Abstract
Flavonoids are natural antioxidants in plants that affect the color of plant tissues. Flavonoids can be divided into eight subgroups, including flavonols, anthocyanins, and proanthocyanidins. The mechanisms of flavonoid synthesis in model plants have been widely studied. However, there are a limited number of reports on the synthesis of flavonoids in the red leaf varieties of woody plants. In this study, we combined morphological observation, pigment content determination, metabolomics, and transcriptomics to investigate the metabolites and gene regulation present in the red and green leaves of Eucommia ulmoides Oliv. The results showed that the red leaves have a lower chlorophyll content and a higher anthocyanin content. Metabonomic analysis identified that the relative content of most flavonoids is up-regulated in red leaves based on UPLC-ESI-MS/MS, which included the apigenin class, quercetin class, kaempferol class, and procyanidins. Transcriptome data suggested that the differentially up-regulated genes are enriched in flavonoid and anthocyanin synthesis pathways, ABC transport, and GST pathways. The integrative analysis of the transcriptome and metabolome showed that the up-regulation of flavonoid metabolism and a high expression of chlorophyll degradation with the down-regulation of chlorophyll biosynthesis genes are detected in E. ulmoides red leaves compared with green leaves. In addition, the co-expression networks implied that cyanidin 3-5-O-diglucoside, EuDR5, EuPAL2, EuDFR11, Eu3MaT1, and EuF3′H are likely associated with the red leaf coloration of E. ulmoides. In summary, this research provided a reference for studying the mechanism of red leaf coloration in woody plants and the use of E. ulmoides red leaves as feedstock for bioactive products.
Highlights
Eucommia ulmoides is native to China and has been extensively cultivated across27 provinces (24◦ 500 –41◦ 500 N, 76◦ 000 –126◦ 000 E) because of its high economic value and adaptability [1]
Our study suggested that the up-regulation of flavonoid biosynthesis and a high expression of chlorophyll degradation with the down-regulation of chlorophyll biosynthesis are detected in E. ulmoides red leaves compared with green leaves
Most flavonoids and anthocyanins were highly accumulated in red leaves
Summary
Eucommia ulmoides is native to China and has been extensively cultivated across27 provinces (24◦ 500 –41◦ 500 N, 76◦ 000 –126◦ 000 E) because of its high economic value and adaptability [1]. E. ulmoides leaf extract is a kind of herbal medicine and potential feedstock for bioactive products, which are used as animal feed additives [2,3,4]. Pharmacological experiments have shown that E. ulmoides leaf extract has antioxidation activity [5]. The flavonol glycosides extracted from E. ulmoides leaves display glycation inhibitory activity [6]. Flavonoids are important antioxidants and highly concentrated in E. ulmoides leaves [7]. A total of 13 flavonoids were identified in green E. ulmoides leaves using ultraperformance liquid chromatography–tandem quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) detection [9]. The mass fraction of the total flavonoid content in E. ulmoides red leaf extract reached 1.7%, which was significantly higher than that in the green leaf variety [10]
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