15N natural abundances and N use by tundra plants

Plant species collected from tundra ecosystems located along a north-south transect from central Alaska to the north coast of Alaska showed large and consistent differences in ¹⁵N natural abundances. Foliar ¹⁵N values varied by about 10% among species within each of two moist tussock tundra sites. Differences in ¹⁵N contents among species or plant groups were consistent across moist tussock tundra at several other sites and across five other tundra types at a single site. Ericaceous species had the lowest ¹⁵N values, ranging between about –8 to –6. Foliar ¹⁵N contents increased progressively in birch, willows and sedges to maximum ¹⁵N values of about +2 in sedges. Soil ¹⁵N contents in tundra ecosystems at our two most intensively studied sites increased with depth and ¹⁵N values were usually higher for soils than for plants. Isotopic fractionations during soil N transformations and possibly during plant N uptake could lead to observed differences in ¹⁵N contents among plant species and between plants and soils. Patterns of variation in ¹⁵N content among species indicate that tundra plants acquire nitrogen in extremely nutrient-poor environments by competitive partitioning of the overall N pool. Differences in plant N sources, rooting depth, mycorrhizal associations, forms of N taken up, and other factors controlling plant N uptake are possible causes of variations in ¹⁵N values of tundra plant species.     

Diversity of mercury resistance plasmids obtained by exogenous isolation from the bacteria of sugar beet in three successive years

Abstract: Self-transmissible plasmids conferring mercury resistance were exogenously isolated from the bacterial populations of sugar beet roots (rhizoplane) and leaves (phyllosphere) into a Pseudomonas putida recipient. Fifty rhizoplane plasmids and 29 phyllosphere plasmids (60–383 kb) were purified. Numerical analysis of plasmid DNA restriction enzyme digest patterns identified five distinct groups. Three of these plasmid groups were isolated from sugar beet crops grown at the same site over three consecutive years, demonstrating their established presence. Each group of plasmids comprised individual isolates with structural additions or deletions. The frequency of exogenous isolation correlated with factors likely to influence plant growth, bacterial activity and the physiological state of donors prior to sampling. All plasmids investigated conferred narrow spectrum mercury resistance with a reductase detoxification mechanism. None of the plasmids conferred resistance to a range of antibiotics, other heavy metals, or to UV, and following transfer to recipient bacteria the range of carbon source utilisation was not altered. This is the first report of the persistence of Pseudomonas spp. plasmid structural types isolated over several years from a terrestrial habitat.     

Enantioselective inhibition of the epidermal growth factor receptor tyrosine kinase by 4-(α-phenethylamino)quinazolines

4-Benzylaminoquinazolines can be potent reversible inhibitors of the EGFR tyrosine kinase at the ATP binding site. Examination of benzylic methylation reveals that an (R)-methyl group is four- to six-fold activating, with an optimal K_i of 630 pM for compound II. In sharp contrast, (S)-methylation causes a > 30 to 500-fold loss of inhibitory activity, showing that the ATP-binding site of the receptor has very low tolerance for even moderate out-of-plane bulk in certain directions. It is suggested that the best of these inhibitors can induce a conformation of the kinase not available to poorer inhibitors.     

Glyphosate-tolerant CP4 and GOX genes as a selectable marker in wheat transformation

The lack of alternative selectable markers in crop transformation has been a substantial barrier for commercial application of agricultural biotechnology. We have developed an efficient selection system for wheat transformation using glyphosate-tolerant CP4 and GOX genes as a selectable marker. Immature embryos of the wheat cultivar Bobwhite were bombarded with two separate plasmids harboring the CP4/GOX and GUS genes. After a 1 week delay, the bombarded embryos were transferred to a selection medium containing 2 mM glyphosate. Embryo-derived calli were subcultured onto the same selection medium every 3 weeks consecutively for 9–12 weeks, and were then regenerated and rooted on selection media with lower glyphosate concentrations. Transgenic plants tolerant to glyphosate were recovered. ELISA assay confirmed expression of the CP4 and GOX genes in R₀ plants. Southern blot analysis demonstrated that the transgenes were integrated into the wheat genomes and transmitted to the following generation. The use of CP4 and GOX genes as a selectable marker provides an efficient, effective, and alternative transformation selection system for wheat.     

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine,USA

We followed the movements of ¹⁵N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha^–1 year^–1 using ¹⁵N-labelled nitric acid solutions (¹⁵N = 344 ) during four successive growing seasons (April–October). The ¹⁵N contents of foliage, bolewood, forests floor and mineral soil (0–5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.     

Determinants of MTT reduction in rat hepatocytes

The determinants of reduction of the dye MTT (3-[4,5dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) in rat hepatocytes have been investigated. NADH, NADPH, and succinate were substrates for MTT reduction in rat liver homogenate, activity being greatest with NADH and least with succinate. Similar results were obtained with submitochondrial particles isolated from rat liver. NAD(P)Hdependent reduction of MTT was also detected in rat liver microsomes and cytosol. Rotenone, at a concentration that inhibited NAD(P)H-dependent MTT reduction in submitochondrial particles, did not inhibit MTT reduction in rat hepatocytes. Malonate, at a concentration that inhibited succinate-dependent MTT reduction in liver homogenate, did not inhibit MTT reduction in rat hepatocytes. Incubation of rat hepatocytes with ethanol or lactate (increase NADH levels), dicoumarol (inhibitor of DT-diaphorase), aminopyrine or hexobarbitone (substrates for the NADPH-requiring cytochrome P450-dependent microsomal monooxygenase) led to significant increases in the level of cellular MTT reduction. From these data, it is concluded that extramitochondrial NAD(P)H is the principal reductant for MTT reduction in rat hepatocytes, with mitochondrial dehydrogenase activity being only a minor contributor. It is also possible that cellular generation of superoxide (as might be expected on redox cycling of endogenous quinones following inhibition of DT diaphorase by dicoumarol) may be another source of MTT reduction. Caution should be exercised in ascribing an alteration in the level of cellular MTT reduction to a change in mitochondrial performance in the absence of corroborating evidence.     

Using Methods in the Field: A Practical Introduction and Casebook; Creative Problem Solving in the Field

Using Methods in the Field:. Practical Introduction and Casebook. Victor C. de Munck and Elisa J. Sobo. eds. Walnut Creek, CA: AltaMira Press, 1998. 288 pp.
Creative Problem Solving in the Field. William Foote Whyte. Walnut Creek, CA: AltaMira Press. 1997. 162 pp.     

Adverse effects of tempol on hidrosaline balance in rats with acute sodium overload

We studied the effects of tempol, an oxygen radical scavenger, on hydrosaline balance in rats with acute sodium overload. Male rats with free access to water were injected with isotonic (control group) or hypertonic saline solution (0.80 mol/l NaCl) either alone (Na group) or with tempol (Na-T group). Hydrosaline balance was determined during a 90 min experimental period. Protein expressions of aquaporin 1 (AQP1), aquaporin 2 (AQP2), angiotensin II (Ang II) and endothelial nitric oxide synthase (eNOS) were measured in renal tissue. Water intake, creatinine clearance, diuresis and natriuresis increased in the Na group. Under conditions of sodium overload, tempol increased plasma sodium and protein levels and increased diuresis, natriuresis and sodium excretion. Tempol also decreased water intake without affecting creatinine clearance. AQP1 and eNOS were increased and Ang II decreased in the renal cortex of the Na group, whereas AQP2 was increased in the renal medulla. Nonglycosylated AQP1 and eNOS were increased further in the renal cortex of the Na-T group, whereas AQP2 was decreased in the renal medulla and was localized mainly in the cell membrane. Moreover, p47-phox immunostaining was increased in the hypothalamus of Na group, and this increase was prevented by tempol. Our findings suggest that tempol causes hypernatremia after acute sodium overload by inhibiting the thirst mechanism and facilitating diuresis, despite increasing renal eNOS expression and natriuresis.