Abstract
Purple coneflower (Echinacea purpurea (L.) Moench) is a popular native North American herbal plant. Its major bioactive compound, chicoric acid, is reported to have various potential physiological functions, but little is known about its biosynthesis. Here, taking an activity-guided approach, we identify two cytosolic BAHD acyltransferases that form two intermediates, caftaric acid and chlorogenic acid. Surprisingly, a unique serine carboxypeptidase-like acyltransferase uses chlorogenic acid as its acyl donor and caftaric acid as its acyl acceptor to produce chicoric acid in vacuoles, which has evolved its acyl donor specificity from the better-known 1-O-β-D-glucose esters typical for this specific type of acyltransferase to chlorogenic acid. This unusual pathway seems unique to Echinacea species suggesting convergent evolution of chicoric acid biosynthesis. Using these identified acyltransferases, we have reconstituted chicoric acid biosynthesis in tobacco. Our results emphasize the flexibility of acyltransferases and their roles in the evolution of specialized metabolism in plants.
Highlights
Purple coneflower (Echinacea purpurea (L.) Moench) is a popular native North American herbal plant
Based on the structural similarity of caftaric acid to chlorogenic acid, we investigated whether hydroxycinnamoylCoA: tartaric acid hydroxycinnamoyl transferase (HTT) could use caffeoyl CoA as an acyl donor to produce caftaric acid as reported[9]
When tartaric acid and caffeoyl CoA were incubated with a crude protein extract from hairy roots of purple coneflower, we detected the production of caftaric acid, indicating the presence of HTT activity
Summary
Purple coneflower (Echinacea purpurea (L.) Moench) is a popular native North American herbal plant. A unique serine carboxypeptidaselike acyltransferase uses chlorogenic acid as its acyl donor and caftaric acid as its acyl acceptor to produce chicoric acid in vacuoles, which has evolved its acyl donor specificity from the better-known 1-O-β-D-glucose esters typical for this specific type of acyltransferase to chlorogenic acid This unusual pathway seems unique to Echinacea species suggesting convergent evolution of chicoric acid biosynthesis. Hydroxycinnamoyl-Coenzyme A: quinate hydroxycinnamoyl transferase (HQT), a BAHD acyltransferase family member, is involved in the biosynthesis of chlorogenic acid in different plant species[10,11,12] Another important acyltransferase group is the serine carboxypeptidase-like (SCPL) family. Two BAHD enzymes, EpHTT and EpHQT are responsible for the biosynthesis of caftaric acid and chlorogenic acid in the cytosol, respectively These two CADs serve as substrates for the generation of chicoric acid in the vacuole by a unique SCPL enzyme. Our results expand the understanding of acyltransferases and highlight the rapid evolution possible with these types of enzymes
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