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

Abstract

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.

Genome-Wide Association Study With Growth-Related Traits and Secondary Metabolite Contents in Red- and White-Heart Chinese Fir

“…Compared with WH, RH has more upstream substrates and downstream glycosylation products, which ultimately changes the color of heartwood. In a previous study, we found that the differentially expressed genes in phenylalanine pathway (PAL, C4H, and 4CL), upstream of flavonoid biosynthesis pathway (CHS, CHI and F3H) and glycosylation (UGT) were all up-regulated in the transition zone of RH compare to WH ( Cao et al, 2022 ).…”

“…In a previous study, we found that the differentially expressed genes in phenylalanine pathway (PAL, C4H, and 4CL), upstream of flavonoid biosynthesis pathway (CHS, CHI and F3H) and glycosylation (UGT) were all up-regulated in the transition zone of RH compare to WH (Cao et al, 2022). By analyzing the 12 phenotypic traits of the 300 core populations screened, it was shown that the traits of this population were little affected by the environment and can be inherited stably (Supplementary Table S1).…”

“…In the present study, no significant correlations were found with other growth-related traits. In previous studies, it was demonstrated that a large amount of metabolic activity occurred in the transition zone of xylem, with the death of ray parenchyma cells and the disappearance of starch grains, the transition zone would transform into heartwood ( Nakaba et al, 2012 ; Cao et al, 2022 ). HY refers to the weight increase of the wood after it is completely soaked in water, which is affected by WBD.…”

“…WBD and HY were measured using formula WBD = 1/( W 1/ W 2 − 0.346) and HY = ( W 1 − W 2)/ W 2, which W 1 and W 2 represent the water-saturated weight and the oven-dry weight, respectively. The contents of six secondary metabolites, including DK, DQ, luteolin, pinocembrin, apigenin, and naringenin were measured by using liquid chromatography with mass spectrometry (LC-MS/MS), and the detailed methods and parameters of LC-MS/MS were described by Cao et al (2022) . V G (genetic variance), V E (environment variance), and residual were measured by using SPSS 24.0, and H B 2 (generalized heritability) was calculated by using formula below:…”

“…Among them, red-heart (RH) Chinese fir is very popular because of its red heartwood color, high density, and has higher content of cedarwood and sclareol when compared with the sapwood ( Duan et al, 2015 ; Yang et al, 2016 ). In our earlier study, it was found that flavonoid metabolites were the main differential metabolites between red- and white-heart Chinese fir heartwood, especially luteolin-7-O-glucuronide-5-O-rhamnoside and luteolin-7-O-glucoside ( Cao et al, 2022 ). Besides, in the previous study, we collected 700 Chinese fir clones, which preserved in the Longshan state forest farm of Guangdong Province.…”