The Biosynthesis of Alkaloids in Root Cultures

Abstract

Alkaloid production by in vitro cultures has a long history. Root cultures of alkaloid-producing species were established more than 50 years ago (Butcher and Street, 1964) but it was not until the 1950s that alkaloid production was demonstrated in such systems (Mitra, 1972; Solt, 1957). Research was limited, however, because of the difficulty of establishing and maintaining root cultures. The advent of dispersed suspension cultures stimulated a period of intense activity and major advances were made in the biochemistry of some alkaloid classes, notably the indole and bisisoquinoline types (Zenk, 1989). These examples proved, unfortunately, to be exceptions rather than the rule and several other important classes, such as the tropanes, quinolines, and pyrrolizidines, were accumulated at only very low levels or not at all. Several groups in the early 1980s reexamined the possibility of using root cultures for alkaloid production and the establishment of cultures of Hyoscyamus niger (Hashimoto and Yamada, 1983) and Senecio vulgaris (Hartmann and Toppel, 1987; Toppel et al., 1987) were reported. In 1986, a further step forward was made when Hamill et al. (1986) reported the use of transformed root cultures for nicotine formation. Four years later, the same authors demonstrated that nicotine accumulation in Nicotiana rustica transformed roots could be increased by genetic engineering (Hamill et al., 1990). The advantage of transformation is that, by engineering the delivery system, genes can be inserted into the plant (Jones et al., 1992). Furthermore, transformed roots have no phytohormone requirement and show good stability in culture.