Two independent biochemical pathways for isopentenyl diphosphate and isoprenoid biosynthesis in higher plants

Abstract

In the early times of isoprenoid research, a single pathway was found for the formation of the C5 monomer, isopentenyl diphosphate (IPP), and 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 higher 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 shown that all chloroplastbound isoprenoids (β‐carotene, phytyl chains of chlorophylls and nona‐prenyl chain of plastoquinone‐9) are formed via the GAP/pyruvate pathway, whereas the cytoplasmic sterols are formed via the acetate/mevalonate pathway. Also, isoprene, emitted by various plants at high light conditions by action of the plastid‐bound isoprene synthase, is formed via the new GAP/pyruvate pathway. Thus, in higher plants, there exist two separate and biochemically different IPP biosynthesis pathways: (1) the novel alternative GAP/pyruvate pathway 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 differences. The novel GAP/pyruvate pathway for IPP formation in plastids appears as a heritage of their prokaryotic, endosymbiotic ancestors.