Metabolite Profiling and Transcriptome Analysis Unveil the Mechanisms of Red-Heart Chinese Fir [Cunninghamia lanceolata (Lamb.) Hook] Heartwood Coloration

Red-heart Chinese fir (Cunninghamia lanceolata) has the advantages of high density and attractive color, making it popular in the market. To date, most studies about stems of woody plants have only been reported at the cytological level because of few living cells. In this study, the xylem was successfully partitioned into three effective sampling areas: sapwood, transition zone, and heartwood. Secondary metabolites, cell survival, and differentially expressed genes in the three sampling areas were, respectively, investigated. First, we identified the phenylpropanoid and flavonoid pathways closely related to color. Based on the chemical structure of secondary metabolites in pathways, two notable directions had been found. Luteolin's glycosylation products might be the key substances that regulated the color of heartwood in red-heart Chinese fir because of the 1,000-fold difference between red-heart and white-heart. We also found pinocembrin and pinobanksin in Chinese fir, which were rarely reported before. At the cytological level, we believed that the transition zone of red-heart Chinese fir was a critical region for color production because of the fewer living ray parenchyma cells. In addition, transcriptome and quantitative reverse transcription PCR (qRT-PCR) proved that genes regulating the entire phenylpropanoid pathway, upstream of the flavonoid pathway, and some glycosyltransferases were significantly upregulated in the transition zone of red-heart and then colored the heartwood by increasing metabolites. This is the first report on the color-related secondary metabolites regulated by differential genes in red-heart Chinese fir. This study will broaden our knowledge on the effects of metabolites on coloring woody plant xylems.