Compartmentalization of Plant Secondary Metabolism

Bifunctional or multifunctional enzymes targeted to alternative subcellular compartments may interact with different substrates to produce unique products. This may partially explain the observed diversity of plant secondary products. Broad enzyme specificities have been observed for O -methyltransferases, glucosyltransferases, P450-dependent monooxygenases, polyketide synthases, and monoterpene synthases. Flavonoid biosynthesis in plants was thought previously to occur exclusively in the cytoplasm although flavonoids could accumulate in distinct subcellular compartments in different tissues. However, at least two of the enzymes of flavonoid biosynthesis occur in the nuclei of Arabidopsis cells, where the flavonoids also accumulate. Although much progress has recently been made toward the deciphering of the compartmentalization of secondary product metabolism, a comprehensive understanding of the spatial relationships among transcripts, enzymes, and biosynthetic products requires further research in several important areas.