Fatou Ndoye

Changes in Land Use System and Environmental Factors Affect Arbuscular Mycorrhizal Fungal Density and Diversity, and Enzyme Activities in Rhizospheric Soils of Acacia senegal (L.) Willd.

The responses of the soil microbial community features associated to the legume tree Acacia senegal (L.) Willd. including both arbuscular mycorrhizal fungal (AMF) diversity and soil bacterial functions, were investigated under contrasting environmental conditions. Soil samples were collected during dry and rainy seasons in two contrasting rainfall sites of Senegal (Dahra and Goudiry, in arid and semiarid zone, resp.). Soils were taken from the rhizosphere of A. senegal both in plantation and natural stands in comparison to bulk soil. A multiple analysis revealed positive correlations between soil physicochemical properties, mycorrhizal potential and enzyme activities variables. The positive effects of A. senegal trees on soil mycorrhizal potential and enzyme activities indicates that in sahelian regions, AMF spore density and diversity as well as soil microbial functions can be influenced by land-use systems (plantation versus natural population of A. senegal) and environmental conditions such as moisture and soil nutrient contents. Our study underlines the importance of prior natural AMF screening for better combinations of A. senegal seedlings with AMF species to achieve optimum plant growth improvement, and for restoration and reforestation of degraded lands.

Phylogeny of Nodulation Genes and Symbiotic Diversity of "Acacia senegal" (L.) Willd. and "A. seyal" (Del.) "Mesorhizobium" Strains from Different Regions of Senegal

Acacia senegal and Acacia seyal are small, deciduous legume trees, most highly valued for nitrogen fixation and for the production of gum arabic, a commodity of international trade since ancient times. Symbiotic nitrogen fixation by legumes represents the main natural input of atmospheric N2 into ecosystems which may ultimately benefit all organisms. We analyzed the nod and nif symbiotic genes and symbiotic properties of root-nodulating bacteria isolated from A. senegal and A. seyal in Senegal. The symbiotic genes of rhizobial strains from the two Acacia species were closed to those of Mesorhizobium plurifarium and grouped separately in the phylogenetic trees. Phylogeny of rhizobial nitrogen fixation gene nifH was similar to those of nodulation genes (nodA and nodC). All A. senegal rhizobial strains showed identical nodA, nodC, and nifH gene sequences. By contrast, A. seyal rhizobial strains exhibited different symbiotic gene sequences. Efficiency tests demonstrated that inoculation of both Acacia species significantly affected nodulation, total dry weight, acetylene reduction activity (ARA), and specific acetylene reduction activity (SARA) of plants. However, these cross-inoculation tests did not show any specificity of Mesorhizobium strains toward a given Acacia host species in terms of infectivity and efficiency as stated by principal component analysis (PCA). This study demonstrates that large-scale inoculation of A. senegal and A. seyal in the framework of reafforestation programs requires a preliminary step of rhizobial strain selection for both Acacia species.

Effect of neighboring cells on cell stiffness measured by optical tweezers indentation

Abstract. We report on the modification of mechanical properties of breast cancer cells when they get in contact with other neighboring cells of the same type. Optical tweezers vertical indentation was employed to investigate cell mechanics in isolated and contact conditions, by setting up stiffness as a marker. Two human breast cancer cell lines with different aggressiveness [MCF-7 (luminal breast cancer) and MDA-MB-231 (basal-like breast cancer)] and one normal immortalized breast cell line HBL-100 (normal and myoepithelial) were selected. We found that neighboring cells significantly alter cell stiffness: MDA-MB-231 becomes stiffer when in contact, while HBL-100 and MCF-7 exhibit softer character. Cell stiffness was probed at three cellular subregions: central (above nucleus), intermediate (cytoplasm), and near the leading edge. In an isolated condition, all cells showed a significant regional variation in stiffness: higher at the center and fading toward the leading edge. However, the regional variation becomes statistically insignificant when the cells were in contact with other neighboring cells. The proposed approach will contribute to understand the intriguing temporal sequential alterations in cancer cells during interaction with their surrounding microenvironment.

Arbuscular mycorrhizal fungi (AMF) communities associated with cowpea in two ecological site conditions in Senegal

The objective of this study was to characterize the diversity of arbuscular mycorrhizal fungal (AMF) communities colonizing the roots of Vigna unguiculata (L.) plants cultivated in two different sites in Senegal. Roots of cowpea plants and soil samples were collected from two fields (Ngothie and Diokoul) in the rural community of Dya (Senegal). Microscopic observations of the stained roots indicated a high colonization rate in roots from Ngothie site as compared to those from Diokoul site. The partial small subunit of ribosomal DNA genes was amplified from the genomic DNA extracted from these roots by polymerase chain reaction (PCR) with the universal primer NS31 and a fungal-specific primer AML2. Nucleotide sequence analysis revealed that 22 sequences from Ngothie site and only four sequences from Diokoul site were close to those of known arbuscular mycorrhizal fungi. Also, 47.6% of the clones from Ngothie site and 89.47% from Diokoul site were not close to known AMF. A total of 15 operational taxonomic units (OUT) were identified. Phylogenetic analyses showed that these clones belonged to the genera Glomus, Sclerocystis, Rhizophagus, Scutellospora, Gigaspora, Racocetra, Acaulospora and Redeckera. The genus Glomus is the most represented with six OTU, representing 40% of all OTU. Key words: Agriculture, Glomeromycota, Vigna unguiculata, diversity, soil origin.