Rhizome dynamics and resource storage in Phragmites australis

Seasonal changes in rhizome concentrations of total nonstructural carbohydrates (TNC), water soluble carbohydrates (WSC), and mineral nutrients (N, P and K) were monitored in two Phragmites australis stands in southern Sweden. Rhizome biomass, rhizome length per unit ground area, and specific weight (weight/ length ratio) of the rhizomes were monitored in one of the stands.Rhizome biomass decreased during spring, increased during summer and decreased during winter. However, changes in spring and summer were small (< 500 g DW m-2) compared to the mean rhizome biomass (approximately 3000 g DW m^–2). Winter losses were larger, approximately 1000 g DW m-2, and to a substantial extent involved structural biomass, indicating rhizome mortality. Seasonal changes in rhizome length per unit ground area revealed a rhizome mortality of about 30% during the winter period, and also indicated that an intensive period of formation of new rhizomes occurred in June.Rhizome concentrations of TNC and WSC decreased during the spring, when carbohydrates were translocated to support shoot growth. However, rhizome standing stock of TNC remained large (> 1000 g m^–2). Concentrations and standing stocks of mineral nutrients decreased during spring/ early summer and increased during summer/ fall. Only N, however, showed a pattern consistent with a spring depletion caused by translocation to shoots. This pattern indicates sufficient root uptake of P and K to support spring growth, and supports other evidence that N is generally the limiting mineral nutrient for Phragmites.The biomass data, as well as increased rhizome specific weight and TNC concentrations, clearly suggests that reloading of rhizomes with energy reserves starts in June, not towards the end of the growing season as has been suggested previously. This resource allocation strategy of Phragmites has consequences for vegetation management.Our data indicate that carbohydrate reserves are much larger than needed to support spring growth. We propose that large stores are needed to ensure establishment of spring shoots when deep water or stochastic environmental events, such as high rhizome mortality in winter or loss of spring shoots due to late season frost, increase the demand for reserves.     

Sequence analysis of two exons from the murine dystrophin locus

We have isolated two genomic clones from the murine dystrophin locus, containing single exons encoding protein sequence from the putative actin-binding domain of the amino-terminus and the terminal portion of the triple helical domain. Using interspecific backcross progeny mice, both clones were shown to be X-linked. Sequence analysis indicated that the amino-terminal clone contains a 173 bp exon exhibiting 90% nucleotide sequence identity to human dystrophin exon 6, whilst the C-terminal clone contains a 61 bp exon with 93% nucleotide sequence identity to the human cDNA sequence.

     

Expression of the Opc protein correlates with invasion of epithelial and endothelial cells by Neisseria meningitidis

Whereas capsulate strains of Neisseria meningitidis are dependent on pili for adhesion to human endothelial and epithelial cells, strains which lacked assembled pili and were partially capsule-deficient adhered to and invaded human endothelial and epithelial cells if they expressed the Opc protein. Bacteria expressing low or undetectable levels of Opc protein failed to adhere to or invade eukaryotic cells. In addition, the presence of OpaAC751 protein on the surface of bacteria did not increase bacterial interactions with host cells. Association of Opc-expressing bacteria was inhibited by antibodies against Opc. Invasion was dependent on the host-cell cytoskeletal activity and was inhibited by cytochalasin D. In some cells, infected at the apical surface, bacteria emerging from basal surface were detected by electron microscopy. Opc is found in diverse meningococci and may represent a common virulence factor which facilitates adherence and invasion by these bacteria.     

A multi-recombination model for the mtDNA rearrangements seen in maize cmsT regenerated plants

Regeneration of plants from maize cytoplasmic male sterile type T (cmsT) callus tissue culture promotes, in some instances, genetic variability in their mitochondrial genomes. These mutations have been analyzed in various cmsT regenerated plants that have or have not regained the male fertile phenotype. A unique multi-recombination model explains the various mitochondrial genome rearrangements. First, recombination involving two different sets of direct repeats gives rise to subgenomic recombinant circles. Second, intermolecular recombination between some selected subgenomes gives rise to a new rearranged master chromosome. The consequence of these events is the formation of a new master chromosome containing sequence deletions and duplications when compared to the progenitor. This new mitochondrial genome seems stable, although it does not contain the entire genetic complexity of the progenitor.     

Deregulation of arginine biosynthesis in Synechococcus sp. PCC7942

Summary In order to deregulate arginine biosynthesis in Synechococcus sp. PCC7942, d-arginine-resistant cell lines were selected following ethyl methanesulfonate mutagenesis of wild-type (WT) cells. Three of these arginine-producing mutant (APM) cell lines, APM1, APM31 and APM40, were putative regulatory mutants based upon secretion of l-arginine into their growth medium. HPLC of lyophilized post-harvest supernatants of APM 31 and 40 resolved two predominant amino acids, arginine and citrulline. In-vitro activity of N-acetylglutamate kinase (NAGK), the proposed regulatory enzyme of the arginine pathway, was about 100-fold less sensitive to l-arginine inhibition in extracts from APM 31 and 40 than the enzyme in WT extracts. The enzyme from APM 1 was 20-fold less sensitive to l-arginine inhibition than WT. The most likely site of mutation in each of the APM cell lines is in the gene for NAGK, rendering the enzymes insensitive to l-arginine feedback control. These strains can be utilized for the phototrophic production of arginine. Offprint requests to: S. E. Bingham     

Seed size,pollination costs and angiosperm success

Summary Seed plants capture pollen before seeds are dispersed and abort unpollinated ovules. As a result, each seed is associated with an accessory cost that represents the costs of pollen capture and the costs of aborted ovules. Accessory costs may explain the minimum seed size among species, because these costs are likely to comprise a greater proportion of total reproductive allocation in species with smaller seeds. For very small propagules, the costs of pollination may not be worth the benefits, perhaps explaining the persistence of pteridophytic reproduction at small propagule sizes. The smallest angiosperm seeds are much smaller than the smallest gymnosperm seeds, both in the fossil record and in the modern flora. This suggests that angiosperms can produce pollinated ovules more cheaply than gymnosperms. Pollination becomes less efficient as a species decreases in abundance, and this loss of efficiency is greater for species with a higher accessory cost per seed. We propose that the greater reproductive efficiency of angiosperms when rare can explain why angiosperm-dominated floras were more speciose than the gymnosperm-dominated floras they replaced.     

Dichogamy,gender variation and bet-hedging inPseudowintera colorata

Summary Temporal patterns of variability in the longevity of the male and female phases of individual flowers and in the gender expression of plants of a dichogamous New Zealand tree,Pseudowintera colorata (Winteraceae), were documented in field studies. Two measures for the duration of phases in a dichogamous flower are distinguished; the nominal phases based on morphological features of the flower, and the effective phases reflecting the duration of their functions. Flower and phase longevity and phenotypic gender varied considerably throughout the season and among individuals. Temporal variability in phenotypic gender was loosely synchronized among the 12 plants sampled. Three effects of an environmental factor (temperature) were noted. First, increased temperatures shortened the duration of the female phase but had no effect on the duration of the male phase. Second, pollination frequency was positively correlated with temperature. These results indirectly suggest that increased pollination may shorten the duration of the female phase. Third, average population maleness, measured as the proportion of open flowers in the population on a given day which were in the male phase, was positively correlated with temperature. It is postulated that temperature indirectly influences temporal patterns of gender expression in the population through its differential effects on the longevity of the male and female phases in individual flowers. A theoretical model of bet-hedging shows that, if the direction of an environmental effect on the proportions of the sexual phases is irreversible, selection favours asynchronous dichogamy and reduces the temporal variability as much as possible. If the direction of the response is reversible, heterodichogamy is favoured.     

Degranulation of eosinophilic granule cells induced by capsaicin and substance P in the intestine of the rainbow trout (Oncorhynchus mykiss Walbaum)

Summary Adult rainbow trout (Oncorhynchus mykiss) were injected intraperitoneally with capsaicin, substance P, serotonin, or a control of saline vehicle or bovine serum albumin (0.5 g/g body weight). Fish were sacrificed 30 min and 1,2 and 4 h post-injection, the gut was dissected out, and a small section of the upper intestine was processed for electron microscopy. A significant proportion of eosinophilic granule cells (EGCs) of the intestine were in close association with non-myelinated neuronal bundles in all fish (4 fish per treatment and time period), but there was no significant difference between treatment or time, suggesting that the association was unaffected by these factors. Close examination of EGC ultrastructure showed that fish treated with capsaicin and substance P exhibited limited degranulation of the EGCs in the stratum compactum and extensive crinophagic-like degranulation in the lamina propria. Cells of the lamina propria contained characteristic multivesicular-like bodies. The degranulation was reminiscent of both mast cell degranulation and endocrine cell crinophagy. EGCs of fish treated with serotonin or a control were unaffected, suggesting that the serotoninergic neurons, believed to be involved in gut motility, were not responsible for degranulation. It is apparent that EGCs of the trout intestine may be under nervous control, as has been demonstrated previously for mammalian mast cells.     

Maturation-related changes in mass and elemental contents of secretory granules as measured by electron-microprobe

Summary The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P<0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.