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.

Quercus variabilis Blume is one of the most ecologically valuable tree species in China and is known to have adaptive mechanisms to climate change. Our objective was to quantify the variation pattern in the fruit morphology of Q. variabilis. Fruit samples were collected from 43 natural populations in autumn of 2019. Our results indicated that the coefficient of variation (CV) of the fruit length (FL) and fruit width (FW) were 10.08% and 11.21%, respectively. There were significant differences in the FL, FW, and fruit length-to-width ratios (FL/FW) among the studied populations. Also, there was a significant positive correlation between the FW and FL. The FL decreased with increasing precipitation in the wettest quarter (PWQ). A concave trend was observed in the variations in FL with the equivalent latitude (ELAT), longitude (LON), annual mean air temperature (MAT), and annual precipitation (AP). A similar concave trend was observed for the FL/FW with LON, MAT, and AP. A positive correlation was observed between the FW, FL and FL/FW, and the ELAT. The cluster analysis revealed five groups of the 43 natural populations. Our study findings suggests that Q. variabilis has high levels of phenotypic plasticity for geographical and climatic factors.

Introduction: To investigate the mechanism of leaf color change at different stages in Populus × euramericana ‘Zhonghuahongye’ (‘Zhonghong’ poplar).Methods: Leaf color phenotypes were determined and a metabolomic analysis was performed on leaves at three stages (R1, R2 and R3).Results: The a*, C* and chromatic light values of the leaves decreased by 108.91%, 52.08% and 113.34%, while the brightness L values and chromatic b* values gradually increased by 36.01% and 13.94%, respectively. In the differential metabolite assay, 81 differentially expressed metabolites were detected in the R1 vs. R3 comparison, 45 were detected in the R1 vs. R2 comparison, and 75 were detected in the R2 vs. R3 comparison. Ten metabolites showed significant differences in all comparisons, which were mostly flavonoid metabolites. The metabolites that were upregulated in the three periods were cyanidin 3,5-O-diglucoside, delphinidin, and gallocatechin, with flavonoid metabolites accounting for the largest proportion and malvidin 3- O-galactoside as the primary downregulated metabolite. The color shift of red leaves from a bright purplish red to a brownish green was associated with the downregulation of malvidin 3-O-glucoside, cyanidin, naringenin, and dihydromyricetin.Discussion: Here, we analyzed the expression of flavonoid metabolites in the leaves of ‘Zhonghong’ poplar at three stages and identified key metabolites closely related to leaf color change, providing an important genetic basis for the genetic improvement of this cultivar.

To explore the mechanism underlying the leaf color variation of Populus × euramericana ‘Zhonghuahongye’ (‘Zhonghong’ poplar) leaves at different maturation stages, we used ‘Zhonghong’ poplar leaves and analyzed the L* (lightness), a* (redness), and b* (yellowness) color difference values and the pigment contents in the three maturation stages. The Illumina HiSeqTM 2000 high−throughput sequencing platform was used for transcriptome sequencing analysis, and leaf color changes during stage transitions were preliminarily explored. Overall, with the increase in L* and b* during leaf development, the a*, C* (colour saturation), and chromatic values decreased, the total anthocyanin content decreased, and the chlorophyll and carotenoid content increased. A total of 11,868 differentially expressed genes were identified by transcriptome sequencing. Comparing the expression differences of structural genes involved in anthocyanin synthesis in the leaves at different stages, we identified 5612 genes in the R1 vs. R2 comparison, 10,083 in the R1 vs. R3 comparison, and 6068 in the R2 vs. R3 comparison (R1, R2, R3 refer to samples obtained on 1 April, 6 April, and 11 April, respectively). Key genes such as DFR (Dihydroflavanol 4−reductase), ANS (anthocyanidin synthase), FLS(flavonol synthase) , CHS(chalcone synthase), BZ1(Bronze 1), bHLH35, and bHLH63 were identified. These structural genes and those that encode transcription factors may be related to the regulation of anthocyanin synthesis. Here, the key genes related to leaf color change in ‘Zhonghong’ poplar were discovered, providing an important genetic basis for the subsequent genetic improvement of ‘Zhonghong’ poplar.

Flavonoids are secondary metabolites that have economic value and are essential for health. Poplar is a model perennial woody tree that is often used to study the regulatory mechanisms of flavonoid synthesis. We used a poplar bud mutant, the red leaf poplar variety 2025 (Populus × euramericana ‘Zhonghuahongye’), and green leaves as study materials and selected three stages of leaf color changes for evaluation. Phenotypic and biochemical analyses showed that the total flavonoid, polyphenol, and anthocyanin contents of red leaves were higher than those of green leaves in the first stage, and the young and tender leaves of the red leaf variety had higher antioxidant activity. The analyses of widely targeted metabolites identified a total of 273 flavonoid metabolites (114 flavones, 41 flavonols, 34 flavonoids, 25 flavanones, 21 anthocyanins, 18 polyphenols, 15 isoflavones, and 5 proanthocyanidins). The greatest difference among the metabolites was found in the first stage. Most flavonoids accumulated in red leaves, and eight anthocyanin compounds contributed to red leaf coloration. A comprehensive metabolomic analysis based on RNA-seq showed that most genes in the flavonoid and anthocyanin biosynthetic pathways were differentially expressed in the two types of leaves. The flavonoid synthesis genes CHS (chalcone synthase gene), FLS (flavonol synthase gene), ANS (anthocyanidin synthase gene), and proanthocyanidin synthesis gene LAR (leucoanthocyanidin reductase gene) might play key roles in the differences in flavonoid metabolism. A correlation analysis of core metabolites and genes revealed several candidate regulators of flavonoid and anthocyanin biosynthesis, including five MYB (MYB domain), three bHLH (basic helix-loop-helix), and HY5 (elongated hypocotyl 5) transcription factors. This study provides a reference for the identification and utilization of flavonoid bioactive components in red-leaf poplar and improves the understanding of the differences in metabolism and gene expression between red and green leaves at different developmental stages.

Eucommia ulmoides ‘Hongye’ is a new ornamental variety of E. ulmoides with excellent red or purple foliage. We found that E. ulmoides ‘Hongye’ exhibited a gradual change from green to red colour under light conditions. However, the colouring mechanism in the leaves of E. ulmoides ‘Hongye’ remains unclear. In this study, we compared the pigment content and leaf colour index of E. ulmoides ‘Hongye’ at five stages with those of E. ulmoides ‘Xiaoye’, which was used as the control variety. The transcriptome sequencing data of the first-period (H1, green) and fifth-period (H5, red) leaves were also analysed and compared. The corresponding gene regulation in anthocyanin-related metabolic pathways was then analysed. Physiological results indicated that the contents of flavonoids and anthocyanins in red leaves (H5) were significantly higher than those in green leaves (H1), whereas the chlorophyll content in red leaves (H5) was lower than that in green leaves (H1). Moreover, the carotenoid content did not significantly differ between the two varieties. A transcriptome analysis identified 4240 differentially expressed genes (DEGs), and 20 of these genes were found to be involved in flavonoid and anthocyanin biosynthesis pathways. The results provide a reference for further study of the leaf colouration mechanism in E. ulmoides.