Virus-like particles in the brown algaStreblonema

Summary Ultrastructural examination ofStreblonema sp. revealed icosahedral virus-like particles (135–150 nm) throughout the cytoplasm of vegetative cells. The densely packed particles consist of an osmiophilic coat around a fibrillar core. Most cytoplasmic organelles are excluded from the regions where the particles are extremely abundant, but no degeneration of plastids, mitochondria or dictyosomes is evident. The virogenic stroma contains many ribosomes and fibers possibly representing DNA strands remaining from the lysed nucleus. No decrease in vigor seemed to be associated with the presence of the particles.     

Male gametophyte in maize: II. Pollen vigor in inbred plants

Summary The competitive ability of pollen from inbred plants in mixed pollinations in this study is not merely maintained but enhanced through successive generations of selfing. The data presented suggest two conclusions: 1) the possible existence of pollen-stylar interactions during successive selfings, which select for certain pollen genotypes, those best suited for rapid growth through self styles; and 2) the presence of sporophytic vigor in the heterotic F₁ sporophyte, or its absence in the depressed F₇ sporophyte, is not necessarily demonstrated in the gametophytic generation, perhaps because it can be overwhelmed by other factors, e.g. gametophytic response to selection.     

The use of photosynthesis inhibitor (DCMU) for in situ metabolic and primary production studies on soft bottom benthos

A selective chemical photosynthesis inhibitor, DCMU (Dichorophenyl-dimethylurea), dissolved in DMSO (Dimethyl sulfoxide) was substituted for the dark incubation method commonly used to measure the oxygen consumption in metabolic and primary production studies. We compared oxygen fluxes during light incubations with DCMU and dark incubations procedure, on soft bottom benthos. For this purpose, we studied the effects of different DCMU concentrations. A concentration of 5 · 10^–5 mol l^–1 inside a clear incubation enclosure completely inhibits photosynthesis without affecting the metabolism of soft bottom benthos.     

Purification and properties of tobacco ferredoxin-dependent glutamate synthase,and isolation of corresponding cDNA clones

Ferredoxin-dependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) was purified to electrophoretic homogeneity from leaves of tobacco (Nicotiana tabacum L.). The holoenzyme is a monomeric flavoprotein with a molecular weight of 164 kDa. Polyclonal rabbit antibodies against the purified enzyme were used to isolate a 450-bp Fd-GOGAT cDNA clone (C₁₆) from a tobacco gt11 expression library. A longer Fd-GOGAT cDNA clone (C₃₅) encoding about 70% of the amino acids of tobacco Fd-GOGAT was isolated from a tobacco gt10 cDNA library using C₁₆ as the probe. The amino-acid sequence of the protein encoded by the Fd-GOGAT cDNA clone C₃₅ was delineated. It is very likely that Fd-GOGAT is encoded by two genes in the amphidiploid genome of tobacco while only a single Fd-GOGAT gene appears to be present in the diploid genome of Nicotiana sylvestris. Two Fd-GOGAT isoenzymes could be distinguished in extracts of tobacco leaf protein. In contrast, a single Fd-GOGAT protein species was detected in leaves of Nicotiana sylvestris speg. et Comes. In tobacco leaves, the 6-kb Fd-GOGAT mRNA is about 50-fold less abundant than chloroplastic glutamine synthetase (EC 6.3.1.2) mRNA. Both Fd-GOGAT mRNA and Fd-GOGAT protein accumulated during greening of etiolated tobacco leaves, and a concomitant increase in Fd-GOGAT activity was observed. These results indicate that tobacco Fd-GOGAT gene expression is light-inducible. Levels of Fd-GOGAT mRNA in tobacco organs other than leaves were below the detection limit of our Northern-blot analysis. Polypeptides of Fd-GOGAT were present in tobacco leaves and, to a lesser extent, in pistils and anthers, but not in corollas, stems and roots. These results support organ specificity in tobacco Fd-GOGAT gene expression.Abbreviations bp base pairs - Fd-GOGAT ferredoxin-dependent glutamate synthase - GS glutamine synthetase - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate The authors wish to thank Juan Luis Gómez Pinchetti (Marine Plant Biotechnology Laboratory) for his assistance during the experiments. This study was supported by grants received from SAREC (Swedish Agency for Research Cooperation with Developing Countries), Carl Tryggers Fund for Scientific Research (K. Haglund), SJFR (Swedish Council for Forestry and Agricultural Research) (M. Björk, M. Pedersén), CITYT Spain (SAB 89-0091 and MAR 91-1237, M. Pedersén) and CICYT Spain (Z. Ramazanov, invited professor of Ministerio de Educatión y Ciencia, Spain). The planning of this cooperation was facilitated by COST-48.     

Carbon and nitrogen metabolism in legume root nodules

The literature concerning the metabolism of carbon compounds during the reduction, assimilation and translocation of nitrogen in root nodules of leguminous plants is reviewed. The reduction of dinitrogen requires an energy source (ATP) and a reluctant which are both supplied by respiratory catabolism of carbohydrates produced by the host plant. Photosynthates are also required to generate the carbon skeletons for amino acid or urcide synthesis during the assimilation of ammonia produced by the bacteria within the nodule tissue. Competition for photosynthates occurs between the bacteroids, nodule tissue and the various vegetative and reproductive sinks in the host plant. The nature of carbon compounds involved in these processes, their routes of metabolism, the mechanisms of control and the partitioning of metabolises between the various sites of utilization are only poorly understood. It is apparent that dinitrogen is reduced to ammonia in the bacteroids. Both fast- and slow-growing strains of Rhizobium possess the Entner-Doudoroff pathway of glucose catabolism, and some, if not all, enzymes of the Emden-Meyerhof pathway. Some bacterial cultures also metabolize carbon through the ketogluconate pathway but only the fast-growing strains of cultured rhizobia possess the key enzyme of the pentose phosphate pathway (6-phosphogluconate dehydrogenase). The host cells are thought to contain the complete Emden-Meyerhof pathway and tricarboxylic acid cycle, which provides the carbon skeletons for assimilation of the ammonia, formed by the bacteroids, into α-amino acids. A pathway of anapleurotic carbon conservation, operative in the host cells, synthesizes oxaloacetic acid through β-carboxylation of phosphoenol pyruvate. This process could be important in the recapture and assimilation of respired CO₂ in the rhizosphere. The main route of assimilation of ammonia produced by the bacteroids would appear to be via the glutamine synthetase-glutamate synthase pathway in the host cells. However, glutamate dehydrogenase may also be involved in ammonia assimilation. These enzymes also occur in in vitro cultures of Rhizobium and in bacteroids where they presumably participate in the synthesis of amino acids for growth of the bacteria or bacteroids. Nitrogen assimilated into glutamine or glutamate is exported from the nodules in a variety of forms, which include asparagine, glutamine, aspartate, homoserine and allantoates, in proportions which depend on the legume species. Studies on regulation of the overall process have focussed on expression of bacteroid genes and on the control of enzyme activity, at the level of nitrogenase and enzymes of nitrogen assimilation in particular. However, due to the wide range of experimental techniques, environmental conditions and plant species which have been used, no clear conclusions can yet be drawn. The pathways of carbon flow in nitrogen metabolism, particularly in relation to the synthesis of ureides and the regulation of carbon metabolism, remain key areas for future research in symbiotic nitrogen fixation.     

Male gametophyte in maize: Influences of the gametophytic genotype

Summary Size and variation in pollen samples were investigated in several successively selfed generations. A significant decrease in mean diameter of pollen grains accompanied inbreeding; also decreased variation in pollen size from individual plants was observed. Since loss of developmental homeostasis in the sporophyte could affect variation in the gametophyte, sporophytic characters were observed as a control. The main conclusion reached in this study was that pollen diameter is influenced by the gametophytic genotype.     

Light and drug induced changes of epiphysial synaptic ribbons

Summary In the present investigation experiments were carried out to determine whether the functionally obscure synaptic ribbons of mammalian pinealocytes can be affected by acute changes in environmental lighting and which chemical processes may be involved in their regulation. Experiments carried out in male guinea-pigs have shown that the amounts of synaptic ribbons are immediately affected by changes in the lighting pattern. Extension of the light period reduced the normally occurring increase, whereas extension of the dark period inhibited the normally occurring decrease in the amount of synaptic ribbons. Results following injections of a number of drugs known to influence pineal function (noradrenaline, L-DOPA, propranolol, reserpine and p-chlorophenylalanine, respectively) suggest that synaptic ribbons may be directly or indirectly regulated by -adrenergic mechanisms.Dedicated to Professor Wolfgang Bargmann on the occasion of his 70th birthday.     

Cell surface changes during the differentiation of Dictyostelium discoideum : Interaction of cells with Concanavalin A

The parameters affecting the agglutination of cells of Dictyostelium discoideum by Concanavalin A (ConA) have been investigated. Under the incubation conditions employed, incubation time does not markedly affect agglutination, but there are distinct optima for cell density and gyration speed. Agglutination does not occur at low temperatures, but the transition temperature between the unagglutinated and fully agglutinated states is markedly influenced by ConA concentration. The rate of aggregation of strain NC-4 is considerably reduced by ConA. In contrast, the differentiation of strain Ax-2 in the presence of ConA is either unaffected or only slightly inhibited, depending on the incubation conditions. Succinylated-ConA binds to the same sites as the unmodified lectin, but has no effect on the differentiation of strain NC-4, suggesting that ConA binding sites are not directly involved in cell-cell contacts vital to the differentiation of D. discoideum. There is a gradual decrease in the susceptibility of cells of D. discoideum to agglutination by ConA as the cells pass from exponential growth phase to stationary growth phase in axenic medium and from vegetative amoebae to aggregates on a solid substratum. These results provide quantitative evidence for a gradual change in carbohydrate containing binding sites during differentiation.