Comparative Analysis of Shikonin and Alkannin Acyltransferases Reveals Their Functional Conservation in Boraginaceae.

Shikonin and its enantiomer, alkannin, are bioactive naphthoquinones produced in several plants of the family Boraginaceae. The structures of these acylated derivatives, which have various short-chain acyl moieties, differ among plant species. The acylation of shikonin and alkannin in Lithospermum erythrorhizon was previously reported to be catalyzed by two enantioselective BAHD acyltransferases, shikonin O-acyltransferase (LeSAT1) and alkannin O-acyltransferase (LeAAT1). However, the mechanisms by which various shikonin and alkannin derivatives are produced in Boraginaceae plants remain to be determined. In the present study, evaluation of six Boraginaceae plants identified 23 homologs of LeSAT1 and LeAAT1, with 15 of these enzymes found to catalyze the acylation of shikonin or alkannin, utilizing acetyl-CoA, isobutyryl-CoA or isovaleryl-CoA as an acyl donor. Analyses of substrate specificities of these enzymes for both acyl donors and acyl acceptors and determination of their subcellular localization using Nicotiana benthamiana revealed a distinct functional differentiation of BAHD acyltransferases in Boraginaceae plants. Gene expression of these acyltransferases correlated with the enantiomeric ratio of produced shikonin/alkannin derivatives in L. erythrorhizon and Echium plantagineum. These enzymes showed conserved substrate specificities for acyl donors among plant species, indicating that the diversity in acyl moieties of shikonin/alkannin derivatives involved factors other than the differentiation of acyltransferases. These findings provide insight into the chemical diversification and evolutionary processes of shikonin/alkannin derivatives.

A Polyphenol Oxidase Catalyzes Aurone Synthesis in Marchantia polymorpha.

Aurones constitute one of the major classes of flavonoids, with a characteristic furanone structure that acts as the C-ring of flavonoids. Members of various enzyme families are involved in aurone biosynthesis in different higher plants, suggesting that during evolution plants acquired the ability to biosynthesize aurones independently and convergently. Bryophytes also produce aurones, but the biosynthetic pathways and enzymes involved have not been determined. The present study describes the identification and characterization of a polyphenol oxidase (PPO) that acts as an aureusidin synthase (MpAS1) in the model liverwort, Marchantia polymorpha. Crude enzyme assays using an M. polymorpha line overexpressing MpMYB14 with high accumulation of aureusidin showed that aureusidin was biosynthesized from naringenin chalcone and converted to riccionidin A. This activity was inhibited by N-phenylthiourea, an inhibitor specific to enzymes of the PPO family. Of the six PPOs highly induced in the line overexpressing MpMyb14, one, MpAS1, was found to biosynthesize aureusidin from naringenin chalcone when expressed in Saccharomyces cerevisiae. MpAS1 also recognized eriodictyol chalcone, isoliquiritigenin and butein, showing the highest activity for eriodictyol chalcone. Members of the PPO family in M. polymorpha evolved independently from PPOs in higher plants, indicating that aureusidin synthases evolved in parallel in land plants.

Two BAHD Acyltransferases Catalyze the Last Step in the Shikonin/Alkannin Biosynthetic Pathway.

Several Boraginaceae plants produce biologically active red naphthoquinone pigments, derivatives of the enantiomers shikonin and alkannin, which vary in acyl groups on their side chains. Compositions of shikonin/alkannin derivatives vary in plant species, but the mechanisms generating the diversity of shikonin/alkannin derivatives are largely unknown. This study describes the identification and characterization of two BAHD acyltransferases, shikonin O-acyltransferase (LeSAT1) and alkannin O-acyltransferase (LeAAT1), from Lithospermum erythrorhizon, a medicinal plant in the family Boraginaceae that primarily produces the shikonin/alkannin derivatives acetylshikonin and ß-hydroxyisovalerylshikonin. Enzyme assays using Escherichia coli showed that the acylation activity of LeSAT1 was specific to shikonin, whereas the acylation activity of LeAAT1 was specific to alkannin. Both enzymes recognized acetyl-CoA, isobutyryl-CoA, and isovaleryl-CoA as acyl donors to produce their corresponding shikonin/alkannin derivatives, with both enzymes showing the highest activity for acetyl-CoA. These findings were consistent with the composition of shikonin/alkannin derivatives in intact L erythrorhizon plants and cell cultures. Genes encoding both enzymes were preferentially expressed in the roots and cell cultures in the dark in pigment production medium M9, conditions associated with shikonin/alkannin production. These results indicated that LeSAT1 and LeAAT1 are enantiomer-specific acyltransferases that generate various shikonin/alkannin derivatives.