The physiological significance of chicoric acid biosynthesis in purple coneflower still needs further exploration. In addition to ultraviolet protection, as the major secondary metabolite of purple coneflower, chicoric acid may possess other biological roles, such as acting as an allelochemical (Jing-Lei et al., 2021). The biosynthesis of chicoric acid not only provides a potent metabolite for plant defense but also alleviates the possible toxic of chlorogenic acid to itself (Fu et al., 2021a, Neetu et al., 2020). Chicoric acid in purple coneflower leaf provides ultraviolet protection through acting as antioxidant and sunscreen and is better than its two substrates, suggesting its biosynthesis’s physiological signification (Fu et al., 2021c).

Plants generate diverse secondary metabolites for defense against biotic and abiotic stresses (Saijo and Loo, 2020). They both use caffeoyl CoA as the acyl donor, produced by hydroxycinnamoyl-CoA: shikimate/quinate hydroxycinnamoyl transferase (HCT) from the phenylpropanoid pathway (Weng and Chapple, 2010). In the upstream pathway, caftaric acid and chlorogenic acid are synthesized by two BAHD acyltransferases, namely hydroxycinnamoyl-CoA: tartaric acid hydroxycinnamoyl transferase (HTT) and hydroxycinnamoyl-CoA: quinate hydroxycinnamoyl transferase (HQT), using tartaric acid and quinic acid as acyl acceptor, respectively (Fu et al., 2021a). Chicoric acid is synthesized from caftaric acid and chlorogenic acid by a special serine carboxypeptidase-like (SCPL) family transferases, chicoric acid synthase (CAS) (Fu et al., 2021a), which has changed its acyl preference from glucose ester to chlorogenic acid (Bontpart et al., 2015, Fu et al., 2021a). Chicoric acid is the major bioactive compound in purple coneflower with various health benefits (Fu et al., 2021b, Lema-Rumińska et al., 2019, Peng et al., 2019). Purple coneflower ( Echinacea purpurea (L.) Moench) is a vital medicinal plant with high commercial interest (Smith et al., 2020), and serves as a popular ornamental plant around the world (Dehestani-Ardakani et al., 2020). In summary, these results demonstrate the metabolic change and its biological meaning during the germination process of purple coneflower seeds.

thaliana than its two substrates, indicating the significance of its biosynthesis. Interestingly, during the seed germination process, chicoric acid was secreted into the culture medium and showed a more substantial growth inhibition effect on A. The upstream biosynthetic genes providing essential precursor caffeoyl CoA was also induced since sprout explain the increase of the sum of these compounds. Accordingly, the structural genes responsible for the biosynthesis of caftaric acid and chicoric acid were upregulated since sprout, just a little head of accumulation of chemicals. The contents of caftaric acid and chicoric acid kept on increasing after sprout, while chlorogenic acid decreased and converted to chicoric acid. Here, through a non-target metabolomic analysis, significant metabolic shifts, especially the dynamics of the contents of chicoric acid and its two substrates, were observed during the seed germination process. The diversity mechanism of these chemicals and the underlying physiological significance remain unknown. Interestingly, the relative contents of chicoric acid and its two substrates in seeds were different from those of other tissues. It is synthesized from caftaric acid and chlorogenic acid (5-caffeoyl quinic acid) by a special acyltransferase. Known as a bioactive compound with various biological functions for plants and humans, chicoric acid is the major secondary metabolite presented in the whole plant of purple coneflower ( Echinacea purpurea (L.) Moench).

Purple coneflower seedlings