Beyond the Obstacles: Molecular Biological Approaches to Improve Productivity of Secondary Metabolites in Plant Cells

After the establishment of genetic engineering techniques, the modification of metabolic pathways and genetic regulation of secondary metabolism became feasible. In fact, the industrial production of secondary metabolites is more feasible if we can improve productivity, either by reducing the culture period or increasing the level of metabolites (Misawa, 1994). Furthermore, it may be possible to improve the quality of metabolites by

FIGURE 11.7 Strategies for overcoming the limitations in the production of metabolites in plant cells. (1) Preexisting pathway, (2) overexpression of rate-limiting (early) step to clarify the regulation and to overproduce metabolites, (3) creation of a new branch pathway to produce novel compound F and to increase the sink strength, (4) reducing an undesired pathway to enhance the metabolic flow to a desired path, (5) overexpression of transcriptional factor(s) to activate the entire biosynthetic pathway simultaneously, (6) mutation/downregulation of gene expression of biosynthetic enzyme to accumulate the intermediate of pathway.

reducing undesired pathways, or introducing new pathways to produce novel compounds, or by completely blocking a pathway to accumulate intermediates. Such metabolic engineering would modify plant cells to become ‘‘green chemical factories’’ to obtain the desired compounds. In the sections below, we discuss how genetic engineering could be useful for removing obstacles for the production of metabolites in cell culture or intact plants; general strategies are shown in Fig. 11.7 (also see Croteau et al., 2000; Verpoorte and Memelink, 2002). One promising result suggests that transformation itself might be effective for overcoming these obstacles. C. roseus cells transformed with Agrobacterium tumefaciens or A. rhizogenes gave a stable production of vindoline, which was not found in cultured cells (O'Keefe et al., 1997). This metabolic activation in transformed cells suggests the possibility that the cell type-specific expression required for alkaloid biosynthesis may not be absolutely necessary under all circumstances.