Clerodane diterpenoids from the roots of Portulaca pilosa

Three trans-clerodane diterpenoids, pilosanol A, B and C, the last compound being a glucoside, have been isolated from the roots of Portulaca pilosa. They show a marked contrast in skeletal type with the constituents of aerial part. Evolutionary changes in the biosynthetic abilities of Portulaca species is discussed.     

Guaianolides and lignans from Centaurea solstitialis subs Schouwii

Aerial parts of Centaurea solstitialis subs schouwii afforded the guaianolides cynaropicrin and aguerin B and the lignans arctigenin and matairesinol. The structure of a third guaianolide previously found also in Centaurea behen was revised.     

Control of nitrogenase inAzospirillum sp.

Many N₂-fixing organisms can turn off nitrogenase activity in the presence of NH₄ ⁺ and turn it on again when the NH₄ ⁺ is exhausted. One of the most interesting systems for accomplishing this is by covalent modification of one subunit of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase (DRAT). The system can be reactivated when NH₄ ⁺ is exhausted, by dinitrogenase reductase activating glycohydrolase (DRAG) which removes the inactivating group. It is fascinating that some species of the genusAzospirillum possess the DRAT and DRAG systems (A. lipoferum andA. brasilense), whereasA. amazonense in the same genus lacks DRAT and DRAG.A. amazonense responds to NH₄ ⁺ but does not exhibit modification of dinitrogenase reductase characteristic of the action of DRAT. However, it has been possible to clone DRAT and DRAG and to introduce them intoA. amazonense, whereupon they become functional in this organism. The DRAT and DRAG system does not appear to function inAcetobacter diazotrophicus, an organism isolated from sugar cane, that fixes N₂ at a pH as low as 3.0.A. diazotrophicus does show a rather sluggish response to NH₄ ⁺. A level of about 10 M NH₄ ⁺ is required to switch off the system. The response to NH₄ ⁺ is influenced by the dissolved oxygen concentration (DOC) as has been reported forAzospirillum sp. A DOC in equilibrium with 0.1 to 0.2 kPa O₂ seems optimal for the response inA. diazotrophicus.     

Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe

Small-scale spatial heterogeneity of soil organic matter (SOM) associated with patterns of plant cover can strongly influence population and ecosystem dynamics in dry regions but is not well characterized for semiarid grasslands. We evaluated differences in plant and soil N and C between soil from under individual grass plants and from small openings in shortgrass steppe. In samples from 0 to 5 cm depth, root biomass, root N, total and mineralizable soil N, total and respirable organic C, C:N ratio, fraction of organic C respired, and ratio of respiration to N mineralization were significantly greater for soil under plants than soil from openings. These differences, which were consistent for two sites with contrasting soil textures, indicate strong differentiation of surface soil at the scale of individual plants, with relative enrichment of soil under plants in total and active SOM. Between-microsite differences were substantial relative to previously reported differences associated with landscape position and grazing intensity in shortgrass steppe. We conclude that microscale heterogeneity in shortgrass steppe deserves attention in investigation of controls on ecosystem and population processes and when sampling to estimate properties at plot or site scales.     

Distribution and immunolocalization of stachyose synthase in Cucumis melo L.

Indirect evidence for the site of stachyose biosynthesis has been provided by determining the occurrence and distribution of stachyose, raffinose and galactinol, the donor of the galactosyl moiety for stachyose synthesis, in Cucumis melo L. cv. Ranjadew. Studies of enzyme activities for the synthesis of these sugars and their distribution in different plant organs and isolates has led to the conclusion that stachyose is synthesized mainly in mature leaves and seeds. Nevertheless, stachyose-synthase activity varied with leaf age, the developmental stage of a plant, the growing season and the plant cultivar used. No stachyose or stachyose-synthase activity could be detected in isolated mesophyll protoplasts and chloroplasts, whereas both were found in a minor-vein-enriched fraction isolated from mature leaves. The conclusion that stachyose biosynthesis is associated with minor veins was confirmed by immunolocalization of the enzyme. Positive specific immunoreactivity of stachyose synthase with polyclonal anti-stachyose-synthase antibodies, labeled with protein A-gold, was detected in intermediary cells of leaf minor veins. The implication of this local synthesis of the main transport sugar for phloem loading in mature leaves of Cucumis melo is discussed.Abbreviation RUBPCase ribulose-1,5-bisphosphate carboxylase This work was supported by Deutsche Forschungsgemeinschaft. The excellent assistance of Ms. B. Müller in preparing the samples for electron microscopy is gratefully acknowledged. The authors thank Professor H.J. Schneider-Poetsch for anti-RuBPCase antibodies.     

Methyljasmonate and α-linolenic acid are potent inducers of tendril coiling

A coiling-inducing factor was isolated from tendrils of Bryonia dioica Jacq. and identified by infrared, ¹H-, ¹³C-nuclear magnetic resonance and mass spectrometry as -linolenic acid. When applied to detached tendrils, exogenous -linolenic acid, but not linoleic acid or oleic acid, induced tendril coiling. Further investigations showed that metabolites of -linolenic acid, jasmonic acid and, even more so, methyljasmonate, are highly effective inducers of tendril coiling in B. dioica. Methyljasmonate was most active when administered by air and, in atmospheric concentrations as low as 40–80 nM, induced a full free-coiling response with kinetics similar to mechanical stimulation. Even atmospheric levels as low as 4–5 nM methyljasmonate were still found to be significantly active. Methyljasmonate could be one of the endogenous chemical signals produced in mechanically stimulated parts of a tendril and, being highly volatile, act as a diffusible gaseous mediator spreading through the intracellular spaces to trigger free coiling of tendrils.Abbreviations EI-MS electron impact-mass spectrometry - HPLC high-performance liquid chromatography - IAA indole-3-acetic acid - NMR nuclear magnetic resonance - TFA trifluoroacetic acid We are indebted to the Deutsche Forschungsgemeinschaft, Bonn and the Fonds der Chemischen Industrie, Frankfurt (literature provision) for their support and to Dr. C. Brückner, Halle, for jasmonic-acid determinations.     

A comparative study into the chemical constitution of cutins and suberins from Picea abies (L.) Karst., Quercus robur L., and Fagus sylvatica L.

The compositions of BF₃/CH₃OH depolymerisates of cutins and suberins from leaf and periderm samples from Picea abies [L.] Karst., Quercus robur L., and Fagus sylvatica L., respectively, were determined by quantitative capillary gas chromatography/mass spectroscopy. Long-chain monobasic, -hydroxymonobasic, dihydroxymonobasic, trihydroxymonobasic and epoxyhydroxymonobasic alkanoic acids constituted the major aliphatic monomers of leaf cutins. The total amounts of cutin monomers ranged from 629 mg · m^–2 (Fagus) to 1350 mg · m^–2 (Quercus). Cutin composition and amounts did not significantly differ between current year and three-year-old needles of Picea. Trans-esterification of periderm samples yielded a much greater variety of aliphatic monomers than obtained from cutins. In addition to the substance classes found with cutins, suberin depolymerisates also contained , -dibasic acids while dihydroxymonobasic acids were lacking. Depolymerisates from periderms taken from different locations on a Picea tree did not differ significantly in their relative composition. The results are discussed in terms of the distinctive characteristics of the aliphatic portions of cutins and suberins, respectively. Discriminant analysis is applied for formulating a quantitative and inarbitrary classification rule for cutins and suberins. The precision, statistical significance and robustness of this classification rule are tested by employing it to a large set of compositional data (70 plant species) from the literature. The relevance of data obtained by depolymerization methods for elucidating the physical structure of cutins and suberins in situ is evaluated.To whom correspondence should be addressedThe authors are indebted to Drs. J. Winkler and H. Krause (Laboratorium für Strukturchemie des Fachbereichs Chemie, Biologie und Geowissenschaften, Technische Universität München, Garching, FRG) for performing capillary gas chromatography-mass spectrometry and their valuable help in the identification of cutin and suberin constituents. The work was supported by grants from the Deutsche Forschungsgemeinschaft and the Bayerisches Staatsministerium für Unterricht, Kultus, Wissenschaft und Kunst.