Functional characterization of two flavone synthase II members in citrus

Neofunctionalization of an OMT cluster dominates polymethoxyflavone biosynthesis associated with the domestication of citrus

Polymethoxyflavones (PMFs) are a class of abundant specialized metabolites with remarkable anticancer properties in citrus. Multiple methoxy groups in PMFs are derived from methylation modification catalyzed by a series of hydroxylases and O-methyltransferases (OMTs). However, the specific OMTs that catalyze the systematic O-methylation of hydroxyflavones remain largely unknown. Here, we report that PMFs are highly accumulated in wild mandarins and mandarin-derived accessions, while undetectable in early-diverging citrus species and related species. Our results demonstrated that three homologous genes, CreOMT3, CreOMT4, and CreOMT5, are crucial for PMF biosynthesis in citrus, and their encoded methyltransferases exhibit multisite O-methylation activities for hydroxyflavones, producing seven PMFs in vitro and in vivo. Comparative genomic and syntenic analyses indicated that the tandem CreOMT3, CreOMT4, and CreOMT5 may be duplicated from CreOMT6 and contributes to the genetic basis of PMF biosynthesis in the mandarin group through neofunctionalization. We also demonstrated that N17 in CreOMT4 is an essential amino acid residue for C3-, C5-, C6-, and C3'-O-methylation activity and provided a rationale for the functional deficiency of OMT6 to produce PMFs in early-diverging citrus and some domesticated citrus species. A 1,041-bp deletion in the CreOMT4 promoter, which is found in most modern cultivated mandarins, has reduced the PMF content relative to that in wild and early-admixture mandarins. This study provides a framework for reconstructing PMF biosynthetic pathways, which may facilitate the breeding of citrus fruits with enhanced health benefits.

Metabolome and Transcriptome Analysis Revealed the Basis of the Difference in Antioxidant Capacity in Different Tissues of Citrus reticulata 'Ponkan'

Citrus is an important type of fruit, with antioxidant bioactivity. However, the variations in the antioxidant ability of different tissues in citrus and its metabolic and molecular basis remain unclear. Here, we assessed the antioxidant capacities of 12 tissues from Citrus reticulata 'Ponkan', finding that young leaves and root exhibited the strongest antioxidant capacity. Secondary metabolites accumulated differentially in parts of the citrus plant, of which flavonoids were enriched in stem, leaf, and flavedo; phenolic acids were enriched in the albedo, while coumarins were enriched in the root, potentially explaining the higher antioxidant capacities of these tissues. The spatially specific accumulation of metabolites was related to the expression levels of biosynthesis-related genes such as chalcone synthase (CHS), chalcone isomerase (CHI), flavone synthase (FNS), O-methyltransferase (OMT), flavonoid-3'-hydroxylase (F3'H), flavonoid-6/8-hydroxylase (F6/8H), p-coumaroyl CoA 2'-hydroxylase (C2'H), and prenyltransferase (PT), among others, in the phenylpropane pathway. Weighted gene co-expression network analysis (WGCNA) identified modules associated with flavonoids and coumarin content, among which we identified an OMT involved in coumarin O-methylation, and related transcription factors were predicted. Our study identifies key genes and metabolites influencing the antioxidant capacity of citrus, which could contribute to the enhanced understanding and utilization of bioactive citrus components.

Combined analysis of inorganic elements and flavonoid metabolites reveals the relationship between flower quality and maturity of Sophora japonica L

Flos Sophorae (FS), or the dried flower buds of Sophora japonica L., is widely used as a food and medicinal material in China. The quality of S. japonica flowers varies with the developmental stages (S1-S5) of the plant. However, the relationship between FS quality and maturity remains unclear. Inductively coupled plasma optical emission spectrometry (ICP-OES) and ultra-high performance liquid chromatography coupled with electrospray ionization-triple quadrupole-linear ion trap mass spectrometry (UPLC-ESI-Q TRAP-MS/MS) were used to analyze inorganic elements and flavonoid metabolites, respectively. A combined analysis of the inorganic elements and flavonoid metabolites in FS was conducted to determine the patterns of FS quality formation. Sixteen inorganic elements and 173 flavonoid metabolites that accumulated at different developmental stages were identified. Notably, 54 flavonoid metabolites associated with the amelioration of major human diseases were identified, and Ca, P, K, Fe, and Cu were postulated to influence flavonoid metabolism and synthesis. This study offers a novel perspective and foundation for the further exploration of the rules governing the quality of plant materials.