Genotypic variation of nodules’ enzymatic activities in symbiotic nitrogen fixation among common bean (Phaseolus vulgaris L.) genotypes grown under salinity constraint

The effect of salt stress, under glasshouse conditions, was studied on plant biomass, nodulation, and activities of acid phosphatases (APase, EC 3.1.3.2) and trehalose 6-phosphate phosphatase (TPP, EC 3.1.3.12) in the symbiosis common bean (Phaseolus vulgaris L.)-rhizobia nodules. Four common bean recombinant inbred lines (147, 115, 104 and 83) were separately inoculated, with CIAT 899 or RhM11 strains and grown in hydroaeroponic culture. Two NaCl levels (0 and 25 mM NaCl plant^?1 week^?1 corresponding, respectively, to the control and the salt treatment) were applied and the culture was assessed during 42 days after their transplantation. The results showed that the nodulation of these lines was not affected by salinity except for the line 83 inoculated with CIAT 899, whose nodule dry weight decreased by 48.24 % compared with the corresponding controls. For the other symbiotic combinations, shoot and root biomasses were not significantly affected by salt constraint. Salinity stress generally reduced acid phosphatise and trehalose phosphate phosphatase activities in nodules that were less affected in plants inoculated with RhM11. Based on our data, it appears that nodule phosphatase activity may be involved in salinity tolerance in common beans and the levels of salt tolerance depend principally on specific combination of the rhizobial strain and the host cultivar.     

Identification at the species and symbiovar levels of strains nodulating Phaseolus vulgaris in saline soils of the Marrakech region (Morocco) and analysis of the otsA gene putatively involved in osmotolerance

Salinity is an increasing problem in Africa affecting rhizobia-legume symbioses. In Morocco, Phaseolus vulgaris is cultivated in saline soils and its symbiosis with rhizobia depends on the presence of osmotolerant strains in these soils. In this study, 32 osmotolerant rhizobial strains nodulating P. vulgaris were identified at the species and symbiovar levels by analysing core and symbiotic genes, respectively. The most abundant strains were closely related to Rhizobium etli and R. phaseoli and belonged to symbiovar phaseoli. A second group of strains was identified as R. gallicum sv gallicum. The remaining strains, identified as R. tropici, belonged to the CIAT 899(T) nodC group, which has not yet been described as a symbiovar. In representative strains, the otsA gene involved in the accumulation of trehalose and putatively in osmotolerance was analysed. The results showed that the phylogeny of this gene was not completely congruent with those of other core genes, since the genus Ensifer was more closely related to some Rhizobium species than others. Although the role of the otsA gene in osmotolerance is not well established, it can be a useful protein-coding gene for phylogenetic studies in the genus Rhizobium, since the phylogenies of otsA and other core genes are coincident at the species level.     

Ecological and Control Techniques for Sand Flies (Diptera: Psychodidae) Associated with Rodent Reservoirs of Leishmaniasis

Background

Leishmaniasis remains a global health problem because of the substantial holes that remain in our understanding of sand fly ecology and the failure of traditional vector control methods. The specific larval food source is unknown for all but a few sand fly species, and this is particularly true for the vectors of Leishmania parasites. We provide methods and materials that could be used to understand, and ultimately break, the transmission cycle of zoonotic cutaneous leishmaniasis.

Methods and Findings

We demonstrated in laboratory studies that analysis of the stable carbon and nitrogen isotopes found naturally in plant and animal tissues was highly effective for linking adult sand flies with their larval diet, without having to locate or capture the sand fly larvae themselves. In a field trial, we also demonstrated using this technique that half of captured adult sand flies had fed as larvae on rodent feces. Through the identification of rodent feces as a sand fly larval habitat, we now know that rodent baits containing insecticides that have been shown in previous studies to pass into the rodents'' feces and kill sand fly larvae also could play a future role in sand fly control. In a second study we showed that rubidium incorporated into rodent baits could be used to demonstrate the level of bloodfeeding by sand flies on baited rodents, and that the elimination of sand flies that feed on rodents can be achieved using baits containing an insecticide that circulates in the blood of baited rodents.

Conclusions

Combined, the techniques described could help to identify larval food sources of other important vectors of the protozoa that cause visceral or dermal leishmaniasis. Unveiling aspects of the life cycles of sand flies that could be targeted with insecticides would guide future sand fly control programs for prevention of leishmaniasis.     

A model of cell cycle behavior dominated by kinetics of a pathway stimulated by growth factors

A modified version of a previously developed mathematical model [Obeyesekere et al., Cell Prolif. (1997)] of the G1-phase of the cell cycle is presented. This model describes the regulation of the G1-phase that includes the interactions of the nuclear proteins, RB, cyclin E, cyclin D, cdk2, cdk4 and E2F. The effects of the growth factors on cyclin D synthesis under saturated or unsaturated growth factor conditions are investigated based on this model. The solutions to this model (a system of nonlinear ordinary differential equations) are discussed with respect to existing experiments. Predictions based on mathematical analysis of this model are presented. In particular, results are presented on the existence of two stablesolutions, i. e., bistability within the G1-phase. It is shown that this bistability exists under unsaturated growth factor concentration levels. This phenomenon is very noticeable if the efficiency of the signal transduction, initiated by the growth factors leading to cyclin D synthesis, is low. The biological significance of this result as well as possible experimental designs to test these predictions are presented.     

Theoretical and experimental evidence for hysteresis in cell proliferation

We propose a mathematical model for the regulation of the G1-phase of the mammalian cell cycle taking into account interactions of cyclin D/cdk4, cyclin E/cdk2, Rb and E2F. Mathematical analysis of this model predicts that a change in the proliferative status in response to a change in concentrations of serum growth factors will exhibit the property of hysteresis: the concentration of growth factors required to induce proliferation is higher than the concentration required to maintain proliferation. We experimentally confirmed this prediction in mouse embryonic fibroblasts in vitro. In agreement with the mathematical model, this indicates that changes in proliferative mode caused by small changes in concentrations of growth factors are not easily reversible. Based on this study, we discuss the importance of proliferation hysteresis for cell cycle regulation.     

Modern pollen-vegetation relationships within a small Mediterranean temporary pool (western Morocco)

Morocco is rich in temporary pools which harbour numerous rare plant species. Long-term conservation of such threatened plant communities should be based on the understanding of their past dynamics. Despite conditions unfavourable to pollen preservation, surface sediments of acidic temporary pools are shown to contain pollen assemblages likely to allow vegetation reconstruction. Knowledge of the modern relationships between pollen and vegetation is, however, necessary for interpreting fossil data in terms of past vegetation. Surface pollen assemblages and floristic surveys of a temporary pool in Benslimane forest, western Morocco, are compared in order to evaluate the pollen record of the local hydrophytic vegetation. Floristic surveys were carried out for 12 years (1996-2008) along two crossing permanent transects. A set of 21 surface-sediment samples, taken along the same transects in 2007, were analysed for pollen. The spatial relationships between vegetation and pollen assemblages are explored by means of multivariate analyses, statistical tests and linear regressions. The calculation of representation indices moreover allows proposing quantitative ways for pollen-based plant-abundance reconstruction.Results reveal that the vegetation structure along the hydrological gradient is well recorded in the pollen assemblages, with: (1) a marginal zone characterised by terrestrial taxa and rare amphibious taxa (Elatine, Pilularia), (2) an intermediate zone of amphibious taxa (Alisma-type, Illecebrum/Paronychia, Isoetes velata-type), and (3) a central zone of aquatics (Myriophyllum alterniflorum, Ranunculus-type). The best correlation between the pollen record and total pool vegetation was found in the centre of the pool, which supports the reliability of the study of a single core from the centre of the pool for the reconstruction of the past dynamics of the local hydrophytic vegetation. Both the qualitative ‘community’ approach (representation indices and indicator pollen taxa) and the quantitative ‘taxa’ approach (correction factors) suggest that reconstructions of past populations can be achieved from a few taxa, namely Isoetes velata-type, Myriophyllum alterniflorum and Ranunculus-type. For these taxa, regression parameters (slope and y-intercept) have been calculated between pollen percentages and plant percentages in present vegetation, and between pollen influxes and plant abundances, respectively. These parameters can be extended to interpret fossil data from other temporary pools within the same region to reconstruct their relative and absolute past plant abundances.     

Model predictions of MDM2 mediated cell regulation

In this work we present a mathematical approach to elucidate possible mechanisms involving mdm2 in the regulation of the cell cycle. It has been experimentally shown that the over-expression of MDM2 leads to decoupling of DNA synthesis with mitosis resulting in polyploidy cells with multiple copies of their genomes. The function of MDM2 that uncouples the DNA synthesis phase (S) and the Mitosis phase (M) is unclear. To answer this question, we first formulate a mathematical model of the dynamics of the cell cycle regulatory proteins during the DNA synthesis phase and mitosis. This model is then tested for bifurcation that produces period doubling cascades that we relate to the biological event of polyploidy. The model formulation, the underlying biology, and the bifurcation results to delineate the unknown function of MDM2 are presented. Based on reproducing known experimental result of polyploidy in MDM2 overexpressed cells, we propose several possible functions of mdm2, i.e., possible interactions with the other cell cycle regulating proteins that will result in uncoupling the S and M phases. We conclude that the most likely unknown function of MDM2 leading to the decoupling of the S and M phases is an obstruction of the activation of Cdc25C by MDM2.     

Impact of grazing on the species richness of plant communities in Mediterranean temporary pools (western Morocco)

The impact of grazing on the vegetation of Moroccan temporary pools has been studied at 2 scales: regional (inter-pools) and local (intra-pools). Half of the 16 forest pools studied is located in a reserve and ungrazed. The other half, located within public forest, is grazed. Vegetation relevés coupled to water-depths measurements were carried out in each pool. The results showed a significant effect of grazing on both scales of analysis. This effect was found in the species composition of the vegetation, which differed between the 2 types of pools, and in the lower species richness and abundance of plant species in the grazed pools. These differences are interpreted as resulting from the selection by herbivores and the differential tolerance of species to disturbance. These impacts are likely to expose certain species to local extinction by reducing their populations.     

Effect of salinity on nodulation,nitrogen fixation and growth of common bean (<Emphasis Type="Italic">Phaseolus vulgaris)</Emphasis> inoculated with rhizobial strains isolated from the Haouz region of Morocco

This study compared growth, nodulation, nitrogen fixation, and nodular enzyme activities in response to salinity in some common bean-rhizobia symbiotic combinations. Seeds of Paulista and Efequince, two varieties of the common bean (Phaseolus vulgaris) were germinated and seedlings were transferred to pots containing vermiculite inoculated with the reference Rhizobium strain CIAT899 or with RhM11 or RhM14, two local strains. Plants were grown in a temperature-controlled glasshouse at 28°C and irrigated with a nutrient solution without NaCl (control) or supplemented with 25 mM NaCl (stressed). Plants were harvested at the flowering stage. The results showed that in controls, inoculation with RhM11 improved plant and nodule growth compared with those inoculated with RhM14 and CIAT 899. NaCl treatment generally had a negative affect on plant and nodule growth. Under the saline treatment, symbiotic nitrogen fixation was not significantly affected in the CIAT899-Paulista, CIAT899-Efequince and RhM11-Paulista combinations. Plant mineral nutrition was negatively affected under salt treatment for all of the tested symbiotic combinations. Inoculation with CIAT899 and RhM11 conferred more plant tolerance to salinity than inoculation with RhM14. The nodular phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH) exhibited higher activities and were less affected by salinity in plants inoculated with the reference strain CIAT899 than those inoculated with local strains. We conclude that plants inoculated with CIAT899 and RhM11 showed more salinity stress tolerance than those inoculated with RhM14.