Engineering Plant Alkaloid Biosynthetic Pathways: Progress and Prospects

With the successful application of molecular genetic methods to the plant alkaloid field, we now have sophisticated tools at our disposal to study regulation of enzymatic biosynthesis, as well as determining the cellular and subcellular localization of these enzymes. The availability of ever-increasing numbers of recombinant enzymes has enabled thorough analyses of selected alkaloid biosynthetic enzymes at the biochemical and structural levels. We are just beginning to use this knowledge to metabolically engineer alkaloid metabolism in plants and in in vitro cultures. Multicellular compartmentation of alkaloid pathways must be considered if meaningful metabolic engineering experiments are to be designed; for example, we will need to use promoters that drive transgene expression in the correct cell types. Regulation of these pathways at the gene and enzyme level is complex and there is still much to be learned about metabolite levels, multienzyme complexes, and pathway interconnections, as we systematically overexpress and suppress gene transcription. Today, pathway engineering in plants remains highly variable. When we perturb cellular physiology, metabolite homeostasis and intra- and intercellular partitioning can be affected in unpredictable ways. Predictive metabolic engineering to generate plants with tailored alkaloid profiles for basic research and for commercial production is clearly a challenge for the future.