Gene Silencing of BnaA09.ZEP and BnaC09.ZEP Confers Orange Color in Brassica napus Flowers

Brassica napus is currently cultivated as an important ornamental crop in China. Flower color has attracted much attention in rapeseed genetics and breeding. Here, we characterize an orange-flowered mutant of B. napus that exhibits an altered carotenoid profile in its petals. As revealed by map-based cloning, the change in color from yellow to orange is attributed to the loss of BnaC09.ZEP (zeaxanthin epoxidase) and a 1,695-bp deletion in BnaA09.ZEP. HPLC analysis, genetic complementation and the CRISPR/Cas9 experiments demonstrated that BnaA09.ZEP and BnaC09.ZEP have similar functions, and the abolishment of both genes led to a substantial increase in lutein content and a sharp decline in violaxanthin content in petals but not leaves. BnaA09.ZEP and BnaC09.ZEP are predominantly expressed in floral tissues, whereas their homologs, BnaA07.ZEP and BnaC07.ZEP mainly function in leaves, indicating redundancy and tissue-specific diversification of BnaZEPs function. Transcriptome analysis in petals revealed the differences in expression of carotenoid and flavonoid biosynthesis-related genes between the mutant and its complementary lines. Flavonoid profiles in the petals of complementary lines were greatly altered compared to the mutant, indicating a potential crosstalk between the regulatory networks underlying the carotenoid and flavonoid pathways. Additionally, our results indicated that there is functional compensation by BnaA07.ZEP and BnaC07.ZEP in the absence of BnaA09.ZEP and BnaC09.ZEP. Cloning and characterization of BnaZEPs provide insights into the molecular mechanisms underlying flower pigmentation in B. napus and would facilitate breeding of B. napus varieties with higher ornamental value.

Keywords: B. napus; carotenoid biosynthesis; flavonoid biosynthesis; functional divergence; map-based cloning; orange-flower; zeaxanthin epoxidase.