Cross-talk between the cytosolic mevalonate and the plastidial methylerythritol phosphate pathways in tobacco bright yellow-2 cells

Isoprenoid biosynthesis in the diatoms Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen)

Isoprenoid biosynthesis in the widespread diatomaceous algae, Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen), results not only in the production of diterpenoids, triterpenoids, and sterols but, unusually for diatoms, also in the production of sesterterpenoids. By using 13C and 2H isotopic labeling techniques followed by NMR and mass spectrometry, specific inhibition of mevalonate (MVA) and methylerythritol (MEP) pathways, and comparison with the natural 13C/12C isotope ratios of the lipids, the different biosynthetic pathways of the sesterterpenes and other isoprenoids have now been determined. Surprisingly, whereas the sesterterpenes (Delta(7(20))-haslenes) in R. setigera were made by the MVA pathway, as were the related triterpenoid rhizenes and desmosterol, in H. ostrearia the structurally similar Delta(6(17))-haslenes and the major sterol, 24-ethylcholest-5-en-3beta-ol, were instead biosynthesized by the MEP route. Phytol was biosynthesized in both diatoms by the MEP route. Subfractionation of R. setigera cells revealed that although phytol was located in the chloroplasts, the haslenes, rhizenes, and sterols were present in the cytoplasm. The observations described here for R. setigera and H. ostrearia show that terpenoid biosynthesis in diatoms is species-dependent and cannot simply be grouped according to structural type. Triterpenes appear to be made by the MVA route as in higher plants, whereas sesterterpenes and sterols can be made by either the MVA or MEP routes. In neither organism were the isoprenoids biosynthesized by leucine metabolism. Sesterterpene and triterpene biosynthesis in diatoms has not been investigated previously.

Cloning, characterization, and immunolocalization of a mycorrhiza-inducible 1-deoxy-d-xylulose 5-phosphate reductoisomerase in arbuscule-containing cells of maize

Colonization of plant roots by symbiotic arbuscular mycorrhizal fungi frequently leads to the accumulation of several apocarotenoids. The corresponding carotenoid precursors originate from the plastidial 2-C-methyl-d-erythritol 4-phosphate pathway. We have cloned and characterized 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), catalyzing the first committed step of the pathway, from maize (Zea mays). Functional identification was accomplished by heterologous expression of sequences coding for the mature protein in Escherichia coli. DXR is up-regulated in maize roots during mycorrhization as shown at transcript and protein levels, but is also abundant in leaves and young seedlings. Inspection of sequenced genomes and expressed sequence tag (EST) databases argue for a single-copy DXR gene. Immunolocalization studies in mycorrhizal roots using affinity-purified antibodies revealed a DXR localization in plastids around the main symbiotic structures, the arbuscules. DXR protein accumulation is tightly correlated with arbuscule development. The highest level of DXR protein is reached around maturity and initial senescence of these structures. We further demonstrate the formation of a DXR-containing plastidial network around arbuscules, which is highly interconnected in the mature, functional state of the arbuscules. Our findings imply a functional role of a still unknown nature for the apocarotenoids or their respective carotenoid precursors in the arbuscular life cycle.

Distinct isoprenoid origins of cis- and trans-zeatin biosyntheses in Arabidopsis

Plants produce the common isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate (DMAPP) through the methylerythritol phosphate (MEP) pathway in plastids and the mevalonate (MVA) pathway in the cytosol. To assess which pathways contribute DMAPP for cytokinin biosynthesis, metabolites from each isoprenoid pathway were selectively labeled with (13)C in Arabidopsis seedlings. Efficient (13)C labeling was achieved by blocking the endogenous pathway genetically or chemically during the feed of a (13)C labeled precursor specific to the MEP or MVA pathways. Liquid chromatography-mass spectrometry analysis demonstrated that the prenyl group of trans-zeatin (tZ) and isopentenyladenine is mainly produced through the MEP pathway. In comparison, a large fraction of the prenyl group of cis-zeatin (cZ) derivatives was provided by the MVA pathway. When expressed as fusion proteins with green fluorescent protein in Arabidopsis cells, four adenosine phosphate-isopentenyltransferases (AtIPT1, AtIPT3, AtIPT5, and AtIPT8) were found in plastids, in agreement with the idea that the MEP pathway primarily provides DMAPP to tZ and isopentenyladenine. On the other hand, AtIPT2, a tRNA isopentenyltransferase, was detected in the cytosol. Because the prenylated adenine moiety of tRNA is usually of the cZ type, the formation of cZ in Arabidopsis seedlings might involve the transfer of DMAPP from the MVA pathway to tRNA. Distinct origins of large proportions of DMAPP for tZ and cZ biosynthesis suggest that plants are able to separately modulate the level of these cytokinin species.

Pseudopterosin Biosynthesis in Symbiodinium sp., the Dinoflagellate Symbiont of Pseudopterogorgia elisabethae

Investigations are reported that identify the biosynthetic source and origins of the pseudopterosins, pharmacologically important diterpene glycosides, in the gorgonian coral Pseudopterogorgia elisabethae. We report here the isolation of physiologically significant levels of endogenous pseudopterosins A, B, C, and D from purified symbionts identified as the dinoflagellate Symbiodinium sp. Biosynthetic studies in photosynthesizing symbiont isolates utilizing 14C-labeled inorganic carbon and the tritiated intermediate geranylgeranyl diphosphate yielded radiochemically pure pseudopterosins A through D and the first committed intermediate, elisabethatriene. The 14CO(2) uptake into the pseudopterosin pathway occurred at low levels compared to the 14CO(2) uptake into total lipids, suggesting a large reserve pool of the compounds. These results reveal for the first time that pseudopterosin biosynthesis is occurring within the algal symbiont and suggests the physiological implications of this biosynthesis.