Abstract
The 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway leads to the synthesis of isopentenyl diphosphate in plastids. It is a major branch point providing precursors for the synthesis of carotenoids, tocopherols, plastoquinone and the phytyl chain of chlorophylls, as well as the hormones abscisic acid and gibberellins. Consequently, disruption of this pathway is harmful to plants. We developed an in vivo bioassay that can measure the carbon flow through the carotenoid pathway. Leaf cuttings are incubated in the presence of a phytoene desaturase inhibitor to induce phytoene accumulation. Any compound reducing the level of phytoene accumulation is likely to interfere with either one of the steps in the MEP pathway or the synthesis of geranylgeranyl diphosphate. This concept was tested with known inhibitors of steps of the MEP pathway. The specificity of this in vivo bioassay was also verified by testing representative herbicides known to target processes outside of the MEP and carotenoid pathways. This assay enables the rapid screen of new inhibitors of enzymes preceding the synthesis of phytoene, though there are some limitations related to the non-specific effect of some inhibitors on this assay.
Highlights
The terms isoprenoid, terpenoid, and terpene are used interchangeably in the literature to refer to a broad class of natural products derived from C5 isopentenyl diphosphate (IPP) [1,2]
It has been suggested that inhibiting the methyl-D-erythritol 4-phosphate (MEP) pathway of plants could be useful in the search of novel herbicides [55] and the bioassay described may be a useful new tool to discover such compounds
It is highly sensitive to clomazone [57], which is important because some plants do not metabolize clomazone to ketoclomazone very rapidly, and their inhibitory effects may not be detected during the time-span of this experiment
Summary
The terms isoprenoid, terpenoid, and terpene are used interchangeably in the literature to refer to a broad class of natural products derived from C5 isopentenyl diphosphate (IPP) [1,2]. Plants produce a myriad of isoprenoids that are functionally important in many physiological and biochemical processes [3,4]. Two independent biosynthetic routes have been selected for the synthesis of these two basic building blocks [8]. In the cytosol and mitochondria, IPP and dimethylallyl diphosphate (DMAPP) are assembled from three molecules of acetyl-CoA by the mevalonate (MVA) pathway. This pathway was first described in the early work of Bloch and Lynen [9,10], and was thought to be the sole source of all terpenoids. It is known that it is responsible for the synthesis of sterols and ubiquinone. The MVA pathway is the subject of several reviews [5,11], and is not the focus of this paper
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