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Rosmarinic acid, amongst other caffeic acid derivatives, belongs to the so called „Labiatengerbstoffe”. They are derived from the metabolism of aromatic amino acids and are important plant responses to herbivores as well as to viral, bacterial or fungal infections. In animal and cell culture experiments these depsides showed interesting effects and it is reported, that they are beneficial for human health. The biosynthetic pathway of rosmarinic acid is elucidated in Lamiaceae. Its precursors are L-phenylalanine and L-tyrosine, which are transformed to rosmarinic acid, an ester of caffeic acid and 3,4-dihydoxyphenyllactic acid, with five soluble cytosolic and three ER-membrane bound enzymes. The key enzyme of this pathway is rosmarinic acid synthase (RAS). It is a hydroxycinnamoyltransferase (HCT), which belongs to the superfamily of BAHD acyltransferases. It catalyses the ester-forming reaction between the coenzyme A-activated 4-coumaric acid and 4-hydroxyphenyllactic acid. This reaction is similar to other known HCT reactions responsible for the formation of chlorogenic acid or caffeoylshikimic acid. There are many other caffeic acid derivatives formed by HCTs or representatives from other enzyme families. The hornwort Anthoceros agrestis, family Anthocerotaceae and division Anthocerophyta, can produce and accumulate rosmarinic acid and other caffeic acid derivatives in cell suspension cultures. Therefore A. agrestis is the earliest known land plant, which can produce rosmarinic acid. However, the biosynthetic pathway of rosmarinic acid production in hornworts is not known.