Consequence of Absence of Nitrate Reductase Activity on Photosynthesis in Nicotiana plumbaginifolia Plants

Chlorate-resistant Nicotiana plumbaginifolia (cv Viviani) mutants were found to be deficient in the nitrate reductase apoprotein (NR⁻nia). Because they could not grow with nitrate as sole nitrogen source, they were cultivated as graftings on wild-type Nicotiana tabacum plants. The grafts of mutant plants were chlorotic compared to the grafts of wild type. Mutant leaves did not accumulate nitrogen and nitrate but contained less malate and more glutamine than wild leaves. They exhibited a slight increase of the proportion of the light-harvesting chlorophyll a/b protein complexes and a lowering of the efficiency of energy transfer between these complexes and the active centers. After a 3 second ¹⁴CO₂ pulse, the total ¹⁴C incorporation of the mutant leaves was approximately 20% of that of the control. The ¹⁴C was essentially recovered in ribulose bisphosphate in these plants. It was consistent with a decline of ribulose bisphosphate carboxylase activity observed in the mutant. After a 3 second ¹⁴CO₂ pulse followed by a 60 second chase with normal CO₂, ¹⁴C was mainly accumulated in starch which was labeled more in the mutant than in the wild type. These results confirm the observation that in the nitrate reductase deficient leaves, chloroplasts were loaded with large starch inclusions preceding disorganization of the photosynthetic apparatus.     

A major difference between the divergence patterns within the lines-1 families in mice and voles

L1 retroposons are represented in mice by subfamilies of interspersed sequences of varied abundance. Previous analyses have indicated that subfamilies are generated by duplicative transposition of a small number of members of the L1 family, the progeny of which then become a major component of the murine L1 population, and are not due to any active processes generating homology within preexisting groups of elements in a particular species. In mice, more than a third of the L1 elements belong to a clade that became active approximately 5 Mya and whose elements are > or = 95% identical. We have collected sequence information from 13 L1 elements isolated from two species of voles (Rodentia: Microtinae: Microtus and Arvicola) and have found that divergence within the vole L1 population is quite different from that in mice, in that there is no abundant subfamily of homologous elements. Individual L1 elements from voles are very divergent from one another and belong to a clade that began a period of elevated duplicative transposition approximately 13 Mya. Sequence analyses of portions of these divergent L1 elements (approximately 250 bp each) gave no evidence for concerted evolution having acted on the vole L1 elements since the split of the two vole lineages approximately 3.5 Mya; that is, the observed interspecific divergence (6.7%-24.7%) is not larger than the intraspecific divergence (7.9%-27.2%), and phylogenetic analyses showed no clustering into Arvicola and Microtus clades.      

Multifactorial diversity sustains microbial community stability

Maintenance of a high degree of biodiversity in homogeneous environments is poorly understood. A complex cheese starter culture with a long history of use was characterized as a model system to study simple microbial communities. Eight distinct genetic lineages were identified, encompassing two species: Lactococcus lactis and Leuconostoc mesenteroides. The genetic lineages were found to be collections of strains with variable plasmid content and phage sensitivities. Kill-the-winner hypothesis explaining the suppression of the fittest strains by density-dependent phage predation was operational at the strain level. This prevents the eradication of entire genetic lineages from the community during propagation regimes (back-slopping), stabilizing the genetic heterogeneity in the starter culture against environmental uncertainty.     

Immunohistochemical localization of irisin in skin,eye, and thyroid and pineal glands of the crested porcupine (Hystrix cristata)

Irisin was first identified in muscle cells. We detected irisin immunoreactivity in various organs of the crested porcupine (Hystrix cristata). In the epidermis, irisin immunoreactivity was localized mainly in stratum basale, stratum spinosum and stratum granulosum layers; immunoreactivity was not observed in the stratum corneum. In the dermis, irisin was found in the external and internal root sheath, cortex and medulla of hair follicles, and in sebaceous glands. Irisin immunoreactivity was found in the neural retina and skeletal muscle fibers associated with the eye. The pineal and thyroid glands also exhibited irisin immunoreactivity.     

The two nitrogen mobilisation- and senescence-associated <Emphasis Type="Italic">GS1</Emphasis> and <Emphasis Type="Italic">GDH</Emphasis> genes are controlled by C and N metabolites

In tobacco, the two enzymes of nitrogen metabolism, cytosolic glutamine synthetase (GS1; E.C.6.3.1.2) and glutamate dehydrogenase (GDH; E.C.1.4.1.2), are induced during leaf senescence, whereas the chloroplastic glutamine synthetase (GS2; E.C.6.3.1.2) and nitrate reductase (NR; E.C.1.6.1.1) are repressed in the course of ageing. In this report, we showed in discs of fully expanded Nicotiana tabacum L. cv. Xanthi leaves that sucrose (Suc) and amino acids were involved in the regulation of the expression of GS1 and GDH genes. Suc induced the expression of GS1 and repressed that of GDH. Therefore, we concluded that in response to Suc, GS1 behaved as an early Senescence Associated Gene (SAG), whereas GDH behaved as a late SAG. Moreover, amino acids induced the expression of both genes. Among the amino acids tested as signal molecules, proline (Pro) and glutamate (Glu) were major inducers of GDH and GS1 expression, respectively. Interestingly, an opposite regulation of GS1 and GS2 by Pro and Glu was shown. The contrary effect of Suc on NIA (NR encoding gene) and GDH mRNA accumulation was also emphasized.     

The role of glycine in determining the rate of glutathione synthesis in poplar. Possible implications for glutathione production during stress

The terminal step of glutathione (GSH) synthesis is the condensation of γ-glutamyl-cysteine (γ-EC) with glycine. Relatively little information exists concerning the importance of photorespiratory glycine in determining the rate of conversion of γ-EC to GSH. Consequently, the effect of exogenous glycine and of illumination on foliar contents of γ-EC and GSH was studied in excised leaves and leaf discs from untransformed poplar ( Populus tremula × P. alba ) and poplar overexpressing γ-glutamylcysteine synthetase (γ-ECS; EC 6.3.2.2). Poplars strongly overexpressing γ-ECS (ggs28) had enhanced levels of γ-EC and GSH compared to untransformed poplars. The relationship between γ-EC and GSH contents in ggs28 was light dependent. In illuminated leaves, GSH contents were up to 50-fold higher than γ-EC. On darkening, γ-EC accumulated markedly and GSH declined, so that the GSH:γ-EC ratio was close to 1. These dark-induced changes were prevented by supplying glycine through the petiole or by incubation of leaf discs on glycine. Dark accumulation of γ-EC in leaf discs from untransformed poplar was also prevented by supplying glycine. Supplying cysteine in the dark to discs from untransformed poplar and ggs28 increased γ-EC levels markedly but GSH levels only slightly. Subsequent illumination caused γ-EC to decrease and GSH to increase. Supplying glycine in concert with cysteine had similar effects to illumination. The data suggest that photorespiratory glycine is essential for GSH synthesis, especially under stress conditions, where increased amounts of GSH are required.