Integrated Metabolome and Transcriptome Analyses Provide New Insights into the Leaf Color Changes in Osmanthus fragrans cv. 'Wucaigui'

Osmanthus fragrans, belonging to the family Oleaceae, is listed as one of the most important traditional ornamental plant species in China. A new cultivar O. fragrans 'Wucaigui' has a very diversified form in terms of leaf colors, in which the leaf color changes from red to yellow-green and finally to dark green. To understand the mechanisms involved in leaf color changes, metabolome and transcriptome studies were performed on leaves at different developmental stages. A total of 79 metabolites, two chlorophyll, 26 carotenoids, and 51 anthocyanins, were detected in the 6 different developmental stages. An orthogonal partial least squares discriminant analysis identified key metabolites at different developmental stages, including lutein, pelargonidin-3-O-(6-O-p-coumaroyl)-glucoside, neoxanthin, and alpha-carotene. A total of 48,837 genes were obtained by transcriptome sequencing, including 3295 novel genes. Using a weighted gene co-expression network analysis to study the correlations between key metabolites and differentially expressed genes, we determined the characteristic modules having the highest correlations with key metabolites and selected associated candidate genes. Five genes (OfSHOU4L, OfATL1B, OfUGE5 OfEIF1AX, and OfUGE3) were finally identified as hub genes using real-time fluorescence quantitative PCR. In addition, we proposed a model based on the changes in key metabolite contents and the network regulatory map during the changes in O. fragrans 'Wucaigui' leaf color. The positive regulation of OfUGE3 led to an increase in the lutein content, which resulted in the leaves changing from grayish brown to moderate brown; during the change from moderate brown to dark greenish-yellow, the positive regulation of three genes (OfHOU4L, OfATL1B, and OfUGE5) increased the content of pelargonidin-3-O-(6-O-p-coumaroyl)-glucoside and the red color of the leaves gradually faded to dark greenish-yellow and then to strong yellow-green; the positive regulation of OfEIF1AX increased the content of neoxanthin; the stages in which the color changed from strong yellow-green to yellow-green and then to moderate olive-green were positively regulated by OfUGE3, which resulted in higher alpha-carotene content. These findings provided new insights into the mechanisms underlying the processes involved in O. fragrans 'Wucaigui' leaf color changes at the metabolic and transcriptional levels. This work seeks to contribute to the development of artificial regulate and control technology in the breeding and production of O. fragrans and other ornamental plants.