Characterization of a Unique Pair of Ferredoxin and Ferredoxin NADP+ Reductase Isoforms That Operates in Non-Photosynthetic Glandular Trichomes

Our recent investigations indicated that isoforms of ferredoxin (Fd) and ferredoxin NADP+ reductase (FNR) play essential roles for the reductive steps of the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway of terpenoid biosynthesis in peppermint glandular trichomes (GTs). Based on an analysis of several transcriptome data sets, we demonstrated the presence of transcripts for a leaf-type FNR (L-FNR), a leaf-type Fd (Fd I), a root-type FNR (R-FNR), and two root-type Fds (Fd II and Fd III) in several members of the mint family (Lamiaceae). The present study reports on the biochemical characterization of all Fd and FNR isoforms of peppermint (Mentha × piperita L.). The redox potentials of Fd and FNR isoforms were determined using photoreduction methods. Based on a diaphorase assay, peppermint R-FNR had a substantially higher specificity constant (kcat/Km) for NADPH than L-FNR. Similar results were obtained with ferricyanide as an electron acceptor. When assayed for NADPH-cytochrome c reductase activity, the specificity constant with the Fd II and Fd III isoforms (when compared to Fd I) was slightly higher for L-FNR and substantially higher for R-FNR. Based on real-time quantitative PCR assays with samples representing various peppermint organs and cell types, the Fd II gene was expressed very highly in metabolically active GTs (but also present at lower levels in roots), whereas Fd III was expressed at low levels in both roots and GTs. Our data provide evidence that high transcript levels of Fd II, and not differences in the biochemical properties of the encoded enzyme when compared to those of Fd III, are likely to support the formation of copious amounts of monoterpene via the MEP pathway in peppermint GTs. This work has laid the foundation for follow-up studies to further investigate the roles of a unique R-FNR-Fd II pair in non-photosynthetic GTs of the Lamiaceae.

Differential expressed analysis of Tripterygium wilfordii unigenes involved in terpenoid backbone biosynthesis

Tripterygium wilfordii Hook. f. is the traditional medicinal plants in China. Triptolide, wilforgine, and wilforine are the bioactive compounds in T. wilfordii. In this study, the contents of three metabolites and transcription levels of 21 genes involved in three metabolites biosynthesis in T. wilfordii were examined using high-performance liquid chromatography and reverse transcription PCR after application of methyl jasmonate (MeJA) on hairy roots in time course experiment (3-24 h). The results indicated that application of MeJA inhibited triptolide accumulation and promoted wilforgine and wilforine metabolites biosynthesis. In hairy roots, wilforgine content reached 693.36 μg/g at 6 h after adding MeJA, which was 2.23-fold higher than control. The accumulation of triptolide and wilforine in hairy roots increased the maximum at 9 h, which was 1.3- and 1.6-folds more than the control. Most of the triptolide secretes into the medium, but wilforgine and wilforine cannot secrete into the medium. The expression levels of unigenes which involved terpenoid backbone biosynthesis exist the correlation with marker metabolites (triptolide, wilforgine and wilforine) after induction by MeJA, and can be then used to infer flux bottlenecks in T. wilfordii secondary metabolites accumulation. These results showed that these genes may have potential applications in the metabolic engineering of T. wilfordii metabolites production.

The amino-terminal conserved domain of 4-hydroxy-3-methylbut-2-enyl diphosphate reductase is critical for its function in oxygen-evolving photosynthetic organisms

4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR), also known as isoprenoid synthesis H (IspH) or lysis-tolerant B (LytB), catalyzes the last step of the methylerythritol phosphate pathway to synthesize isopentenyl diphosphate and dimethylallyl diphosphate. The structure and reaction mechanism of IspH have been actively investigated in Escherichia coli but little is known in plants. Compared with the bacterial IspH, cyanobacterial and plant HDRs all contain an extra N-terminal conserved domain (NCD) that is essential for their function. Tyr72 in the NCD and several plant-specific residues around the central active site are critical for Arabidopsis HDR function. These results suggest that the structure and reaction mechanism of HDR/IspH may be different between plants and bacteria. The E. coli IspH is an iron-sulfur protein that is sensitive to oxygen. It is possible that the cyanobacterial HDR may independently evolve from the common ancestor of prokaryotes to obtain the NCD, which may protect the enzyme from high concentration of oxygen during photosynthesis.

Both methylerythritol phosphate and mevalonate pathways contribute to biosynthesis of each of the major isoprenoid classes in young cotton seedlings

In higher plants, both the methylerythritol phosphate (MEP) and mevalonate (MVA) pathways contribute to the biosynthesis of isoprenoids. However, despite a significant amount of research on the activity of these pathways under different conditions, the relative contribution of each to the biosynthesis of diverse isoprenoids remains unclear. In this work, we examined the formation of several classes of isoprenoids in cotton (Gossypium hirsutum L.). After feeding [5,5-(2)H2]-1-deoxy-D-xylulose ([5,5-(2)H2]DOX) and [2-(13)C]MVA to intact cotton seedlings hydroponically, incorporation into isoprenoids was analyzed by MS and NMR. The predominant pattern of incorporation followed the classical scheme in which C5 units from the MEP pathway were used to form monoterpenes (C10), phytol side chains (C20) and carotenoids (C40) while C5 units from the MVA pathway were used to form sesquiterpenes (C15), terpenoid aldehydes (C15 and C25) and steroids/triterpenoids (C30). However, both pathways contributed to all classes of terpenoids, sometimes substantially. For example, the MEP pathway provided up to 20% of the substrate for sterols and the MVA pathway provided as much as 50% of the substrate for phytol side chains and carotenoids. Incorporation of C5 units from the MEP pathway was highest in cotyledons, compared to true leaves, and not observed at all in the roots. Incorporation of C5 units from the MVA pathway was highest in the roots (into sterols) and more prominent in the first true leaves than in other above-ground organs. The relative accumulation of label in intermediates vs. end products of phytosterol metabolism confirmed previous identification of slow steps in this pathway.

Carotenoids, versatile components of oxygenic photosynthesis

Carotenoids (CARs) are a group of pigments that perform several important physiological functions in all kingdoms of living organisms. CARs serve as protective agents, which are essential structural components of photosynthetic complexes and membranes, and they play an important role in the light harvesting mechanism of photosynthesizing plants and cyanobacteria. The protection against reactive oxygen species, realized by quenching of singlet oxygen and the excited states of photosensitizing molecules, as well as by the scavenging of free radicals, is one of the main biological functions of CARs. X-ray crystallographic localization of CARs revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Characterization of a CAR-less cyanobacterial mutant revealed that while the absence of CARs prevents the formation of PSII complexes, it does not abolish the assembly and function of PSI. CAR molecules assist in the formation of protein subunits of the photosynthetic complexes by gluing together their protein components. In addition to their aforementioned indispensable functions, CARs have a substantial role in the formation and maintenance of proper cellular architecture, and potentially also in the protection of the translational machinery under stress conditions.

Methylerythritol phosphate pathway to isoprenoids: kinetic modeling and in silico enzyme inhibitions in Plasmodium falciparum

The methylerythritol phosphate (MEP) pathway of Plasmodium falciparum (P. falciparum) has become an attractive target for anti-malarial drug discovery. This study describes a kinetic model of this pathway, its use in validating 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR) as drug target from the systemic perspective, and additional target identification, using metabolic control analysis and in silico inhibition studies. In addition to DXR, 1-deoxy-d-xylulose 5-phosphate synthase (DXS) can be targeted because it is the first enzyme of the pathway and has the highest flux control coefficient followed by that of DXR. In silico inhibition of both enzymes caused large decrement in the pathway flux. An added advantage of targeting DXS is its influence on vitamin B1 and B6 biosynthesis. Two more potential targets, 2-C-methyl-d-erythritol 2,4-cyclodiphosphate synthase and 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase, were also identified. Their inhibition caused large accumulation of their substrates causing instability of the system. This study demonstrates that both types of enzyme targets, one acting via flux reduction and the other by metabolite accumulation, exist in P. falciparum MEP pathway. These groups of targets can be exploited for independent anti-malarial drugs.