2 INDEPENDENT BIOCHEMICAL PATHWAYS FOR ISOPENTENYL DIPHOSPHATE AND ISOPRENOID BIOSYNTHESIS IN HIGHER-PLANTS

In the early times of isoprenoid research, a single pathway was found for the formation of the C-5 monomer, isopentenyl diphosphate (IPP), a nd this acetate/mevalonate pathway was supposed to occur ubiquitously in all living organisms. Now, 40 years later, a totally different IPP biosynthesis route has been detected in eubacteria, green algae and hi gher plants. In this new pathway glyceraldehyde 3-phosphate (GAP) and pyruvate are precursors of isopentenyl diphosphate, but not acetyl-CoA and mevalonic acid. In green tissues of three higher plants it was sh own that all chloroplast-bound isoprenoids (beta-carotene, phytyl chai ns of chlorophylls and nona-prenyl chain of plastoquinone-9) are forme d via the GAP/pyruvate pathway, whereas the cytoplasmic sterols are fo rmed via the acetate/mevalonate pathway. Also, isoprene, emitted by va rious plants at high light conditions by action of the plastid-bound i soprene synthase, is formed via the new GAP/pyruvate pathway. Thus, in higher plants, there exist two separate and biochemically different I PP biosynthesis pathways: (1) the novel alternative GAP/pyruvate pathw ay apparently bound to the plastidic compartment and (2) the classical cytoplasmic acetate/mevalonate pathway. This new GAP/pyruvate pathway for IPP formation allows a reasonable interpretation of previous odd results concerning the biosynthesis of chloroplast isoprenoids, which, so far, had mainly been interpreted assuming compartmentation differe nces. The novel GAP/pyruvate pathway for IPP formation in plastids app ears as a heritage of their prokaryotic, endosymbiotic ancestors.