A tormentor in the quest for plant p53-like proteins

Pretreatment with salicylic acid (SA), an inducer of plant disease resistance, enhanced the capacity of parsley cells for the induction of a rapid K⁺/pH response and the subsequent coumarin (phytoalexin) secretion. In SA-primed cells, a low elicitor dose induced these two responses to a similar extent as did a high elicitor dose in non-primed cells. These observations suggest that the SA-mediated augmentation of the early K⁺/pH response may contribute to the enhancement of subsequent coumarin secretion. As the amphotericin B-induced K⁺/pH response was not enhanced in SA-primed cells, it is concluded that signaling components that are improved by priming are located between elicitor signal perception and the plasma membrane transporters mediating the K⁺/pH response.     

Homologous priming in chemotactic peptide-stimulated neutrophils

The kinetics and dose-dependence of activation of human neutrophils exposed to sequential additions of the chemotactic peptide n-formyl-methionyl-leucyl-phenylalanine (fMLP) have been investigated by multiwell microplate assays. Treatment of neutrophils with medium–high doses (from 10^?8 to 5 × 10^?7 M) of fMLP caused activation of superoxide anion (O) production, but prevented further activation by a subsequent addition of an optimal dose (from 10^?7 M to 5 × 10^?7 M) of fMLP. These findings represent an example of cell desensitization, or adaptation. However, neutrophils treated with low, sub-stimulatory doses (from 10^?10 to 5 × 10^?9 M) of the peptide and then treated with optimal doses of fMLP exhibited an O production that was two to three-fold higher than that induced by the same optimal doses on untreated cells. A similar phenomenon of homologous priming of the oxidative metabolism of neutrophil has not previously been described or characterized. Priming was maximal after about 30 min of incubation with fMLP, which differed from desensitization, which required only a few minutes. Homologous priming was not confined to O production, but was also observed with the release of the granule enzyme, lysozyme. Low doses of fMLP were also capable of triggering an increase of intracellular free Ca²⁺ and of fMLP membrane receptors, which are possible mechanisms responsible for priming.     

The Ca<Superscript>2+</Superscript>-dependent Activator Protein for Secretion CAPS: Do I Dock or do I Prime?

The “Ca²⁺-dependent activator protein for secretion” (CAPS) is a protein which reconstitutes regulated secretion in permeabilized neuroendocrine cells. It is generally accepted that CAPS plays an important role in the release of the contents of dense core vesicles in the nervous system as well as in a variety of other secretory tissues. At which step in the exocytotic process CAPS functions as well as its role in the fusion of synaptic vesicles is still under dispute. A recent growth spurt in the CAPS field has been fueled by genetic approaches in Caenorhabditis elegans and Drosophila as well as the application of knockout and knockdown approaches in mouse cells and in cell lines, respectively. We have attempted to review the body of work that established CAPS as an important regulator of secretion and to describe new information that has furthered our understanding of how CAPS may function. We discuss the conclusions, point out areas where controversy remains, and suggest directions for future experiments.     

Mineralization of organically bound nitrogen in soil as influenced by plant growth and fertilization

Summary A loam soil containing an organic fraction labelled with¹⁵N was used for pot experiments with spring barley, rye-grass and clover. The organically bound labelled N was mineralized at a rate corresponding to a half-life of about 9 years. Fertilization with 106 and 424 kgN/ha of unlabelled N in the form of KNO₃ significantly increased uptake of labelled N from the soil in barley and the first harvest of rye-grass crops. The fertilized plants removed all the labelled NH₄ and NO₃ present in the soil, whereas the unfertilized plants removed only about 80%. The second, third and fourth harvests of the unfertilized rye-grass took up more labelled N than the fertilized rye-grass. The total uptake in the four harvests was similar whether the plants were fertilized or not. Application of KCl to barley plants in amounts equivalent to that of KNO₃ resulted in a small but insignificant increase in uptake of labelled N. The uptake of labelled N in the first harvest of clover which was not fertilized but inoculated with Rhizobium was similar to that of the fully fertilized rye-grass indicating that the biological fixation of N had the same effect as addition of N-fertilizer. N uptake in the following harvests was lower and the total uptake by four harvests of clover was similar to that of rye-grass. There was no indication that fertilization with KNO₃ accelerated the mineralization of the organically bound labelled N. The observed apparent ‘priming effect’ of the fertilizer on the uptake of labelled N was compensated by subsequent crops and harvests, and it seems to arise from a more thorough search of the soil volume by a better developed root system of the fertilized plants.     

Progressive N limitation of plant response to elevated CO2: a microbiological perspective

A major uncertainty in predicting long-term ecosystem C balance is whether stimulation of net primary production will be sustained in future atmospheric CO₂ scenarios. Immobilization of nutrients (N in particular) in plant biomass and soil organic matter (SOM) provides negative feedbacks to plant growth and may lead to progressive N limitation (PNL) of plant response to CO₂ enrichment. Soil microbes mediate N availability to plants by controlling litter decomposition and N transformations as well as dominating biological N fixation. CO₂-induced changes in C inputs, plant nutrient demand and water use efficiency often have interactive and contrasting effects on microbes and microbially mediated N processes. One critical question is whether CO₂-induced N accumulation in plant biomass and SOM will result in N limitation of microbes and subsequently cause them to obtain N from alternative sources or to alter the ecosystem N balance. We reviewed the experimental results that examined elevated CO₂ effects on microbial parameters, focusing on those published since 2000. These results in general show that increased C inputs dominate the CO₂ impact on microbes, microbial activities and their subsequent controls over ecosystem N dynamics, potentially enhancing microbial N acquisition and ecosystem N retention. We reason that microbial mediation of N availability for plants under future CO₂ scenarios will strongly depend on the initial ecosystem N status, and the nature and magnitude of external N inputs. Consequently, microbial processes that exert critical controls over long-term N availability for plants would be ecosystem-specific. The challenge remains to quantify CO₂-induced changes in these processes, and to extrapolate the results from short-term studies with step-up CO₂ increases to native ecosystems that are already experiencing gradual changes in the CO₂ concentration.     

Vesicle pools, docking, priming, and release

The release of neurotransmitter from synaptic vesicles represents the final event by which presynapses send their chemical signal to the receiving postsynapses. Prior to fusion, synaptic vesicles undergo a series of maturation events, most notably the membrane-delimited docking and priming steps. Physiological and optical experiments with high-time resolution have allowed the distinction of vesicles in different maturation states with respect to fusion, the so-called vesicle pools. In this review, we define the various vesicle pools and discuss pathways leading into and out of these pools. We also provide an overview of an array of proteins that have been identified or are speculated to play a role in the transition between the various vesicle pools.Work in the lab of the authors is supported by grants from the Deutsche Forschungsgemeinschaft and the European Community and by local funding (HOMFOR).     

Dual effects of formylpeptides on the adhesion of endotoxin-primed human neutrophils

Neutrophils, treated with sequential additions of bacterial products such as endotoxin (E. Coli lipopolysaccharide, LPS) and the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP), undergo to metabolic activation and express membrane-anchoring proteins that promote adhesion to serum-coated culture wells. By investigating the dose–response relationships of these phenomena, we have found that: (a) resting neutrophils do not produce a significant amount of superoxide (O) and show only minimal adhesion to serum-coated plastic surfaces; (b) fully activatory doeses (> 5 × 10^?8 M ) of fMLP induce the release of O and a significant increase of the cell adhesion; (c) pretreatment of the cells for 1 h with LPS augments cell adhesion to serum-coated culture wells in the absence of further stimulation and primes the neutrophils to enhanced fMLP-dependent O release; (d) addition of low, substimulatory doses of fMLP (from 10^?10 M to 5 × 10^?9 M ) inhibits and reverses the adhesion of LPS-treated cells, (e) high fMLP doses (> 10^?7 M ) are additive to LPS in promoting adhesion. Phorbol-myristate acetate (> 10^?9M) increased adhesion in both normal and LPS-treated neutrophils, but low doses of this stimulant did not inhibit adhesion. Low doses (10^?9 M ) of fMLP increased intracellular cyclic AMP in both normal and LPS-treated neutrophils, suggesting that stimulus-induced rises in cAMP may be the negative signal responsible for down-modulation of adhesion. Low (5 × 10^?9 M ) and high (5 × 10^?7 M ) fMLP doses induced the same increase of expression of CD11/CD18 integrins, indicating that the inhibition of adhesion caused by low doses is not due to quantitative down-regulation of integrins. These findings may provide an in vitro model of the complex biological events involved in the regulation of neutrophil adhesion.     

Rainfall input, throughfall and stemflow of nutrients in a central African rain forest dominated by ectomycorrhizal trees

Incident rainfall is a major source of nutrient input to a forest ecosystem and the consequent throughfall and stemflow contribute to nutrient cycling. These rain-based fluxes were measured over 12 mo in two forest types in Korup National Park, Cameroon, one with low (LEM) and one with high (HEM) ectomycorrhizal abundances of trees. Throughfall was 96.6 and 92.4% of the incident annual rainfall (5370 mm) in LEM and HEM forests respectively; stemflow was correspondingly 1.5 and 2.2%. Architectural analysis showed that ln(funneling ratio) declined linearly with increasing ln(basal area) of trees. Mean annual inputs of N, P, K, Mg and Ca in incident rainfall were 1.50, 1.07, 7.77, 5.25 and 9.27 kg ha^–1, and total rain-based inputs to the forest floor were 5.0, 3.2, 123.4, 14.4 and 37.7 kg ha^–1 respectively. The value for K is high for tropical forests and that for N is low. Nitrogen showed a significantly lower loading of throughfall and stemflow in HEM than in LEM forest, this being associated in the HEM forest with a greater abundance of epiphytic bryophytes which may absorb more N. Incident rainfall provided c. 35% of the gross input of P to the forest floor (i.e., rain-based plus small litter inputs), a surprisingly high contribution given the sandy P-poor soils. At the start of the wet season leaching of K from the canopy was particularly high. Calcium in the rain was also highest at this time, most likely due to washing off of dry-deposited Harmattan dusts. It is proposed that throughfall has an important 'priming' function in the rapid decomposition of litter and mineralization of P at the start of the wet season. The contribution of P inputted from the atmosphere appears to be significant when compared to the rates of P mineralization from leaf litter.     

A slowing effect on visual search by advance information in pigeons (Columba livia): a comparison with humans (Homo sapiens)

A comparative study was conducted to investigate whether the search for a target letter was facilitated when the target and prime (preceding stimulus) letters were identical. Pigeons (Section 2) and human participants (Section 3) were first trained to search for “A” among “Y”s and “E” among “D”s in a condition in which a square shape appeared as the prime (Neutral condition). In subsequent testing, a prime was identical either to the corresponding target (Target-priming condition) or to the distractor (Distractor-priming condition). Humans and pigeons responded differently to the two priming conditions. On early trials, the Target prime facilitated search in humans, reducing reaction times (RTs) to targets. In pigeons, however, RTs were longer with Target primes, suggesting that pre-exposure to target letters may directly inhibit the search for targets in subsequent search displays. Furthermore, pre-exposure to the distractor letters may inhibit the processing of the distractor. On later trials, RTs of humans were faster in both priming conditions than in the Neutral condition, suggesting that expectation of a target facilitated search (“Y” predicted “A” and “D” predicted “E”). In contrast, the pigeons showed no evidence of expectation-based facilitation, with constant slowing effects of the Target prime extending across sessions. Possible mechanisms underlying such a slowing priming effect in pigeons were discussed.     

Soil organic matter dynamics under soybean exposed to elevated [CO2]

It is unclear how changing atmospheric composition will influence the plant–soil interactions that determine soil organic matter (SOM) levels in fertile agricultural soils. Positive effects of CO₂ fertilization on plant productivity and residue returns should increase SOM stocks unless mineralization or biomass removal rates increase in proportion to offset gains. Our objectives were to quantify changes in SOM stocks and labile fractions in prime farmland supporting a conventionally managed corn–soybean system and the seasonal dynamics of labile C and N in soybean in plots exposed to elevated [CO₂] (550 ppm) under free-air concentration enrichment (FACE) conditions. Changes in SOM stocks including reduced C/N ratios and labile N stocks suggest that SOM declined slightly and became more decomposed in all plots after 3 years. Plant available N (>273 mg N kg⁻¹) and other nutrients (Bray P, 22–50 ppm; extractable K, 157–237 ppm; Ca, 2,378–2,730 ppm; Mg, 245–317 ppm) were in the high to medium range. Exposure to elevated [CO₂] failed to increase particulate organic matter C (POM-C) and increased POM-N concentrations slightly in the surface depth despite known increases (≈30%) in root biomass. This, and elevated CO₂ efflux rates indicate accelerated decay rates in fumigated plots (2001: elevated [CO₂]: 10.5 ± 1.2 μmol CO₂ m⁻² s⁻¹ vs. ambient: 8.9 ± 1.0 μmol CO₂ m⁻² s⁻¹). There were no treatment-based differences in the within-season dynamics of SOM. Soil POM-C and POM-N contents were slightly greater in the surface depth of elevated than ambient plots. Most studies attribute limited ability of fumigated soils to accumulate SOM to N limitation and/or limited plant response to CO₂ fertilization. In this study, SOM turnover appears to be accelerated under elevated [CO₂] even though soil moisture and nutrients are non-limiting and plant productivity is consistently increased. Accelerated SOM turnover rates may have long-term implications for soil’s productive potential and calls for deeper investigation into C and N dynamics in highly-productive row crop systems.