Xyloglucan is released by plants and promotes soil particle aggregation.

Soil is a crucial component of the biosphere and is a major sink for organic carbon. Plant roots are known to release a wide range of carbon-based compounds into soils, including polysaccharides, but the functions of these are not known in detail. Using a monoclonal antibody to plant cell wall xyloglucan, we show that this polysaccharide is secreted by a wide range of angiosperm roots, and relatively abundantly by grasses. It is also released from the rhizoids of liverworts, the earliest diverging lineage of land plants. Using analysis of water-stable aggregate size, dry dispersion particle analysis and scanning electron microscopy, we show that xyloglucan is effective in increasing soil particle aggregation, a key factor in the formation and function of healthy soils. To study the possible roles of xyloglucan in the formation of soils, we analysed the xyloglucan contents of mineral soils of known age exposed upon the retreat of glaciers. These glacial forefield soils had significantly higher xyloglucan contents than detected in a UK grassland soil. We propose that xyloglucan released from plant rhizoids/roots is an effective soil particle aggregator and may, in this role, have been important in the initial colonization of land

Electronic states and phases of KxC60 from photoemission and X-ray absorption spectroscopy

HIGH-resolution photoemission and soft X-ray absorption spectroscopies have provided valuable information on the electronic structure near the Fermi energy in the superconducting copper oxide compounds 1-4, helping to constrain the possible mechanisms of superconductivity. Here we describe the application of these techniques to K(x)C60, found recently to be superconducting below 19.3 K for x almost-equal-to 3 (refs 5-7). The photoemission and absorption spectra as a function of x can be fitted by a linear combination of data from just three phases, C60, K3C60, and K6C60, indicating that there is phase separation in our samples. The photoemission spectra clearly show a well defined Fermi edge in the K3C60 phase with a density of states of 5.2 x 10(-3) electrons eV-1 angstrom-3 and an occupied-band width of 1.2 eV, suggesting that this phase may be a weakly coupled BCS-like (conventional) superconductor. The C1s absorption spectra show large non-rigid-band shifts between the three phases with half and complete filling, in the K3C60 and K6C60 phases respectively, of the conduction band formed from the lowest unoccupied molecular orbital of C60. These observations clearly demonstrate that the conduction band has C 2p character. The non-rigid-band shift coupled with the anomalous occupied-band width implies that there is significant mixing of the electronic states of K and C60 in the superconducting phase

Ion homeostasis in the Chloroplast

peer reviewedThe chloroplast is an organelle of high demand for macro- and micro-nutrient ions, which are required for the maintenance of the photosynthetic process. To avoid deficiency while preventing excess, homeostasis mechanisms must be tightly regulated. Here, we describe the needs for nutrient ions in the chloroplast and briefly highlight their functions in the chloroplastidial metabolism. We further discuss the impact of nutrient deficiency on chloroplasts and the acclimation mechanisms that evolved to preserve the photosynthetic apparatus. We finally present what is known about import and export mechanisms for these ions. Whenever possible, a comparison between cyanobacteria, algae and plants is provided to add an evolutionary perspective to the description of ion homeostasis mechanisms in photosynthesis

Greigite: a true intermediate on the polysulfide pathway to pyrite

The formation of pyrite (FeS(2)) from iron monosulfide precursors in anoxic sediments has been suggested to proceed via mackinawite (FeS) and greigite (Fe(3)S(4)). Despite decades of research, the mechanisms of pyrite formation are not sufficiently understood because solid and dissolved intermediates are oxygen-sensitive and poorly crystalline and therefore notoriously difficult to characterize and quantify. In this study, hydrothermal synchrotron-based energy dispersive X-ray diffraction (ED-XRD) methods were used to investigate in situ and in real-time the transformation of mackinawite to greigite and pyrite via the polysulfide pathway. The rate of formation and disappearance of specific Bragg peaks during the reaction and the changes in morphology of the solid phases as observed with high resolution microscopy were used to derive kinetic parameters and to determine the mechanisms of the reaction from mackinawite to greigite and pyrite. The results clearly show that greigite is formed as an intermediate on the pathway from mackinawite to pyrite. The kinetics of the transformation of mackinawite to greigite and pyrite follow a zero-order rate law indicating a solid-state mechanism. The morphology of greigite and pyrite crystals formed under hydrothermal conditions supports this conclusion and furthermore implies growth of greigite and pyrite by oriented aggregation of nanoparticulate mackinawite and greigite, respectively. The activation enthalpies and entropies of the transformation of mackinawite to greigite, and of greigite to pyrite were determined from the temperature dependence of the rate constants according to the Eyring equation. Although the activation enthalpies are uncharacteristic of a solid-state mechanism, the activation entropies indicate a large increase of order in the transition state, commensurate with a solid-state mechanism

The Main Functions of Plastids.

International audiencePlastids are semiautonomous organelles like mitochondria, and derive from a cyanobacterial ancestor that was engulfed by a host cell. During evolution, they have recruited proteins originating from the nuclear genome, and only parts of their ancestral metabolic properties were conserved and optimized to limit functional redundancy with other cell compartments. Furthermore, large disparities in metabolic functions exist among various types of plastids, and the characterization of their various metabolic properties is far from being accomplished. In this review, we provide an overview of the main functions, known to be achieved by plastids or shared by plastids and other compartments of the cell. In short, plastids appear at the heart of all main plant functions