly increased in dark teas. These results suggest that various microbial-fermented methods greatly affect the metabolic profile of dark tea, which can provide useful information for dark tea biochemistry research.

Reduced expression of CsPH8, a P-type ATPase gene, is the major factor leading to the low citrate accumulation in citrus leaves

Citrate is an important intermediate product for the biosynthesis of several metabolites in plants. As two important organs of the citrus plant, fruits and leaves have their own metabolites characteristics; among them, citrate is normally high in fruit juice sacs (JS) and low in leaves. In this study, citrate content and transcript levels of citrate synthesis, transport, storage, and utilization related genes were compared between leaves and fruit JS of Citrus reticulata cv. 'Huagan No. 2', C. grandis cv. 'Hirado Buntan', and C. sinensis cv. 'Anliu'. Results indicated that the citrate content in fruit JS was significantly higher than in leaves of each cultivar. Only the relative mRNA levels of a P-type proton pump gene, CsPH8, was significantly lower in leaves than in fruit JS of three citrus cultivars, while other genes related to citrate biosynthesis, transport, storage, and utilization were highly expressed in leaves as compared to fruit JS. Furthermore, CsPH8 transient and stable transformation in leaves indicated that the change in citrate content is highly consistent with the change of CsPH8 transcript levels. Taken together, our results strongly suggest that the low accumulation of citrate in citrus leaves is mainly due to the low expression level of CsPH8; additionally, the high level of expression of citrate-utilizing genes would prevent citrate accumulation in the leaf organ.