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.     

Megagyrolithes ardescensis N. Gen., N. SP.,Trace fossile nouvelle du Valanginien d'Ardeche (France)

A new trace fossil corresponding to a spiral burrow is described in the Upper Valanginian of the Ardèche area (France). It seems to be restricted to marine hemipelagic facies (outer shelf and upper slope).     

Relationship between the phagocytic inhibition of the rat reticuloendothelial system and the anticholinesterasic effect of an insecticide, Carbaryl (author's transl)]

Phagocytic activity of the reticuloendothelial system (RES) and blood cholinesterase activity were determined in male rats after veinous administrations of carbaryl and 1-naphthol, a carbaryl metabolite. The various parameters were measured 1, 24, 48 and 72 hours after administration of the following four doses per 100 g body weight : 1.875, 3.75, 7.5 and 15 mumol. 1. Results showed an inhibition of the RES phagocytic activity (clearance of colloidal carbon) after carbaryl administration; although 1.875 mumol/100 g had no effect, the other doses inhibited RES activity, blockade time being a function of the dose given. The phagocytic function had returned to normal 72 hr after carbaryl administration. 2. Reductions in spleen weight and protein content were observed together with the RES blockade. 3. At all four doses, the anticholinesterase effect was already apparent one hour after carbaryl administration. 4. 1-naphthol, one of carbaryl's chief metabolites, had no effect either on the RES or on the different parameters studied. These results show a relationship between the phagocytic inhibition of the reticuloendothelial system and the anticholinesterasic effect by carbaryl. They suggest an inhibition of some esterases of macrophages interfering with the phagocytosis.     

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.     

AN AUTORADIOGRAPHIC AND HISTOCHEMICAL LOCALIZATION OF SULFATED POLYSACCHARIDES IN EUCHEUMA NUDUM (RHODOPHYTA)1

The predominant sulfated polysaccharide, ?-carrageenan, was localized in the middle lamella of epidermal, cortical and medullary cells of Eucheumanudum J. Agardh. Autoradiographic studies with ³⁵SO₄= indicated that the label was first incorporated in the inner wall and ultimately deposited in the middle lamella of all cells, and in an outer wall layer of the epidermal cells. There was no evidence for cytoplasmic incorporation of the label. The middle lamella stained with alcian blue, was γ-metachromatic with toluidina blue O and bound diaminobenzidine-osmium tetroxide. This region was also positive with periodic acid-Schiff's (PAS) ragent, possibly demonstrating cellulose and/or a nonsulfated precursor of ?-carrageenan. A proposed model for extracellular sulfation includes production and secretion of a nonsulfated polygalactan and sulfotransferase enzyme(s) by the golgi apparatus and endoplasmic reticulum, respectively. Free sulfate in the wall would be bound to the precursor polysaccharide, with much of the resulting carrageenan migrating to the middle lamella facilitating mutual cell growth.