Physiological, biochemical and transcriptomic analyses reveal the mechanism of variation in color traits in Panax japonicus fruits

Panax japonicus C. A. Mey. is a rare and endangered Class II important protected wild medicinal plant in China with a wide range of pharmacological activities. The development of the fruit is special, only the apical capitulum can develop into fruit normally, and the color trait changes during the maturation process showed a pattern from green to red and from red to purple-black. In order to reveal the phenomenon of color trait changes during fruit development, this paper analyzed the correlation between color trait changes and pigment contents (chlorophyll, carotenoids) and secondary metabolite contents (anthocyanin, flavonoids, and total phenols) of the fruits, and used transcriptome sequencing to explore the correlation between color trait changes and synthetic genes, and then verified the fluorescence quantification with qRT-PCR. The results showed that chlorophyll was gradually converted and carotenoids began to appear in the pericarp when the fruit entered the color change stage from the green fruit stage; the content of anthocyanin, flavonoids and total phenolic substances gradually increased during fruit ripening. Transcriptome sequencing obtained transcriptome data of 15 samples from five periods of color trait changes in Panax japonicus fruits 93.94 Gbp, with 462,117 functionally annotated Total Unigenes; the total number of DETs generated by intercomparison of the five periods (G, Y, R, B, P) was 17,895 (up-regulated 9435 and down-regulated 8460). Weighted gene co-expression network analysis (WGCNA) indicated that 448 were hub differential genes (DETs) associated with common involvement in the color changes (carotenoids, anthocyanins, flavonoids, total phenols) in the fruits of Duchess. GO and KEGG were enriched to participate in the photosynthesis-antenna protein pathway 19; involved in flavonoid biosynthesis a total of 205. The results of qRT-PCR fluorescence quantitative validation showed that chlorophyll degradation related 6 candidate genes PjCAB and fruit color change flavonoid biosynthesis 6 candidate genes PjF3'H, PjCHI, PjCHS, PjDFR, PjANS, and PjC3'H at five periods were consistent with the transcriptome sequencing results.