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Theme 5: Bioprospecting of Indian Gentianaceae plants - Canscora decussata and Swertia chirata for phytopharmaceutical uses

Canscora decussata and Swertia chirata are well known Indian medicinal plants of Gentianaceae family. In addition to other phytochemicals, these plants are known to produce a wide range of xanthone compounds. In India, nervous ailments are one of the major health hazards leading to a high rate of mental disability and even mortality. Extracts of C. decussata is reported to produce memory power; fresh juice is given to insanity, epilepsy and nervous debility. The chemical structures of the xanthone compounds reported from S. chirata and C. decussata are broadly similar to those found in several species of European Gentianaceae membersshowing inhibitory activities against AChE and MAO, the two enzymes that regulate nervous stability in human brain. Therefore attempts will be made to test if the major xanthones that will be detected from the extracts of C. decussata and S. chirata have AChE and MAO inhibitory properties. The outcome of this project may provide important clues for widening the uses of these plants and in turn make an impact on the growing markets of C. decussata and S. chirata.

Limited availability of these plants throughout the year encouraged us to adapt an ex situ conservation strategy by establishing in vitro cultures. These in vitro plantlets are being screened for their bioactive constituents. TLC and HPLC analyses of extracts of in vitro grown tissues confirmed the presence of mangiferin as one of the most abundant glycosylated xanthones, along with several other xanthone aglycones, such as, decussatin. These plant extracts are now being screened for in vitro antioxidant activity using 2, 2-diphenylpicrylhydrazyl (DPPH) assay. Currently work is in progress for in-depth metabolite profiling for identification of bioactive xanthones.

A common precursor of xanthones is the rare 3-hydroxybenzoic acid which serves as substrate for benzophenone synthase (BPS).  Interestingly, 3-hydroxybenzoic acid arises from an early intermediate of the shikimate pathway and not via cinnamic acid or benzoic acid. Therefore, one of our current objectives is to look for a putative shikimate dehydratase enzyme activity from these cultures that convert shikimic acid to 3-hydroxybenzoic acid. The BPS enzyme is postulated to be type III polyketide synthases which form identical intermediates but then catalyze alternative ring closures, thereby contributing to the biosynthesis of xanthones. Very recently we have initiated research for cDNA cloning and functional expression of BPS enzyme. Furthermore, the temporal and spatial regulation of the BPS enzyme will be studied, preferably in those Gentianaceous species which accumulate both xanthones and amarogentin, such as Swertia chirata.

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