Differential Regulation of an OsIspH1, the Functional 4-Hydroxy-3-Methylbut-2-Enyl Diphosphate Reductase, for Photosynthetic Pigment Biosynthesis in Rice Leaves and Seeds

Deoxyxylulose 5-Phosphate Synthase Does Not Play a Major Role in Regulating the Methylerythritol 4-Phosphate Pathway in Poplar

The plastidic 2-C-methylerythritol 4-phosphate (MEP) pathway supplies the precursors of a large variety of essential plant isoprenoids, but its regulation is still not well understood. Using metabolic control analysis (MCA), we examined the first enzyme of this pathway, 1-deoxyxylulose 5-phosphate synthase (DXS), in multiple grey poplar (Populus × canescens) lines modified in their DXS activity. Single leaves were dynamically labeled with 13CO2 in an illuminated, climate-controlled gas exchange cuvette coupled to a proton transfer reaction mass spectrometer, and the carbon flux through the MEP pathway was calculated. Carbon was rapidly assimilated into MEP pathway intermediates and labeled both the isoprene released and the IDP+DMADP pool by up to 90%. DXS activity was increased by 25% in lines overexpressing the DXS gene and reduced by 50% in RNA interference lines, while the carbon flux in the MEP pathway was 25-35% greater in overexpressing lines and unchanged in RNA interference lines. Isoprene emission was also not altered in these different genetic backgrounds. By correlating absolute flux to DXS activity under different conditions of light and temperature, the flux control coefficient was found to be low. Among isoprenoid end products, isoprene itself was unchanged in DXS transgenic lines, but the levels of the chlorophylls and most carotenoids measured were 20-30% less in RNA interference lines than in overexpression lines. Our data thus demonstrate that DXS in the isoprene-emitting grey poplar plays only a minor part in controlling flux through the MEP pathway.

HDR, the last enzyme in the MEP pathway, differently regulates isoprenoid biosynthesis in two woody plants

Dimethylallyl diphosphate (DMADP) and isopentenyl diphosphate (IDP) serves as the universal C5 precursors of isoprenoid biosynthesis in plants. These compounds are formed by the last step of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, catalyzed by (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase (HDR). In this study, we investigated the major HDR isoforms of two woody plant species, Norway spruce (Picea abies) and gray poplar (Populus × canescens), to determine how they regulate isoprenoid formation. Since each of these species has a distinct profile of isoprenoid compounds, they may require different proportions of DMADP and IDP with proportionally more IDP being needed to make larger isoprenoids. Norway spruce contained two major HDR isoforms differing in their occurrence and biochemical characteristics. PaHDR1 produced relatively more IDP than PaHDR2 and it encoding gene was expressed constitutively in leaves, likely serving to form substrate for production of carotenoids, chlorophylls, and other primary isoprenoids derived from a C20 precursor. On the other hand, Norway spruce PaHDR2 produced relatively more DMADP than PaHDR1 and its encoding gene was expressed in leaves, stems, and roots, both constitutively and after induction with the defense hormone methyl jasmonate. This second HDR enzyme likely forms a substrate for the specialized monoterpene (C10), sesquiterpene (C15), and diterpene (C20) metabolites of spruce oleoresin. Gray poplar contained only one dominant isoform (named PcHDR2) that produced relatively more DMADP and the gene of which was expressed in all organs. In leaves, where the requirement for IDP is high to make the major carotenoid and chlorophyll isoprenoids derived from C20 precursors, excess DMADP may accumulate, which could explain the high rate of isoprene (C5) emission. Our results provide new insights into the biosynthesis of isoprenoids in woody plants under conditions of differentially regulated biosynthesis of the precursors IDP and DMADP.

Gene expression profile during seed development of Bixa orellana accessions varying in bixin pigment

Diverse morphological, cellular and physiological changes occur during seed maturation in Bixa orellana when the seed tissues form specialized cell glands that produce reddish latex with high bixin amounts. Transcriptomic profiling during seed development in three B. orellana accessions (P12, N4 and N5) with contrasting morphologic characteristics showed enrichment in pathways of triterpenes, sesquiterpenes, and cuticular wax biosynthesis. WGCNA allows groups of all identified genes in six modules the module turquoise, the largest and highly correlated with the bixin content. The high number of genes in this module suggests a diversification of regulatory mechanisms for bixin accumulation with the genes belonging to isoprene, triterpenes and carotene pathways, being more highly correlated with the bixin content. Analysis of key genes of the mevalonate (MVA) and the 2C-methyl-D-erythritol-4-phosphate (MEP) pathways revealed specific activities of orthologs of BoHMGR, BoFFP, BoDXS, and BoHDR. This suggests that isoprenoid production is necessary for compounds included in the reddish latex of developing seeds. The carotenoid-related genes BoPSY2, BoPDS1 and BoZDS displayed a high correlation with bixin production, consistent with the requirement for carotene precursors for apocarotenoid biosynthesis. The BoCCD gene member (BoCCD4-4) and some BoALDH (ALDH2B7.2 and ALDH3I1) and BoMET (BoSABATH1 and BoSABATH8) gene members were highly correlated to bixin in the final seed development stage. This suggested a contributing role for several genes in apocarotenoid production. The results revealed high genetic complexity in the biosynthesis of reddish latex and bixin in specialized seed cell glands in different accessions of B. orellana suggesting gene expression coordination between both metabolite biosynthesis processes.