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The metabolism of massively accumulated chlorogenic acid is crucial for the successful germination of purple coneflower (Echinacea purpurea (L.) Menoch). A serine carboxypeptidase-like (SCPL) acyltransferase (chicoric acid synthase, CAS) utilizes chlorogenic acid to produce chicoric acid during germination. However, it seems that the generation of chicoric acid lags behind the decrease in chlorogenic acid, suggesting an earlier route of chlorogenic acid metabolism. We discovered another chlorogenic acid metabolic product, 3,5-dicaffeoylquinic acid, which is produced before chicoric acid, filling the lag phase. Then, we identified two additional typical clade IA SCPL acyltransferases, named chlorogenic acid condensing enzymes (CCEs), that catalyze the biosynthesis of 3,5-dicaffeoylquinic acid from chlorogenic acid with different kinetic characteristics. Chlorogenic acid inhibits radicle elongation in a dose-dependent manner, explaining the potential biological role of SCPL acyltransferases-mediated continuous chlorogenic acid metabolism during germination. Both CCE1 and CCE2 are highly conserved among Echinacea species, supporting the observed metabolism of chlorogenic acid to 3,5-dicaffeoylquinic acid in two Echinacea species without chicoric acid accumulation. The discovery of SCPL acyltransferase involved in the biosynthesis of 3,5-dicaffeoylquinic acid suggests convergent evolution. Our research clarifies the metabolism strategy of chlorogenic acid in Echinacea species and provides more insight into plant metabolism.
Huang et al. (Fri,) studied this question.
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