Comparative Transcriptome and Pigment Analyses Reveal Changes in Gene Expression Associated with Flavonol Metabolism in Yellow Camellia

The accumulation of various pigments leads to the formation of different flower colors in plants. However, the regulation mechanism of yellow flower formation and flower color differences between Camellia nitidssima C.W.Chi (CN) and its hybrids C. 'Zhenghuangqi' (ZHQ), C. 'Huangxuanlv' (HXL), and C. 'Xinshiji' (XSJ), remains largely unknown. Here, we showed that the content of two flavonols, quercetin-7-O-glucoside (Qu7G) and quercetin-3-O-glucoside (Qu3G), was positively correlated with the yellow degree of petals in CN and its three hybrids. Additionally, we performed a comparative transcriptomic analysis of petals of the four yellow camellia plants, which revealed 322 common upregulated and 866 common downregulated DEGs (differentially expressed genes) in the CN vs. ZHQ, CN vs. HXL, and CN vs. XSJ comparison groups. Their regulatory pathway analysis showed that flavonol biosynthesis genes (FLSs and GTs) and transcriptional regulatory genes MYBs were all expressed higher in CN than its three hybrids, which corresponded to differences in the flavonol content among the four yellow camellias. Further, two ethylene synthesis genes (ACSs, ACO) and three ethylene signaling genes (EIN2s, EIN3, ERFs) were all upregulated in the yellow petals of CN. In conclusion, the expression of flavonol-related genes and flavonols (Qu7G and Qu3G) accumulation could play a key role in the formation of yellow flowers in camellia, and the ethylene pathway might be involved in the regulation of yellow flower formation of camellias. This work describes the possible regulatory pathway of yellow camellia, thereby laying a foundation for future verification of genes linked to flower coloring and the breeding of yellow camellia.