Overexpression of Arabidopsis thaliana farnesyl diphosphate synthase (FPS1S) in transgenic Arabidopsis induces a cell death/senescence-like response and reduced cytokinin levels

To investigate the contribution of farnesyl diphosphate synthase (FPS) to the overall control of the mevalonic acid pathway in plants, we have generated transgenic Arabidopsis thaliana overexpressing the Arabidopsis FPS1S isoform. Despite high levels of FPS activity in transgenic plants (8- to 12-fold as compared to wild-type plants), the content of sterols and the levels of 3-hydroxy-3-methylglutaryl-CoA reductase activity in leaves were similar to those in control plants. Plants overexpressing FPS1S showed a cell death/senescence-like phenotype and grew less vigorously than wild-type plants. The onset and the severity of these phenotypes directly correlated with the levels of FPS activity. In leaves of plants with increased FPS activity, the expression of the senescence activated gene SAG12 was prematurely induced. Transgenic plants grown in the presence of either mevalonic acid (MVA) or the cytokinin 2-isopentenyladenine (2-iP) recovered the wild-type phenotype. Quantification of endogenous cytokinins demonstrated that FPS1S overexpression specifically reduces the levels of endogenous zeatin-type cytokinins in leaves. Altogether these results support the notion that increasing FPS activity without a concomitant increase of MVA production leads to a reduction of IPP and DMAPP available for cytokinin biosynthesis. The reduced cytokinin levels would be, at least in part, responsible for the phenotypic alterations observed in the transgenic plants. The finding that wild-type and transgenic plants accumulated similar increased amounts of sterols when grown in the presence of exogenous MVA suggests that FPS1S is not limiting for sterol biosynthesis.     

Genetic evidence of branching in the isoprenoid pathway for the production of isopentenyl diphosphate and dimethylallyl diphosphate in Escherichia coli

An alternative mevalonate-independent pathway for isoprenoid biosynthesis has been recently discovered in eubacteria (including Escherichia coli) and plant plastids, although it is not fully elucidated yet. In this work, E. coli cells were engineered to utilize exogenously provided mevalonate and used to demonstrate by a genetic approach that branching of the endogenous pathway results in separate synthesis of the isoprenoid building units isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In addition, the IPP isomerase encoded by the idi gene was shown to be functional in vivo and to represent the only possibility for interconverting IPP and DMAPP in this bacterium.     

Carotenoid biosynthesis during tomato fruit development: regulatory role of 1-deoxy-D-xylulose 5-phosphate synthase

Plant isoprenoids represent a heterogeneous group of compounds which play essential roles not only in growth and development, but also in the interaction of plants with their environment. Higher plants contain two pathways for the biosynthesis of isoprenoids: the mevalonate pathway, located in the cytosol/endoplasmic reticulum, and the recently discovered mevalonate-independent pathway (Rohmer pathway), located in the plastids. In order to evaluate the function of the Rohmer pathway in the regulation of the synthesis of plastidial isoprenoids, we have isolated a tomato cDNA encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS), the first enzyme of the pathway. We demonstrate in vivo activity and plastid targeting of plant DXS. Expression analysis of the tomato DXS gene indicates developmental and organ-specific regulation of mRNA accumulation and a strong correlation with carotenoid synthesis during fruit development. 1-Deoxy-D-xylulose feeding experiments, together with expression analysis of DXS and PSY1 (encoding the fruit-specific isoform of phytoene synthase) in wild-type and yellow flesh mutant fruits, indicate that DXS catalyses the first potentially regulatory step in carotenoid biosynthesis during early fruit ripening. Our results change the current view that PSY1 is the only regulatory enzyme in tomato fruit carotenogenesis, and point towards a coordinated role of both DXS and PSY1 in the control of fruit carotenoid synthesis.     

A mutant pyruvate dehydrogenase E1 subunit allows survival of Escherichia coli strains defective in 1-deoxy-D-xylulose 5-phosphate synthase

The 2-C-methyl-D-erythritol 4-phosphate pathway has been proposed as a promising target to develop new antimicrobial agents. However, spontaneous mutations in Escherichia coli were observed to rescue the otherwise lethal loss of the first two enzymes of the pathway, 1-deoxy-D-xylulose 5-phosphate (DXP) synthase (DXS) and DXP reductoisomerase (DXR), with a relatively high frequency. A mutation in the gene encoding the E1 subunit of the pyruvate dehydrogenase complex was shown to be sufficient to rescue the lack of DXS but not DXR in vivo, suggesting that the mutant enzyme likely allows the synthesis of DXP or an alternative substrate for DXR.     

The orchids of Timor: checklist and conservation status

A checklist of the Orchidaceae of Timor is presented, with emphasis on the eastern half of the island (East Timor), based on historical herbarium collections and recent botanical explorations. This list comprises 38 genera with 66 species, including 15 new genera and 32 new species records for this island. Moreover, four new species are described: Bulbophyllum sundaicum , Habenaria ankylocentron , Habenaria cauda‐porcelli , and Pterostylis timorensis . Of these, we consider the finding of a new species of Pterostylis to be especially noteworthy, because this species seems to be more closely related to certain Australian members of the genus than to the Malesian ones, suggesting earlier contacts of Timor with Australia. Four new synonyms are proposed: Calanthe veratrifolia var. timorensis J.J.Sm. (C. triplicata), Habenaria cornuta Span. (H. giriensis), H. grandis Benth. ex Ridl. (Peristylus goodyeroides), and H. mutica Span. (H. elongata). The best represented genus is Habenaria, with 13 species, followed by Dendrobium with four, and Bulbophyllum with three. Because of insufficient or sterile material, it was not possible to identify, or describe as new, 20 different taxa. The conservation status of the ten endemic species, plus six possible new undescribed species and two non‐endemic, but threatened, species, was assessed using the World Conservation Union (IUCN) criteria, and categories of threat were proposed. Seven endemic species are considered to be Critically Endangered and two Endangered. One of the nonendemic species is considered to be Critically Endangered, and the other Endangered. The survival of some of these species might be less insecure if an effective application of Regulation project N.2000/19 on protected areas (UNTAET/REG/2000/19) was implemented and maintained, because most of these species were collected in areas considered for protection under this Regulation. Further studies are required, however, in order to complete our knowledge of the diversity and population dynamics of this interesting part of Timor's biodiversity. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157 , 197–215.     

Annual carbon fixation in terrestrial populations of Nostoc commune (Cyanobacteria) from an Antarctic dry valley is driven by temperature regime

Nostoc commune Vaucher (a cyanobacterium) is a very conspicuous terrestrial primary producer in Victoria Land, continental Antarctica. Because polar ecosystems are considered to be especially sensitive to environmental changes, understanding the environmental constraints on net carbon (C) fixation by N. commune is necessary to determine the effects of environmental changes on the ecological functioning of ice‐free areas of the continent. A model describing net C fixation in terrestrial populations of N. commune in an Antarctic dry valley was constructed using field and laboratory measurements in which N. commune colonies were exposed to different combinations of incident irradiance (400–700 nm), temperature, and degree of desiccation. For desiccated N. commune mats with water content ≤ 30% saturation, net C fixation was highly variable between replicates and could not be modelled. However, for colonies at > 30% saturation, rates of net C fixation and dark respiration depended strongly on irradiance and temperature. Net C fixation reached a maximum rate of 21.6 μg C m^{− 2} s^{− 1} at irradiance of approximately 250 μmol m^{− 2} s^{− 1} and the optimum temperature of 20.5 °C. Agreement between predicted short‐term net C fixation and field and laboratory measurements allowed estimation of total seasonal fixation, using previously published environmental data. Annual net C fixation was estimated in the range 14.5–21.0 g C fixed m^{− 2}Nostoc mat, depending on year/season. Estimates for different seasons correlated with thermal time (accumulated hours above 0 °C during the year) rather than irradiance, in contrast to communities in local lacustrine environments, where irradiance is the main driver of primary productivity. In the terrestrial habitat, N. commune appears to compromise between an ability to capitalize on short periods of higher temperature and efficient utilization of lower irradiance at low temperature. The relationship between thermal time and net annual C fixation by N. commune is strongly linear.     

Frog liver glycogen synthase. In vitro and in vivo interconversions between I and D forms.

1. Frog liver has enzymatic systems able to interconvert glycogen synthase. 2. D to I conversion is achieved in vitro by incubation at 30 degrees C. ATP, ADP, inorganic phosphate and glycogen are inhibitors of this conversion, whereas glucose-6-P and Mg2+ stimulate it. 3. I to D conversion in vitro depends on ATP-Mg2+. Cyclic-AMP activates this conversion, while glucose-6-P inhibits it. 4. Injection of glucose, ribose, mannose, fructose, galactose, and cortisone into frogs increase liver percentage of I activity. 5. Glucagon and adrenaline decrease percentage of I activity.