Feeding of Precursors and Bioconversions

Production can be increased by addition of precursors to the culture media, in which cases the precursors are not metabolized in the medium and, after uptake, appear in the right compartment of the plant cell. For C. roseus cultures, for example, it was found that increased indole alkaloid production was obtained after feeding with L-tryptophan, tryptamine, secologanin, loganin, loganic acid, or shikimic acid (20). Cell cultures have also been used for biotransformations, for example, the conversion of (-)-codeinone to (-)-codeine in Papaver somniferum cultures (100). For the tropane alkaloids a large number of precursor feeding and biotransformation studies with cultures of various solanaceous plants have been performed (see below). 

Bioconversion rates can be optimized by using immobilized cells, cell-free preparations, or immobilized (purified) enzymes. Furuya et al. lOI) reported the reduction of codeinone to be more efficient with immobilized P. somniferum cells than with suspended cells. Strictosidine could be produced in large quantities using immobilized strictosidine synthase 102). The potential for this use of plant cell cultures is enormous however, the number of successful applications is still limited. 

Fixation of plant cells in a matrix of, for example, polyurethane foam or entrapment of the cells in calcium alginate beads provides an artificial surrounding for the cells, which protects them from hydrodynamic stress, high cell densities inside the matrix also allow cell to cell contact and communication. Inside the immobilized matrix nutrient and product gradients may occur. Furthermore, immobilized biomass is easily separated from the medium, which is useful in production and biotransformation systems. Immobilization of plant cells has been reviewed 103,104). 

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