The genus Morinda belongs to the family Rubiaceae. Among the many species comprising this genus, six are of some pharmaceutical and technical importance. One of these is Morinda citrifolia which occurs in India and Southeast Asia. Its leaves and roots are used in the treatment of hypertension or as a diuretic and laxative. A more recent study shows that extracts of the roots exhibit an analgesic and probably sedative effect on mice. Morinda lucida, another plant dealt with in this Chapter, grows in central Africa. Natives of central Africa use the plant as a diuretic, purgative and in the treatment of leprosy, fever, malaria, yellow fever, diarrhea and dysentery.
The technical use of Morinda plants as a dye is based on the occurrence of anthraquinone pigments in the roots. The pigments isolated from both M. citrifolia and M. lucida plants are listed in Thomson (1987). A publication by Demagos et al. (1981) and reviews by Wijnsma and Verpoorte (1986) as well as by van den Berg and Labadie (1989) contain later additions to the array of known anthraquinones. While roots are the main source of anthraquinones, pigments are also present in the heartwood, leaves and even flowers. The anthraquinones present in cell suspension cultures of both M. lucida () and M. citrifolia () have been identified. Main pigments in the M. lucida cell culture are anthraquinone glycosides such as lucidin primveroside, soranjidiole prim-veroside and morindine. The pigments were quantitatively determined by HPCL. From the M. citrifolia cell suspension culture, rubiadin, morindone, and lucidin (which occurs only in minor amount in the intact plant), nordamnacanthal and alizarin have been isolated. It was later shown that lucidin and morindone occur also as their primverosides. In addition, seven anthraquinone aglycones were also detected in this study, six of which were hitherto unknown compounds. Similar observations were made by Wijnsma et al. (1986): Seven known and five new anthraquinones were detected in Cinchonapubescens cell cultures, whereas seven known and eight new ones were found in Cinchona ledgeriana. It is a common observation that natural products occurring in cell cultures are new compounds which have not yet been isolated from intact plants. The potential to gain new and hitherto unknown compounds from cell cultures is only insufficiently exploited. The capability of cell cultures to produce certain metabolites may change during subculture (van den Berg and Labadie 1989). This would explain why alizarin and nordamnacanthal originally isolated from M. citrifolia cell cultures were not detectable during a later assessment of the constituents of the same M. citrifolia cell culture strain.
Anthraquinones are not the only natural products present in plants of the family Rubiaceae. Purines, tannins, naphthols, coumarins and iridoids are examples. Nevertheless, cell cultures derived from Rubiaceae plants have been analyzed for the presence of natural products other than anthraquinones in rare cases only. This may be due to the fact that anthraquinones are coloured compounds, that high producing cell lines are easily selected visually and that the chemistry and spectroscopy of anthraquinones are well understood. One of the exceptions is the Galium mollugo cell culture which was found to contain prenylated naphthalene derivatives such as 3,4-dihydro-2,2-dimethylnaphtho-1, 2b-pyran, 1-naphthylisopentenylether, mollugin (4) and 1,4-dihydroxy-3prenyl-2-naphthoic acid methyl ester diglucoside (4).
In the same culture, however, asperuloside, an iridoid occurring in the intact plant, was not detectable.
A photoautotrophic cell suspension culture has been raised from M. lucida leaves. The lipoquinones (phylloquinone, i.e. vitamin Kl, plastoquinone, ubiquinone, and tocopherol, i.e. vitamin E) were isolated, identified and determined quantitatively. The quantity of lipoquinones matched that of the intact leaf.
Summary and Conclusions
Cell cultures of Rubiaceae plants, including Morinda, readily produce large amounts of anthraquinones. The production can be influenced by different medium components (such as amino acids and carbohydrates) and especially by hormones. Anthraquinones are produced in heterotrophic, but not in autotrophic cell cultures. The latter type of culture contains Hpoquinones. The switch from autotrophy to heterotrophy correlates with degradation of Hpoquinones and induction of anthraquinone synthesis.
The cell cultures are suitable systems to extract enzymes of anthraquinone biosynthesis. The initial steps in anthraquinone synthesis parallel bacterial menaquinone (vitamin K2) biosynthesis at the substrate level. The enzymes involved, however, differ remarkably.
Selections from the book: Medicinal and Aromatic Plants VIII (1995).