Possible interference of fertilization in the natural recovery of a declining sugar maple stand in southern Quebec

A five year study was conducted in a 100–120 year old even-aged sugar maple stand in southern Quebec (46°07N 73° 56W; 305 m altitude) to explore the effect of different fertilization formulations aimed at 1) correcting the most common nutrient deficiencies observed in declining maple stands (K and Mg), 2) decreasing soil acidity, and 3) simulating enrichment with atmospheric N. Seven fertilizer mixtures were applied in the spring of 1987: 400 kg ha^-1 of K₂SO₄, CaCO₃, CaMg(CO₃)₂, (NH₄)₂SO₄, complete fertilizer (Maplegro) and 800 kg ha^-1 of an equal mixture of K₂SO₄+CaCO₃ or K₂SO₄+CaMg(CO₃)₂. The site was divided into twenty-four 25×25 m plots and treatments including control were replicated three times. Leaves and soils (organic and mineral) were sampled in 1987, 1988 and 1991. Trees were cored at 1.2 m to measure their response in diameter growth. The application of K₂SO₄+CaMg(CO₃)₂ was the only treatment that significantly increased (+13%) the average growth rate over the five year period after fertilization. The application of (NH₄)₂SO₄, Maplegro, CaMg(CO₃)₂ and K₂SO₄ reduced growth relative to the control for the five year period by 29, 24, 20 and 12 %, respectively. Positive and negative effects on growth can be explained mainly in terms of changes in leaf K. Both the application of Maplegro and (NH₄)₂SO₄ increased soil P availability. Overall, the rate of growth showed a cubic pattern of change over the 5 year period with peaks in 1988 and 1991. Trees in control plots went from a limiting foliar status of Ca and Mg, and surplus N in 1987 to a surplus of Ca and Mg, and lower N concentration in 1991. Our results suggest that nutrient deficiencies observed at our site were associated with a disturbance of the biogeochemical cycle of nutrients rather than soil nutrient depletion.Abbreviations BS base saturation - CEC cation exchange capacity - DRIS diagnosis and recommendation integrated system     

Clathrin heavy chain is required for pinocytosis, the presence of large vacuoles, and development in Dictyostelium

To investigate the intracellular role of the clathrin heavy chain in living cells, we have used "antisense" RNA to engineer mutant Dictyostelium discoideum cells that are severely deficient in clathrin heavy chain expression. Immunoblots stained with an anti-clathrin heavy chain antiserum revealed that mutant cells contained undetectable amounts of clathrin heavy chain protein. Similarly, Northern blots showed an absence of clathrin heavy chain mRNA. Clathrin heavy chain-deficient Dictyostelium cells were viable, but exhibited growth rates twofold slower than parental cells. Whereas many morphological features of the mutant cells were normal, mutant cells lacked coated pits and coated vesicles. Clathrin-deficient cells were also missing large translucent vacuoles that serve as endosomes and contractile vacuoles. In the absence of clathrin heavy chain, mutant cells displayed three distinct functional defects: (a) impairment in endocytosis of fluid phase markers, but competence in another endocytic pathway, the phagocytosis of solid particles; (b) defects in osmoregulation; and (c) inability to complete the starvation-induced development cycle.     

Habitat associations of Eurasian Skylarks Alauda arvensis breeding on Irish farmland and implications for agri-environment planning

Capsule Skylarks breeding in Ireland prefer extensive grassland habitats and almost completely avoid tillage habitats.

Aims To describe the distribution and habitat use of breeding Skylarks in Ireland, particularly in lowland agricultural habitats, and to use this information to inform conservation measures for this species.

Methods Countryside Bird Survey (CBS) and Farmland Bird Project (FBP) data were examined to determine large-scale (national) distribution and habitat selection, in addition to smaller-scale (farm- and field-level) habitat use. The CBS is a national breeding bird monitoring scheme involving 397 1-km squares. The FBP collected detailed bird and habitat data from 122 farms.

Results CBS and FBP data both showed significant regional differences in breeding Skylark densities, with the highest relative abundances in the northwest and west. Dry grassland/grass moor habitats supported the highest densities of breeding Skylarks in the CBS, which were significantly higher than in improved grassland or tillage. At the farm-level, Skylark numbers were positively related to wetland habitats but negatively associated with trees in field boundaries, dense ground vegetation and overall density of farm boundaries. At the field-scale, larger fields and unimproved grasslands were preferred.

Conclusion Agri-environment measures tailored to region-specific requirements and to the relatively local habitat preferences of target species are required if population declines of species of conservation concern, including Skylarks, are to be reversed.     

Molecular characterization of rabE, a developmentally regulated Dictyostelium homolog of mammalian rab GTPases

The superfamily of small GTPases includes a subgroup, rab proteins, thought to function in the regulation of discrete steps of membrane traffic. Using a screen based on the polymerase chain reaction, we identified six partial gene sequences of novel rab genes from the soil slime mold amoeba, Dictyostelium. Stretches of conserved sequence for these genes identified them clearly as rab GTPases; unique sequences showed these were novel rab genes. A full-length clone for one gene, which we named rabE, was characterized in detail. The coding sequence of rabE was 1.1 kb in length and contained three introns. RNAse protection analysis revealed rabE expression to be under developmental regulation, with an onset of message expression after 8 h of development. Comparison of the rabE amino acid sequence with the database showed that its unique domains were most similar to the products of four mammalian rab genes. Interestingly, only the rabE protein and its four mammalian homologs contained the sequence WDIAGQE, a variation of a conserved GTPase domain. The WDIAGQE motif thus defines a subgroup of rab proteins. Identification of a Dictyostelium homolog of this group of proteins opens an experimental system to explore the structure and function of this group of WDIAGQE-containing rab proteins.     

Factors controlling the uptake of yeast copper/zinc superoxide dismutase into mitochondria

We have previously shown that a fraction of yeast copper/zinc-superoxide dismutase (SOD1) and its copper chaperone CCS localize to the intermembrane space of mitochondria. In the present study, we have focused on the mechanism by which SOD1 is partitioned between cytosolic and mitochondrial pools. Using in vitro mitochondrial import assays, we show that only a very immature form of the SOD1 polypeptide that is apo for both copper and zinc can efficiently enter the mitochondria. Moreover, a conserved disulfide in SOD1 that is essential for activity must be reduced to facilitate mitochondrial uptake of SOD1. Once inside the mitochondria, SOD1 is converted to an active holo enzyme through the same post-translational modifications seen with cytosolic SOD1. The presence of high levels of CCS in the mitochondrial intermembrane space results in enhanced mitochondrial accumulation of SOD1, and this apparently involves CCS-mediated retention of SOD1 within mitochondria. This retention of SOD1 is not dependent on copper loading of the enzyme but does require protein-protein interactions at the heterodimerization interface of SOD1 and CCS as well as conserved cysteine residues in both molecules. A model for how CCS-mediated post-translational modification of SOD1 controls its partitioning between the mitochondria and cytosol will be presented.     

Membrane traffic and cytokinesis

Cytokinesis, the last step in cell division, is a process common to all eukaryotic life forms. The many mechanisms cells use to divide one parent cell into two progeny reflect the diversity of eukaryotic life. Despite the varied mechanisms cells use, increasing evidence demonstrates that many different cells use 'classical' membrane trafficking proteins for cytokinesis. This review highlights recent evidence for roles for membrane trafficking proteins in cytokinesis.     

Heterodimer formation between superoxide dismutase and its copper chaperone

Copper, zinc superoxide dismutase (SOD1) is activated in vivo by the copper chaperone for superoxide dismutase (CCS). The molecular mechanisms by which CCS recognizes and docks with SOD1 for metal ion insertion are not well understood. Two models for the oligomerization state during copper transfer have been proposed: a heterodimer comprising one monomer of CCS and one monomer of SOD1 and a dimer of dimers involving interactions between the two homodimers. We have investigated protein-protein complex formation between copper-loaded and apo yeast CCS (yCCS) and yeast SOD1 for both wild-type SOD1 (wtSOD1) and a mutant SOD1 in which copper ligand His 48 has been replaced with phenylalanine (H48F-SOD1). According to gel filtration chromatography, dynamic light scattering, analytical ultracentrifugation, and chemical cross-linking experiments, yCCS and this mutant SOD1 form a complex with the correct molecular mass for a heterodimer. No higher order oligomers were detected. Heterodimer formation is facilitated by the presence of zinc but does not depend on copper loading of yCCS. The complex formed with H48F-SOD1 is more stable than that formed with wtSOD1, suggesting that the latter is a more transient species. Notably, heterodimer formation between copper-loaded yCCS and wtSOD1 is accompanied by SOD1 activation only in the presence of zinc. These findings, taken together with structural, biochemical, and genetic studies, strongly suggest that in vivo copper loading of yeast SOD1 occurs via a heterodimeric intermediate.