Branch growth of riparian cottonwoods: a hydrologically sensitive dendrochronological tool

 The conservation of riparian (river valley flood plain) forests relies on the provision of instream flows that are sufficient to sustain tree growth. In the present study, annual branch growth increments were investigated as an indicator of environmental favorability for riparian cottonwoods. Trees of three species, Populus angustifolia, P. balsamifera, and P. deltoides, and their natural interspecific hybrids, were studied at five sites along the Oldman and South Saskatchewan rivers in Alberta, Canada. Annual branch growth increments for the interval from 1983 to 1992 were positively correlated with stream flows (r ² = 0.79 at Lethbridge) and slightly negatively correlated with weather variables that contribute to water demand: evaporation, temperature, wind, and/or sunshine. The combination of January to May stream flow (water supply) and June evaporation (water demand) almost entirely accounted for the branch growth variation across years (r ² = 0.91 at Lethbridge). Tree ring increments were also investigated but were less closely correlated than branch increments across trees or with stream flow. Branch growth increments thus provide an accurate but short duration (1 or 2 decades) record of environmental favorability for growth. The close correlation between branch growth and stream flow indicates that water is the principal limitation to growth of these riparian cottonwoods and that these trees obtained their water from a source linked to the stream, the riparian water table. Analyses of branch increments should provide a management tool for (i) determining instream flow needs for riparian cottonwoods and (ii) analyzing impacts of stream flow alterations due to river damming or water diversion. Received: 8 May 1997 / Accepted: 23 September 1997     

Three rhamnosyltransferases responsible for assembly of the A-band D-rhamnan polysaccharide in Pseudomonas aeruginosa: a fourth transferase, WbpL, is required for the initiation of both A-band and B-band lipopolysaccharide synthesis

The Pseudomonas aeruginosa A-band lipopolysaccharide (LPS) molecule has an O-polysaccharide region composed of trisaccharide repeat units of α1 → 2, α1 → 3, α1 → 3 linked D -rhamnose (Rha). The A-band polysaccharide is assembled by the α-D -rhamnosyltransferases, WbpX, WbpY and WbpZ. WbpZ probably transfers the first Rha residue onto the A-band accepting molecule, while WbpY and WbpX subsequently transfer two α1 → 3 linked Rha residues and one α1 → 2 linked Rha respectively. The last two transferases are predicted to be processive, alternating in their activities to complete the A-band polymer. The genes coding for these transferases were identified at the 3′ end of the A-band biosynthetic cluster. Two additional genes, psecoA and uvrD, border the 3′ end of the cluster and are predicted to encode a co-enzyme A transferase and a DNA helicase II enzyme respectively. Chromosomal wbpX, wbpY and wbpZ mutants were generated, and Western immunoblot analysis demonstrates that these mutants are unable to synthesize A-band LPS, while B-band synthesis is unaffected. WbpL, a transferase encoded within the B-band biosynthetic cluster, was previously proposed to initiate B-band biosynthesis through the addition of Fuc2NAc (2-acetamido-2,6-dideoxy-D -galactose) to undecaprenol phosphate (Und-P). In this study, chromosomal wbpL mutants were generated that did not express A band or B band, indicating that WbpL initiates the synthesis of both LPS molecules. Cross-complementation experiments using WbpL and its homologue, Escherichia coli WecA, demonstrates that WbpL is bifunctional, initiating B-band synthesis with a Fuc2NAc residue and A-band synthesis with either a GlcNAc (N-acetylglucosamine) or GalNAc (N-acetylgalactosamine) residue. These data indicate that A-band polysaccharide assembly requires four glycosyltransferases, one of which is necessary for initiating both A-band and B-band LPS synthesis.     

Notes from some crypt watchers: regulation of renewal in the mouse intestinal epithelium

The mouse intestinal epithelium undergoes rapid renewal throughout life, thereby requiring continuous coordination of its cellular proliferation, differentiation, and death programs. Recent advances in our understanding of this process have highlighted some of the molecules that regulate renewal and their potential roles in gut neoplasia.     

Selective Perturbation of Apical Membrane Traffic by Expression of Influenza M2, an Acid-activated Ion Channel, in Polarized Madin–Darby Canine Kidney Cells

The function of acidification along the endocytic pathway is not well understood, in part because the perturbants used to modify compartmental pH have global effects and in some cases alter cytoplasmic pH. We have used a new approach to study the effect of pH perturbation on postendocytic traffic in polarized Madin–Darby canine kidney (MDCK) cells. Influenza M2 is a small membrane protein that functions as an acid-activated ion channel and can elevate the pH of the trans-Golgi network and endosomes. We used recombinant adenoviruses to express the M2 protein of influenza virus in polarized MDCK cells stably transfected with the polymeric immunoglobulin (Ig) receptor. Using indirect immunofluorescence and immunoelectron microscopy, M2 was found to be concentrated at the apical plasma membrane and in subapical vesicles; intracellular M2 colocalized partly with internalized IgA in apical recycling endosomes as well as with the trans-Golgi network marker TGN-38. Expression of M2 slowed the rate of IgA transcytosis across polarized MDCK monolayers. The delay in transport occurred after IgA reached the apical recycling endosome, consistent with the localization of intracellular M2. Apical recycling of IgA was also slowed in the presence of M2, whereas basolateral recycling of transferrin and degradation of IgA were unaffected. By contrast, ammonium chloride affected both apical IgA and basolateral transferrin release. Together, our data suggest that M2 expression selectively perturbs acidification in compartments involved in apical delivery without disrupting other postendocytic transport steps.     

A Splice-Isoform of Vesicle-associated Membrane Protein-1 (VAMP-1) Contains a Mitochondrial Targeting Signal

Screening of a library derived from primary human endothelial cells revealed a novel human isoform of vesicle-associated membrane protein-1 (VAMP-1), a protein involved in the targeting and/or fusion of transport vesicles to their target membrane. We have termed this novel isoform VAMP-1B and designated the previously described isoform VAMP-1A. VAMP-1B appears to be an alternatively spliced form of VAMP-1. A similar rat splice variant of VAMP-1 (also termed VAMP-1B) has recently been reported. Five different cultured cell lines, from different lineages, all contained VAMP-1B but little or no detectable VAMP-1A mRNA, as assessed by PCR. In contrast, brain mRNA contained VAMP-1A but no VAMP-1B. The VAMP-1B sequence encodes a protein identical to VAMP-1A except for the carboxy-terminal five amino acids. VAMP-1 is anchored in the vesicle membrane by a carboxy-terminal hydrophobic sequence. In VAMP-1A the hydrophobic anchor is followed by a single threonine, which is the carboxy-terminal amino acid. In VAMP-1B the predicted hydrophobic membrane anchor is shortened by four amino acids, and the hydrophobic sequence is immediately followed by three charged amino acids, arginine-arginine-aspartic acid. Transfection of human endothelial cells with epitope-tagged VAMP-1B demonstrated that VAMP-1B was targeted to mitochondria whereas VAMP-1A was localized to the plasma membrane and endosome-like structures. Analysis of C-terminal mutations of VAMP-1B demonstrated that mitochondrial targeting depends both on the addition of positive charge at the C terminus and a shortened hydrophobic membrane anchor. These data suggest that mitochondria may be integrated, at least at a mechanistic level, to the vesicular trafficking pathways that govern protein movement between other organelles of the cell.     

Roadside Revegetation in Glacier National Park, U.S.A.: Effects of Herbicide and Seeding Treatments

From 1992 to 1995 we experimentally evaluated the effectiveness of several revegetation treatments along a segment of Going-to-the-Sun Highway in Glacier National Park, U.S.A. This segment, reconstructed during the spring and summer of 1992, is bordered by fescue prairie vegetation and is known to be susceptible to invasion by several alien species, including Centaurea maculosa (spotted knapweed) and Phleum pratense (common timothy). We used a split plot study design to evaluate the effectiveness of herbicide and seeding treatments on assisting recovery of native flora and limiting the establishment of alien species. The herbicide treatment consisted of a yearly herbicide spray application of clopyralid (3,6-dichloropicolinic acid). Five seeding treatments were evaluated, three of which included an indigenous graminoid-forb seed mix. Percent canopy coverages of four species groups—alien graminoids, native graminoids, alien forbs, and native forbs—were determined in July 1995. In addition, community-level patterns in sprayed plots and unsprayed plots were compared with a reference site of native fescue prairie. Herbicide treatments decreased mean canopy coverage of alien forbs (treated = 4.2%, untreated = 23.4%) and increased mean canopy coverage of native graminoids slightly (treated = 6.3%, untreated = 4.0%). But herbicide treatments reduced mean coverage of native forbs (treated = 3.9%, untreated = 8.9%) and likely increased coverage of alien graminoids. Treatments that included a fall 1992 seed mix increased native graminoid coverages 2.8–4.6 times, although coverages were still lower than those attained by alien graminoids. Native and alien forb coverage appeared unaffected by seeding treatments. Species composition was less diverse in sprayed plots and more dominated by alien grasses than in unsprayed plots and the reference site. Areas for additional study are suggested, including seed bank assays to determine treatment effects on recruitment of alien versus native species and the use of native graminoids to create low-diversity communities with high canopy coverages to resist establishment of alien species.     

Ocular and regional cerebral blood flow in aging Fischer-344 rats

Vascularremodeling and changes in vascular responsiveness occur in the ratcerebrum with old age. This includes reductions in cerebral arteriolarnumerical density, cross-sectional area, distensibility, the relativeproportion of distensible elements in the cerebral arteriolar wall, andreduced endothelium-dependent relaxation. The purpose of this study wasto test the hypothesis that old age results in an increase in vascularresistance and, correspondingly, a decrease in blood flow to ocular,regional cerebral, and spinal tissue in the rat. Blood flow wasmeasured in the eye, olfactory bulb, left and right cerebrum, pituitary gland, midbrain, pons, cerebellum, medulla, and spinal cord of juvenile(2-mo-old, n = 6), adult (6-mo-old,n = 7), and aged (24-mo-old,n = 7) male Fischer-344 rats. Arterialpressure and blood flow were used to calculate vascular resistance.Vascular resistance in the eye of aged rats (6.03 ± 1.08 mmHg · ml¹ · min · 100 g) was higher than that in juvenile (3.83 ± 0.38 mmHg · ml¹ · min · 100 g) and adult rats (3.12 ± 0.24 mmHg · ml¹ · min · 100 g). Similarly, resistance in the pons of older rats (2.24 ± 0.55 mmHg · ml¹ · min · 100 g) was greater than in juvenile (0.66 ± 0.06 mmHg ·ml¹ · min · 100 g) and adult rats (0.80 ± 0.11 mmHg · ml¹ · min · 100 g). In contrast, vascular resistance in the pituitary gland was lowerin the aged rats (juvenile, 3.09 ± 0.22; adult, 2.79 ± 0.42;aged, 1.73 ± 0.32 mmHg · ml¹ · min · 100 g, respectively). Vascular resistance was not different in othercerebral tissues or in the spinal cord in the aged rats. These datasuggest that regional cerebral and spinal blood flow and vascularresistance remain largely unchanged in conscious aged rats at rest butthat elevations in ocular vascular resistance and, correspondingly,decreases in ocular perfusion with advanced age could have seriousadverse effects on visual function.