Metabolic Flux Analysis of Plastidic Isoprenoid Biosynthesis in Poplar Leaves Emitting and Nonemitting Isoprene

摘要

The plastidic 2-C-methyl-d-erythritol-4-phosphate () pathway is one of the most important pathways in plants and produces a large variety of essential isoprenoids. Its regulation, however, is still not well understood. Using the stable isotope ¹³C-labeling technique, we analyzed the carbon fluxes through the pathway and into the major plastidic isoprenoid products in isoprene-emitting and transgenic isoprene-nonemitting () gray poplar (Populus × canescens). We assessed the dependence on temperature, light intensity, and atmospheric [CO2]. Isoprene biosynthesis was by far (99%) the main carbon sink of pathway intermediates in mature gray poplar leaves, and its production required severalfold higher carbon fluxes compared with leaves with almost zero isoprene emission. To compensate for the much lower demand for carbon, leaves drastically reduced the overall carbon flux within the pathway. Feedback inhibition of 1-deoxy-d-xylulose-5-phosphate synthase activity by accumulated plastidic dimethylallyl diphosphate almost completely explained this reduction in carbon flux. Our data demonstrate that short-term biochemical feedback regulation of 1-deoxy-d-xylulose-5-phosphate synthase activity by plastidic dimethylallyl diphosphate is an important regulatory mechanism of the pathway. Despite being relieved from the large carbon demand of isoprene biosynthesis, plants redirected only approximately 0.5% of this saved carbon toward essential nonvolatile isoprenoids, i.e. β-carotene and lutein, most probably to compensate for the absence of isoprene and its antioxidant properties.