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 some limited Boraginaceae species, exhibits unique pharmacological properties and is also 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, and the copper-requiring biosynthetic step is shikonin-specific, unlike the byproduct benzoquinones. Despite the long history of shikonin biosynthesis studies for over 40 years, the copper-involved reaction has been still unknown. Here, we explored candidate genes associated with shikonin production using a PCR-select subtraction experiment. Genes encoding polyphenol oxidase (PPO), a dicopper-dependent oxidoreductase, was highlighted by the strong 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 (LePPOs), only LePPO1 showed a close correlation with shikonin production. Then, 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, while the content of deoxyshikonofuran, a hydroquinone-type shunt product that branches after 3''-hydroxygeranylhydroquinone in the shikonin biosynthetic pathway, remained unaffected in the LePPO1-ge lines. These findings address the question why a copper ion is crucial for shikonin biosynthesis and suggest that LePPO1 participates in naphthalene ring formation. Interestingly, LePPO1 is localized in plastids, whereas shikonin is accumulated in the apoplast.