Some new aspects of isoprenoid biosynthesis in plants--a review

Plants are capable of synthesizing a myriad of isoprenoids and prenyl lipids. Much attention has been focused on 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the enzyme that synthesizes mevalonate and is generally considered responsible for the regulation of substrate flux to isoprenoids. In contrast to vertebrates, where there seems to exist only one HMGR gene, in plants a small family of isogenes appears differentially expressed in regard to location and time. Much less is known in plants about the preceding steps, viz. the conversion of acetyl-CoA to HMG-CoA. An enzyme system has been isolated from radish that can catalyze this transformation, and which shows some unusual properties in vitro. The intracellular localization of the early steps of isoprenoid biosynthesis in plant cells is still a matter of debate. The various observations and hypotheses derived from incorporation and inhibition studies are somewhat contradictory, and an attempt is being made to rationalize various findings that do not at first seem compatible. There are good arguments in favor of an exclusively cytoplasmic formation of isopentenyl pyrophosphate (IPP) via mevalonic acid, but other studies and observations suggest an independent formation in plastids. Other possibilities are being considered, such as the existence of independent (compartmentalized) biosynthetic pathways of IPP formation via the so-called Rohmer pathway. Substrate channeling through the formation of end product-specific multienzyme complexes (metabolons) with no release of substrate intermediates will also be discussed.