The Deoxyxylulose Phosphate Pathway for the Biosynthesis of Plastidic Isoprenoids: Early Days in Our Understanding of the Early Stages of Gibberellin Biosynthesis

The identification of a novel pathway for isopentenyl diphosphate synthesis by Rohmer, Arigoni and colleagues in the early 1990's has led to a reappraisal of terpenoid biosynthesis in many organisms. It is now apparent that in plants there are two biosynthetic routes to isopentenyl diphosphate-the classical mevalonate pathway in the cytosol and the deoxyxylulose phosphate pathway in plastids. Sesquiterpenoids and sterols are predominantly synthesized in the cytosol by the mevalonate pathway whereas monoterpenoids, diterpenoids, the phytol side-chain of chlorophyll, carotenoids, and the nonaprenyl side-chain of plastoquinone-9 are synthesized within plastids by the deoxyxylulose phosphate pathway. Our assumptions that the early stages of gibberellin biosynthesis are plastid-localized has led to several attempts to demonstrate that the deoxyxylulose phosphate pathway is the biosynthetic route to gibberellins. Although definitive evidence is still not available there is a growing body of evidence, mostly from transgenic plants and from the use of the inhibitor, fosmidomycin, that gibberellins are synthesized from deoxyxylulose phosphate-derived isopentenyl diphosphate. However, there is evidence that a small amount of cross-talk between the two pathways may occur, implying that the pathways are not totally autonomous. Implications for the regulation of the early stages of gibberellin biosynthesis are discussed.