Last-Century Increases in Intrinsic Water-Use Efficiency of Grassland Communities Have Occurred over a Wide Range of Vegetation Composition,Nutrient Inputs,and Soil pH

Last-century climate change has led to variable increases of the intrinsic water-use efficiency (W_i; the ratio of net CO₂ assimilation to stomatal conductance for water vapor) of trees and C₃ grassland ecosystems, but the causes of the variability are not well understood. Here, we address putative drivers underlying variable W_i responses in a wide range of grassland communities. W_i was estimated from carbon isotope discrimination in archived herbage samples from 16 contrasting fertilizer treatments in the Park Grass Experiment, Rothamsted, England, for the 1915 to 1929 and 1995 to 2009 periods. Changes in W_i were analyzed in relation to nitrogen input, soil pH, species richness, and functional group composition. Treatments included liming as well as phosphorus and potassium additions with or without ammonium or nitrate fertilizer applications at three levels. W_i increased between 11% and 25% (P < 0.001) in the different treatments between the two periods. None of the fertilizers had a direct effect on the change of W_i (ΔW_i). However, soil pH (P < 0.05), species richness (P < 0.01), and percentage grass content (P < 0.01) were significantly related to ΔW_i. Grass-dominated, species-poor plots on acidic soils showed the largest ΔW_i (+14.7 μmol mol⁻¹). The ΔW_i response of these acidic plots was probably related to drought effects resulting from aluminum toxicity on root growth. Our results from the Park Grass Experiment show that W_i in grassland communities consistently increased over a wide range of nutrient inputs, soil pH, and plant community compositions during the last century.The intrinsic water-use efficiency (W_i) of plants is controlled by photosynthetic carbon assimilation and stomatal conductance via the leaf-level coupling of CO₂ and water fluxes. A general, but variable, increase of W_i under rising atmospheric CO₂ has been observed in long-term studies (Peñuelas et al., 2011; Franks et al., 2013; Saurer et al., 2014), but little is known about other environmental or ecosystem factors, which may interact with the effect of increasing CO₂ on W_i. An improved understanding of putative interactive mechanisms is important because changes in W_i may have significant effects on the global terrestrial carbon and water cycles (Gedney et al., 2006; Betts et al., 2007). This study explores the interactive effects of the increase in atmospheric CO₂ (observed over the last century), nutrient loading, and soil pH together with other related effects on plant species richness and functional group composition on the coupling of plant CO₂ and water fluxes in a seminatural grassland in southeastern England.W_i is a leaf-level efficiency that has also been termed potential water-use efficiency or physiological water-use efficiency, as it excludes the direct influence of vapor pressure deficit (VPD), a parameter determined by environmental conditions, on leaf-level water-use efficiency (Farquhar et al., 1989; Franks et al., 2013). W_i reports the relationship between net CO₂ assimilation rate (A_n) and stomatal conductance for water vapor (g_H2O):(1)According to the first law of Fick, A_n can be given as the product of the stomatal conductance for CO₂ (g_CO2) and the concentration gradient between the atmosphere (c_a) and the leaf internal gas space (c_i): A_n= g_CO2 (c_a – c_i). Using g_CO2 (c_a – c_i) instead of A_n in Equation 1, replacement of g_H2O/g_CO2 by the numerical value of g_H2O/g_CO2 (1.6) and rearrangement yields the following alternative expression of W_i:(2)Equation 2 reveals that past changes of W_i must have been controlled by two parameters: the change of c_a and the concurrent change of 1 – c_i/c_a, the relative gradient for CO₂ diffusion into the leaf (Franks et al., 2013). A change in the relative gradient is determined by the changes in A_n relative to g_H2O, as leaves respond to changing c_a and other environmental factors. In particular, Equation 2 shows that any variation in the climate change response of W_i is determined by the c_i/c_a response, if the comparison is made for vegetation at the same location and in the same period of time.Studies with C₃ vegetation, including trees/forests and C₃ grasslands, have revealed a general increase of W_i in the last century (Bert et al., 1997; Duquesnay et al., 1998; Feng, 1999; Arneth et al., 2002; Saurer et al., 2004; Barbosa et al., 2010; Köhler et al., 2010; Andreu-Hayles et al., 2011). In many cases, c_i/c_a, estimated by ¹³C discrimination (Farquhar et al., 1989), varied relatively little. Indeed, it has been suggested, based on theoretical grounds and empirical evidence from studies over geological/evolutionary to short time scales, that adaptive feedback responses will tend to maintain c_i/c_a approximately constant (Ehleringer and Cerling, 1995; Franks et al., 2013), as plants optimize carbon gain with respect to water loss (Cowan and Farquhar, 1977). Yet, c_i/c_a-dependent variation in the W_i response to climate change has also been noted (Peñuelas et al., 2011; Köhler et al., 2012) over the last century, indicating that additional factors, perhaps including other global change drivers, can modify the W_i response over this time scale, at least transiently. A meta-analysis by Peñuelas et al. (2011) reports c_i/c_a-dependent increases of W_i for different forests between 6% and 36% from the early 1960s to 2000s. A recent study by Saurer et al. (2014) on European forest trees found increases in W_i ranging from 1% to 53% during the last century. The strongest increase of W_i was recorded in regions where summer soil-water availability decreased in the last century. For different grassland communities, the c_i/c_a-dependent increases of W_i varied between 13% and 28% at one site (Köhler et al., 2012) from 1915 to 2009. Evidently, such variation can have important repercussions for the coupling of terrestrial CO₂ and water fluxes. Yet, little is known about the mechanism(s) underlying the variation.At the Park Grass Experiment (PGE) at Rothamsted, England, Köhler et al. (2012) observed a nitrogen supply-dependent enhancement of the W_i response on plots receiving nitrate fertilizer and maintained at a near-neutral soil pH by liming. However, the actual relationship between nitrogen supply and W_i response did not hold when the unlimed control (soil pH approximately 5.2) was included in the comparison. Remarkably, however, there was a significant positive relationship between the grass content of the community and the W_i response of the experimental plots in the investigation. These results suggested that the effect of nutrient supply on the W_i response of the grassland communities was indirect, perhaps working via effects on soil pH and/or vegetation composition (plant species richness or functional group composition).The PGE provides a unique opportunity to study century-scale variation in the c_i/c_a-dependent variation of W_i for a wide range of diverse grassland communities. Much of the extant ecosystem-scale variability of plant species richness and soil pH in temperate grasslands of Europe (Ceulemans et al., 2014) is included in the range of plot-scale plant species richness and soil pH at the PGE (which is reported in this investigation). The different long-term applications of fertilizer and lime over the past century have resulted in substantial changes in soil pH, species richness, and grass content on the experimental plots, but in most cases, within-plot changes over the study period considered here (1915–2009) were comparatively small (Crawley et al., 2005; Silvertown et al., 2006). All experimental plots are located at the same site and are exposed to the same weather conditions. Consequently, trends in climate as a direct driver for differences in W_i between plots can be ruled out.Here, we explore putative mechanisms underlying eventual c_i/c_a-dependent variation of W_i during the last century at the PGE by, first, quantifying the sustained effect of a wide range of contrasting fertilizer treatments (n = 16) on the change of W_i during the last century and, second, analyzing the relationships between the observed W_i response of treatments and the respective nutrient status, soil pH, plant species richness, and plant functional group composition of the grassland communities.     

A network of assembly factors is involved in remodeling rRNA elements during preribosome maturation

Eukaryotic ribosome biogenesis involves ∼200 assembly factors, but how these contribute to ribosome maturation is poorly understood. Here, we identify a network of factors on the nascent 60S subunit that actively remodels preribosome structure. At its hub is Rsa4, a direct substrate of the force-generating ATPase Rea1. We show that Rsa4 is connected to the central protuberance by binding to Rpl5 and to ribosomal RNA (rRNA) helix 89 of the nascent peptidyl transferase center (PTC) through Nsa2. Importantly, Nsa2 binds to helix 89 before relocation of helix 89 to the PTC. Structure-based mutations of these factors reveal the functional importance of their interactions for ribosome assembly. Thus, Rsa4 is held tightly in the preribosome and can serve as a “distribution box,” transmitting remodeling energy from Rea1 into the developing ribosome. We suggest that a relay-like factor network coupled to a mechano-enzyme is strategically positioned to relocate rRNA elements during ribosome maturation.     

Assessment of three Resistance-Nodulation-Cell Division drug efflux transporters of Burkholderia cenocepacia in intrinsic antibiotic resistance

Background  

Burkholderia cenocepacia are opportunistic Gram-negative bacteria that can cause chronic pulmonary infections in patients with cystic fibrosis. These bacteria demonstrate a high-level of intrinsic antibiotic resistance to most clinically useful antibiotics complicating treatment. We previously identified 14 genes encoding putative Resistance-Nodulation-Cell Division (RND) efflux pumps in the genome of B. cenocepacia J2315, but the contribution of these pumps to the intrinsic drug resistance of this bacterium remains unclear.     

Fluorescence Spectroscopy as a Tool to Unravel the Dynamics of Protein Nanoparticle Formation by Liquid Antisolvent Precipitation

Protein-based particles are very promising colloidal systems for protection and controlled release applications in the food, cosmetics and pharmaceutical sector. One technique to produce these protein colloidal particles is liquid antisolvent precipitation (LAS). Despite the simplicity and versatility of LAS, not much is known about the protein conformational changes and interactions that are at the basis of the particle formation process. In this study, steady state fluorescence experiments using intrinsic fluorophores were evaluated as a tool to unravel the dynamics of the protein nanoparticle formation. Colloidal whey protein isolate and gliadin particles were produced by LAS. Changes in particle diameter (distribution), polydispersity index and photophysical properties of intrinsic fluorophores were monitored as a function of antisolvent concentration. By combining dynamic light scattering with photophysical data, a model of the changes occurring during particle formation and disintegration could be proposed. The results suggest that particle formation and disintegration are fully reversible processes during which the main changes in protein conformation (around the fluorescent probes) occur at the same antisolvent concentrations. In principle, steady state fluorescence measurements using intrinsic probes can indeed be used to effectively report on (part of the) conformational changes for both protein systems under study.     

Conserved central domains control the quaternary structure of type I and type II Hsp40 molecular chaperones

Heat shock protein (Hsp)40s play an essential role in protein metabolism by regulating the polypeptide binding and release cycle of Hsp70. The Hsp40 family is large, and specialized family members direct Hsp70 to perform highly specific tasks. Type I and Type II Hsp40s, such as yeast Ydj1 and Sis1, are homodimers that dictate functions of cytosolic Hsp70, but how they do so is unclear. Type I Hsp40s contain a conserved, centrally located cysteine-rich domain that is replaced by a glycine- and methionine-rich region in Type II Hsp40s, but the mechanism by which these unique domains influence Hsp40 structure and function is unknown. This is the case because high-resolution structures of full-length forms of these Hsp40s have not been solved. To fill this void, we built low-resolution models of the quaternary structure of Ydj1 and Sis1 with information obtained from biophysical measurements of protein shape, small-angle X-ray scattering, and ab initio protein modeling. Low-resolution models were also calculated for the chimeric Hsp40s YSY and SYS, in which the central domains of Ydj1 and Sis1 were exchanged. Similar to their human homologs, Ydj1 and Sis1 each has a unique shape with major structural differences apparently being the orientation of the J domains relative to the long axis of the dimers. Central domain swapping in YSY and SYS correlates with the switched ability of YSY and SYS to perform unique functions of Sis1 and Ydj1, respectively. Models for the mechanism by which the conserved cysteine-rich domain and glycine- and methionine-rich region confer structural and functional specificity to Type I and Type II Hsp40s are discussed.     

Sympatric diploid and hexaploid cytotypes of Senecio carniolicus (Asteraceae) in the Eastern Alps are separated along an altitudinal gradient

We explored the fine-scale distribution of cytotypes of the mountain plant Senecio carniolicus along an altitudinal transect in the Eastern Alps. Cytotypes showed a statistically significant altitudinal segregation with diploids exclusively found in the upper part of the transect, whereas diploids and hexaploids co-occurred in the lower range. Analysis of accompanying plant assemblages revealed significant differences between cytotypes along the entire transect but not within the lower part only, where both cytotypes co-occur. This suggests the presence of ecological differentiation between cytotypes with the diploid possessing the broader ecological niche. No tetraploids were detected, indicating the presence of strong crossing barriers. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Production of bioactive peptides in an in vitro system

An in vitro system for the preparation of bioactive peptides is described. This system couples three different posttranslational modification enzymes, prohormone convertases (PCs), carboxypeptidase E, and peptidyl alpha-amidating enzyme, to transform recombinant precursors into bioactive peptides. Three different precursors, mouse proopiomelanocortin (mPOMC), rat proenkephalin (rPE), and human proghrelin, were used as model systems. The conversion of mPOMC and rPE to smaller peptide products was measured by radioimmunoassay. After optimization of the system, excellent efficiency was obtained: about 85% of starting mPOMC was converted to des-acetyl alpha-melanocyte-stimulating hormone (alpha-MSH). For proenkephalin, 75 and 96% yields were obtained for the opioid peptides Met-RGL and Met-enk, respectively. Cell-based assays demonstrated that in-vitro-generated des-acetyl alpha-MSH successfully activated the melanocortin 4 receptor. Proghrelin digestion was used to screen the specificity of PC cleavage and to confirm the cleavage site by mass spectroscopy. Mature ghrelin was produced by human furin, mouse prohormone convertase 1, and human prohormone convertase 7 but not by mouse prohormone convertase 2. These results demonstrate that our in vitro system (1) can produce peptides in quantities sufficient to carry out functional analyses, (2) can be used to determine the specificity of proprotein convertases on recombinant precursors, and (3) has the potential to identify novel peptide functions on both known and orphan G-protein-coupled receptors.     

The entire N-terminal half of TatC is involved in twin-arginine precursor binding

Translocation of twin-arginine precursor proteins across the cytoplasmic membrane of Escherichia coli requires the three membrane proteins TatA, TatB, and TatC. TatC and TatB were shown to be involved in precursor binding. We have analyzed in vitro a number of single alanine substitutions in tatC that were previously shown to compromise in vivo the function of the Tat translocase. All tatC mutants that were defective in precursor translocation into cytoplasmic membrane vesicles concomitantly interfered with precursor binding not only to TatC but also to TatB. Hence structural changes of TatC that affect precursor targeting simultaneously abolish engagement of the twin-arginine signal sequence with TatB and block the formation of a functional Tat translocase. Since these phenotypes were observed for tatC mutations spread over the first half of TatC, this entire part of the molecule must globally be involved in precursor binding.     

Perceptual load-dependent neural correlates of distractor interference inhibition

Background

The load theory of selective attention hypothesizes that distractor interference is suppressed after perceptual processing (i.e., in the later stage of central processing) at low perceptual load of the central task, but in the early stage of perceptual processing at high perceptual load. Consistently, studies on the neural correlates of attention have found a smaller distractor-related activation in the sensory cortex at high relative to low perceptual load. However, it is not clear whether the distractor-related activation in brain regions linked to later stages of central processing (e.g., in the frontostriatal circuits) is also smaller at high rather than low perceptual load, as might be predicted based on the load theory.

Methodology/Principal Findings

We studied 24 healthy participants using functional magnetic resonance imaging (fMRI) during a visual target identification task with two perceptual loads (low vs. high). Participants showed distractor-related increases in activation in the midbrain, striatum, occipital and medial and lateral prefrontal cortices at low load, but distractor-related decreases in activation in the midbrain ventral tegmental area and substantia nigra (VTA/SN), striatum, thalamus, and extensive sensory cortices at high load.

Conclusions

Multiple levels of central processing involving midbrain and frontostriatal circuits participate in suppressing distractor interference at either low or high perceptual load. For suppressing distractor interference, the processing of sensory inputs in both early and late stages of central processing are enhanced at low load but inhibited at high load.     

Protein kinase CK2 links polyamine metabolism to MAPK signalling in Drosophila

MAPK signalling is a complex process not only requiring the core components Raf, MEK and Erk, but also many proteins like the scaffold protein KSR and several kinases to specifically localize, modulate and fine-tune the outcome of the pathway in a cell context specific manner. In mammals, protein kinase CK2 was shown to bind to the scaffold protein KSR and to phosphorylate Raf proteins at a conserved serine residue in the negative-charge regulatory (N−) region, thereby facilitating maximal activity of the MAPK signalling pathway. In this work we show that in Drosophila CK2 is also bound to KSR. However, despite the presence of a corresponding serine residue in the N-region of DRaf, CK2-mediated phosphorylation of DRaf takes place on a serine residue at the N-terminus and is required for Erk activation. Previous work identified polyamines as regulators of CK2 kinase activity. The main cellular source of polyamines is the catabolism of amino acids. Evidence is provided that phosphorylation of DRaf by CK2 is modulated by polyamines, with spermine being the most potent inhibitor of the reaction. We suggest that CK2 is able to monitor intracellular polyamine levels and translates this information to modulate MAPK signalling.