Pathways Associated With Rosmarinic Acid Biosynthesis

It has been shown that two aromatic amino acids, phenylalanine and tyrosine, are precursors of RA biosynthesis (37). Using radioactive phenylalanine and tyrosine, it was established that they are incorporated into caffeic acid and 3,4-dihydroxyphenyllactic acid moieties, respectively (37). Steps in RA biosynthesis originating from phenylalanine and tyrosine have been characterized (Fig. 2) (13,14,30,38, 39). In several cell cultures, the activity of phenylalanine-ammonia-lyase was correlated to RA (13,14). Using An-chusa officinalis cell suspension cultures, it was reported that tyrosine aminotransferase catalyzes the first step of the transformation of tyrosine to 3,4-dihydroxyphenyllac-tic acid. Several isoforms of tyrosine aminotransferase were found to be active in cell suspension cultures of An-chusa officinalis (13,39). Prephenate aminotransferase in Anchusa cell suspension cultures was found to be important; its activity was affected by 3,4-dihydroxyphenyllactic acid (40). Other enzymes of late steps in the RA biosynthesis pathway, such as hydroxyphenylpyruvate reductase and RA synthase, were isolated and characterized in cell cultures of Coleus blumei (41—43).

Recently, microsomal hydroxylase activities that introduce hydroxyl groups at positions 3 and 3' of the aromatic rings of ester 4-coumaroyl-4'-hydroxyphenyllactate to give rise to RA were isolated (30). This led to the proposed complete biosynthetic pathway for RA biosynthesis originating from phenylalanine and tyrosine.

These reports point to good success in understanding RA biosynthesis from phenylalanine and tyrosine using various cell suspension cultures. However, several major issues have to be addressed to gain access and to control the interacting metabolic fluxes critical to RA biosynthesis for subsequent metabolic engineering. There are major gaps in understanding and significant questions to be answered: (1) What is the role of primary metabolism, particularly the pentose phosphate pathway? (2) How is the pentose phosphate pathway regulated during RA synthesis? (3) What is the role of light in regulating RA biosynthesis? (4) How can the problems associated with genetic instability of undifferentiated callus cultures be resolved? (5) How can the understanding of metabolic pathways and subsequent engineering of efficient RA biosynthesis be used to develop elite varieties for traditional and contemporary agricultural production systems?

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