CRISPR/Cas9-mediated genome editing reveals the involvement of a polyphenol oxidase in the shikonin-specific biosynthesis in Lithospermum erythrorhizon
Shikonin, a 1,4-naphthoquinone derivative produced by several Boraginaceae species, exhibits unique pharmacological properties and is used as a natural dye. The regulatory factors of shikonin production have been demonstrated using a cell culture system of Lithospermum erythrorhizon. Among these factors, copper is known to be the strongest enhancer of shikonin production. Although shikonin biosynthesis has been studied for over 40 years, the steps of naphthalene ring formation are still unknown, as is the reason for the effect of copper. In this study, we explored candidate genes associated with shikonin production using a PCR-select subtraction experiment. Polyphenol oxidase (PPO), a dicopper-dependent oxidoreductase, was highlighted because it showed synchronous expression with shikonin production. Transcriptome analysis of hairy roots and cultured cells of this plant revealed that, of the five PPO genes expressed in L. erythrorhizon, only PPO1 showed a strong correlation with shikonin production. Next, we generated genome-edited hairy roots of LePPO1 using CRISPR/Cas9-mediated mutagenesis to analyze its impact on shikonin derivative and other specialized metabolite production. The results showed that shikonin content was markedly reduced in all LePPO1-ge lines. Interestingly, the content of deoxyshikonofuran, a hydroquinone derivative and shunt product that branches after GHQ-3''-OH in the shikonin biosynthetic pathway, remained unaffected in the LePPO1-ge lines. These findings suggest that LePPO1 participates in naphthalene ring formation and explain why a copper ion is crucial for shikonin biosynthesis.