Early Steps in Isoprenoid Biosynthesis: Multilevel Regulation of the Supply of Common Precursors in Plant Cells

Isoprenoids are produced in all organisms but are especially abundant and diverse in plants. Two separate pathways operate in plant cells to synthesize prenyl diphosphate precursors common to all isoprenoids. Cytosolic and mitochondrial precursors are produced by the mevalonic acid (MVA) pathway whereas the recently discovered methylerythritol phosphate (MEP) pathway is located in plastids. However, both pathways may participate in the synthesis of at least some isoprenoids under certain circumstances. Although genes encoding all the enzymes from both pathways have already been cloned, little is known about the regulatory mechanisms that control the supply of isoprenoid precursors. Genetic approaches are providing valuable information on the regulation of both pathways. Thus, recent data from overexpression experiments in transgenic plants show that several enzymes share control over the metabolic flux through the MEP pathway, whereas a single regulatory step has been proposed for the MVA pathway. Identification of Arabidopsis thaliana mutants that are resistant to the inhibition of the MVA and the MEP pathways is a promising approach to uncover mechanisms involved in the crosstalk between pathways. The characterization of some of these mutants impaired in light perception and signaling has recently provided genetic evidence for a role of light as a key factor to modulate the availability of isoprenoid precursors in Arabidopsis seedlings. The picture emerging from recent data supports that a complex regulatory network appears to be at work in plant cells to ensure the supply of isoprenoid precursors when needed.