Eucalyptus recruitment in degraded woodlands: no benefit from elevated soil fertility

Grasses and forbs compete heavily with young tree seedlings for available resources, greatly reducing tree seedling establishment success. Soil nutrient enrichment associated with agricultural intensification can increase the growth of both herbaceous and woody lifeforms growing in isolation, but may change the balance of competitive advantage when growing together. The effects of nitrogen and phosphorus enrichment on pasture biomass and competition with two Australian grassy woodland trees (Eucalyptus albens and Eucalyptus microcarpa) was investigated in a field plot trial. Soil nutrients increased pasture biomass, but had no measurable effect on tree growth in our experiment. Competition from pasture species, even at low levels, led to high tree seedling mortality and greatly reduced tree seedling growth compared with pasture-free plots. However, when pasture-free plots were excluded from the analysis, tree seedling leaf area was not strongly correlated with herbaceous biomass. Tree seedling establishment was severely restricted even at the lowest levels of pasture biomass. We conclude that increased soil fertility resulted in a competitive advantage to the pasture, and does not improve tree seedling establishment when grown either with or without exotic herbaceous pasture (grassy understorey) species.     

Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA

Forest fires often result in a series of biogeochemical processes that increase soil nitrate (NO₃ ^?) concentrations for several years; however, the dynamic nature of inorganic nitrogen (N) cycling in the plant–microbe–soil complex makes it challenging to determine the direct causes of increased soil NO₃ ^?. We measured gross inorganic N transformation rates in mineral soils 2 years after wildfires in three central Idaho coniferous forests to determine the causes of the elevated soil NO₃ ^?. We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and carbon (C) availability. We found no significant differences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO₃ ^? uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO₃ ^? caused slightly elevated NO₃ ^? concentrations in the burned soils. C availability was positively correlated with microbial NO₃ ^? uptake rates. Despite reduced microbial NO₃ ^? uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO₃ ^? concentrations 2 years post fire. Soil NH₄ ⁺ concentrations between the treatments were not significantly different (P = 0.673). However, gross NH₄ ⁺ production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively correlated with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region.     

N:P Ratio and the Nature of Nutrient Limitation in Calluna-Dominated Heathlands

There is growing evidence from different sources that prolonged high N deposition causes a shift from nitrogen (N) limitation to nitrogen and phosphorus (P) co-limitation or even P limitation in many terrestrial ecosystems. However, the number of ecosystems where the type of limitation has been directly tested by longer-term full-factorial field experiments is very limited. We conducted a 5-year fertilization experiment with N and P in the Lüneburger Heide (NW Germany) to test the hypothesis that, following decades of elevated atmospheric N inputs, plant growth in dry lowland heaths may have shifted from N to N–P co-limitation or P limitation. We also tested whether the plant tissue N:P ratio reflects the type of nutrient limitation in a continental lowland heathland. Experimental plots dominated by Calluna vulgaris received regular additions of N (50 kg N ha⁻¹ y⁻¹), P (20 kg P ha⁻¹ y⁻¹), a combination of both, or water only (control) from 2004 to 2008. Over the whole study period, a highly significant positive N effect on shoot length was found, thus indicating N limitation. We conclude that a clear shift from N limitation to N–P co-limitation or P limitation has not yet occurred. Tissue N:P ratios showed a high temporal variability and no relationship between tissue N:P ratio and the shoot length response of Calluna to nutrient addition was found. The N:P tool is thus of limited use at the local scale and within the range of N:P ratio observed in this study, and should only be used as a rough indicator for the prediction of the type of nutrient limitation in lowland heathland on a larger geographical scale with a broader interval of N:P ratio.     

Wood-feeding beetles and soil nutrient cycling in burned forests: implications of post-fire salvage logging

• 1 Rising economic demands for boreal forest resources along with current and predicted increases in wildfire activity have increased salvage logging of burned forests. Currently, the ecological consequences of post‐fire salvage logging are insufficiently understood to develop effective management guidelines or to adequately inform policy decision‐makers.
• 2 We used both field and laboratory studies to examine the effects of post‐fire salvage logging on populations of the white‐spotted sawyer Monochamus scutellatus scutellatus (Say) (Coleoptera: Cerambycidae) and its ecological function in boreal forest.
• 3 Monochamus s. scutellatus adults were relatively abundant in both burned and clear‐cut logged sites but were absent from salvage logged sites.
• 4 An in situ mesocosm experiment showed that the abundance of M. s. scutellatus larvae in burned white spruce bolts was linked to changes in total organic nitrogen and carbon in mineral soil.
• 5 Organic nutrient inputs in the form of M. s. scutellatus frass increased mineral soil microbial respiration rates by more than three‐fold and altered the availability of nitrogen. Changes in nitrogen availability corresponded with decreased germination and growth of Epilobium angustifolium and Populus spp. but not Calamagrostis canadensis.
• 6 Although the present study focused on local scale effects, the reported findings suggest that continued economic emphasis on post‐fire salvage logging may have implications beyond the local scale for biodiversity conservation, nutrient cycling and plant community composition in forest ecosystems recovering from wildfire.

     

Clostridium botulinum C2 toxin is internalized by clathrin‐ and Rho‐dependent mechanisms

Clostridium botulinum C2 toxin is an ADP‐ribosyltransferase, causing depolymerization of the actin cytoskeleton in eukaryotic cells. The C2 toxin is a binary toxin consisting of the enzymatic subunit C2I and the binding subunit C2II. Proteolytical activation of the binding subunit triggers the formation of heptameric structures (C2IIa), which bind to cellular receptors. C2I is able to bind to C2IIa oligomers, and it has been suggested that the whole complex is internalized by a raft‐dependent mechanism. Here we analysed by which mechanism C2 toxin is endocytosed. In HeLa cells expressing a dominant‐negative dynamin mutant, cytotoxicity and C2 toxin uptake were blocked. Furthermore, siRNA‐mediated knockdown of flotillins or inhibition of Arf6 function, proteins suggested to be involved in dynamin‐independent endocytosis, did not affect C2 toxicity. Knockdown of caveolin did not inhibit endocytosis of C2 toxin, whereas inhibition of clathrin function reduced the uptake of C2 toxin and delayed the cytotoxic effect. Finally, we found evidence for a Rho‐mediated uptake of C2 toxin. In conclusion, C2 toxin is endocytosed by dynamin‐dependent mechanisms and we provide evidence for involvement of clathrin and Rho.     

Predicting MHC class I epitopes in large datasets

Background  

Experimental screening of large sets of peptides with respect to their MHC binding capabilities is still very demanding due to the large number of possible peptide sequences and the extensive polymorphism of the MHC proteins. Therefore, there is significant interest in the development of computational methods for predicting the binding capability of peptides to MHC molecules, as a first step towards selecting peptides for actual screening.     

Functional characterization of the protein C A267T mutation: evidence for impaired secretion due to defective intracellular transport

Background  

Activated protein C (PC) is a serine protease that regulates blood coagulation by inactivating coagulation factors Va and VIIIa. PC deficiency is an autosomally inherited disorder associated with a high risk of recurrent venous thrombosis. The aim of the study was to explore the mechanisms responsible for severe PC deficiency in a patient with the protein C A267T mutation by in-vitro expression studies.     

Plasticity of human Achilles tendon mechanical and morphological properties in response to cyclic strain

The purpose of the current study in combination with our previous published data (Arampatzis et al., 2007) was to examine the effects of a controlled modulation of strain magnitude and strain frequency applied to the Achilles tendon on the plasticity of tendon mechanical and morphological properties. Eleven male adults (23.9±2.2 yr) participated in the study. The participants exercised one leg at low magnitude tendon strain (2.97±0.47%), and the other leg at high tendon strain magnitude (4.72±1.08%) of similar frequency (0.5 Hz, 1 s loading, 1 s relaxation) and exercise volume (integral of the plantar flexion moment over time) for 14 weeks, 4 days per week, 5 sets per session. The exercise volume was similar to the intervention of our earlier study (0.17 Hz frequency; 3 s loading, 3 s relaxation) allowing a direct comparison of the results. Before and after the intervention ankle joint moment has been measured by a dynamometer, tendon–aponeurosis elongation by ultrasound and cross-sectional area of the Achilles tendon by magnet resonance images (MRI). We found a decrease in strain at a given tendon force, an increase in tendon–aponeurosis stiffness and tendon elastic modulus of the Achilles tendon only in the leg exercised at high strain magnitude. The cross-sectional area (CSA) of the Achilles tendon did not show any statistically significant (P>0.05) differences to the pre-exercise values in both legs. The results indicate a superior improvement in tendon properties (stiffness, elastic modulus and CSA) at the low frequency (0.17 Hz) compared to the high strain frequency (0.5 Hz) protocol. These findings provide evidence that the strain magnitude applied to the Achilles tendon should exceed the value, which occurs during habitual activities to trigger adaptational effects and that higher tendon strain duration per contraction leads to superior tendon adaptational responses.     

Direct Observation of the Uncapping of Capping Protein-capped Actin Filaments by CARMIL Homology Domain 3

Bulk solution assays have shown that the isolated CARMIL homology 3 (CAH3) domain from mouse and Acanthamoeba CARMIL rapidly and potently restores actin polymerization when added to actin filaments previously capped with capping protein (CP). To demonstrate this putative uncapping activity directly, we used total internal reflection microscopy to observe single, CP-capped actin filaments before and after the addition of the CAH3 domain from mouse CARMIL-1 (mCAH3). The addition of mCAH3 rapidly restored the polymerization of individual capped filaments, consistent with uncapping. To verify uncapping, filaments were capped with recombinant mouse CP tagged with monomeric green fluorescent protein (mGFP-CP). Restoration of polymerization upon the addition of mCAH3 was immediately preceded by the complete dissociation of mGFP-CP from the filament end, confirming the CAH3-driven uncapping mechanism. Quantitative analyses showed that the percentage of capped filaments that uncapped increased as the concentration of mCAH3 was increased, reaching a maximum of ∼90% at ∼250 nm mCAH3. Moreover, the time interval between mCAH3 addition and uncapping decreased as the concentration of mCAH3 increased, with the half-time of CP at the barbed end decreasing from ∼30 min without mCAH3 to ∼10 s with a saturating amount of mCAH3. Finally, using mCAH3 tagged with mGFP, we obtained direct evidence that the complex of CP and mCAH3 has a small but measurable affinity for the barbed end, as inferred from previous studies and kinetic modeling. We conclude that the isolated CAH3 domain of CARMIL (and presumably the intact molecule as well) possesses the ability to uncap CP-capped actin filaments.