BIOSYNTHESIS OF ISOPRENOIDS IN HIGHER-PLANT CHLOROPLASTS PROCEEDS VIAA MEVALONATE-INDEPENDENT PATHWAY

Isopentenyl diphosphate (IPP) is the biological C-5 precursor of isopr enoids. By labeling experiments using [1-C-13]glucose, higher plants w ere shown to possess two distinct biosynthetic routes for IPP biosynth esis: while the cytoplasmic sterols were formed via the acetate/mevalo nate pathway, the chloroplast-bound isoprenoids (beta-carotene, lutein , prenyl chains of chlorophylls and plastoquinone-9) were synthesized via a novel IPP biosynthesis pathway (glyceraldehyde phosphate/pyruvat e pathway) which was first found in eubacteria and a green alga. The d ichotomy in isoprenoid biosynthesis in higher plants allows a reasonab le interpretation of previous odd and inconclusive results concerning the biosynthesis of chloroplast isoprenoids, which so far had mainly b een interpreted in the frame of models using compartmentation of the m evalonate pathway.