Samba Ndao Sylla

Changes in soil diversity and global activities following invasions of the exotic invasive plant, Amaranthus viridis L., decrease the growth of native sahelian Acacia species

The objectives of this study were to determine whether the invasive plant Amaranthus viridis influenced soil microbial and chemical properties and to assess the consequences of these modifications on native plant growth. The experiment was conducted in Senegal at two sites: one invaded by A. viridis and the other covered by other plant species. Soil nutrient contents as well as microbial community density, diversity and functions were measured. Additionally, five sahelian Acacia species were grown in (1) soil disinfected or not collected from both sites, (2) uninvaded soil exposed to an A. viridis plant aqueous extract and (3) soil collected from invaded and uninvaded sites and inoculated or not with the arbuscular mycorrhizal (AM) fungus Glomus intraradices. The results showed that the invasion of A. viridis increased soil nutrient availability, bacterial abundance and microbial activities. In contrast, AM fungi and rhizobial development and the growth of Acacia species were severely reduced in A. viridis-invaded soil. Amaranthus viridis aqueous extract also exhibited an inhibitory effect on rhizobial growth, indicating an antibacterial activity of this plant extract. However, the inoculation of G. intraradices was highly beneficial to the growth and nodulation of Acacia species. These results highlight the role of AM symbiosis in the processes involved in plant coexistence and in ecosystem management programs that target preservation of native plant diversity.

Phenotypic and genotypic diversity of rhizobia nodulating Pterocarpus erinaceus and P. lucens in Senegal.

A total of fifty root nodules isolates of fast-growing and slow growing rhizobia from Pterocarpus ennaceus and Pterocarpus lucens respectively native of sudanean and sahelian regions of Senegal were characterized. These isolates were compared to representative strains of known rhizobial species. Twenty-two new isolates were slow growers and twenty-eight were fast growers. A polyphasic approach was performed including comparative total protein sodium dodecyl sulphate polyacrylamide gel (SDS-PAGE) profile analysis; 16S rDNA and 16S-23S rDNA intergenic spacer (IGS) sequence analysis. By SDS-PAGE the slow growing isolates grouped in one major cluster containing reference strains of Bradyrhizobium sp. including strains isolated in Africa, in Brazil and in New Zealand. Most of the fast-growing rhizobia grouped in four different clusters or were separate strains related to Rhizobium and Mesorhizobium strains. The 16S rDNA and 16S-23S rDNA IGS sequences analysis showed accurately the differentiation of fast growing rhizobia among the Rhizobium and Mesorbizobium genospecies. The representative strains of slow growing rhizobia were identified as closely related to Bradyrbizobium elkanii and Bradyrhizobium japonicum. Based on 16S rDNA sequence analysis, one slow growing strain (ORS199) was phylogenetically related to Bradyrbizobium sp. (Lupinus) and Blastobacter denitrificans. This position of ORS 199 was not confirmed by IGS sequence divergence. We found no clear relation between the diversity of strains, the host plants and the ecogeographical origins.

Genetic diversity and distribution of Bradyrhizobium and Azorhizobium strains associated with the herb legume Zornia glochidiata sampled from across Senegal.

Herb legumes have great potential for rehabilitation of semi-arid degraded soils in Sahelian ecosystems as they establish mutualistic symbiosis with N(2)-fixing rhizobia. A phylogenetic analysis was performed for 78 root nodule bacteria associated with the common Sahelian herb legume Zornia glochidiata Reichb ex DC in Senegal. Based on ITS (rDNA16S-23S) and recA sequences, these strains were shown to belong to the two genera Bradyrhizobium and Azorhizobium. Strains of this latter, although frequent, formed small and ineffective nodules and suggested a parasitism rather than a symbiotic association. A potential negative effect of Azorhizobium on Zornia growth was tested for when inoculated alone or in association with a Bradyrhizobium strain. Bradyrhizobium isolates were distributed in four groups. Groups A and B were two sister clades in a larger monophyletic group also including Bradyrhizobium liaoningense, Bradyrhizobium yuanmingense, and Bradyrhizobium japonicum. Strains of cluster D fell in a sister clade of the photosynthetic Bradyrhizobium sp. group, including ORS278, whereas group C appeared to be divergent from all known Bradyrhizobium clusters. Amplified fragment length polymorphism (AFLP) clustering was congruent with ITS and recA phylogenies, but displayed much more variability. However, within the main Bradyrhizobium clades, no obvious relationship could be detected between clustering and geographical origin of the strains. Each sub-cluster included strains sampled from different locations. Conversely, Azorhizobium strains showed a tendency in the phylogeny to group together according to the site of sampling. The predominance of ineffective Azorhizobium strains in the nodules of Zornia roots, the large Bradyrhizobium genetic diversity and the geographical genetic diversity pattern are explored.

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.

Assessment of nitrogen fixation in Pterocarpus erinaceus and P. lucens using 15N labeling techniques

A greenhouse experiment was conducted to measure N2 fixation in Pterocarpus erinaceus Poir. and P. lucens Lepr. using both A value and direct isotope dilution techniques. A nonfixing plant, Dialium guineense Willd. served as a reference plant. Seven months after planting, the amount of total N accumulated in all plants was 0.46 and 0.34 g plant−1 for P. erinaceus and P. lucens, respectively. Regardless of the method of calculation, total N fixed in the whole plant was 0.16 g for P. erinaceus. and 0.12 g for P. lucens, and the proportion of nitrogen derived from the atmosphere was in the range of 35%. Thus Pterocarpus species should be considered as leguminous trees with low nitrogen fixation potential.

A pantropically introduced tree is followed by specific ectomycorrhizal symbionts due to pseudo-vertical transmission

Global trade increases plant introductions, but joint introduction of associated microbes is overlooked. We analyzed the ectomycorrhizal fungi of a Caribbean beach tree, seagrape (Coccoloba uvifera, Polygonacaeae), introduced pantropically to stabilize coastal soils and produce edible fruits. Seagrape displays a limited symbiont diversity in the Caribbean. In five regions of introduction (Brazil, Japan, Malaysia, Réunion and Senegal), molecular barcoding showed that seagrape mostly or exclusively associates with Scleroderma species (Basidiomycota) that were hitherto only known from Caribbean seagrape stands. An unknown Scleroderma species dominates in Brazil, Japan and Malaysia, while Scleroderma bermudense exclusively occurs in Réunion and Senegal. Population genetics analysis of S. bermudense did not detect any demographic bottleneck associated with a possible founder effect, but fungal populations from regions where seagrape is introduced are little differentiated from the Caribbean ones, separated by thousands of kilometers, consistently with relatively recent introduction. Moreover, dry seagrape fruits carry Scleroderma spores, probably because, when drying on beach sand, they aggregate spores from the spore bank accumulated by semi-hypogeous Scleroderma sporocarps. Aggregated spores inoculate seedlings, and their abundance may limit the founder effect after seagrape introduction. This rare pseudo-vertical transmission of mycorrhizal fungi likely contributed to efficient and repeated seagrape/Scleroderma co-introductions.

Growth Response of Different Species and Provenances of Jujube Seedlings to Inoculation with Arbuscular Mycorrhizal Fungi

Many species of Ziziphus are underutilized crops despite their potential usage in agroforestry systems. The aim of our study was to evaluate the effects of arbuscular mycorrhizal fungi (AMF) on the growth and mineral nutrition of Ziziphus spp. and provenances of Z. mauritiana in greenhouse conditions. Three isolates of AMF were used: Rhizophagus irregularis IR27, Funneliformis mosseae DAOM227131, and Rhizophagus irregularis DAOM197198. They were inoculated on seven species of Ziziphus (Z. mauritiana, Z. lotus, Z. spina-christi, Z. mucronata, Z. amphibia, Z. abyssinica, and Z. sphaerocarpa) and six provenances of Z. mauritiana (Senegal, Mali, Mauritania, Burkina Faso, Niger, and India). Plants were grown in nursery receiving 20 g of crude inoculum of AMF. Two greenhouse experiments were set up: (1) a 4×6 factorial design consisting of three AMF, one control (disinfected soil without inoculum), and six provenances of Z. mauritiana and (2) a 4×7 factorial design consisting of three AMF, one control and seven Ziziphus spp. Inoculation by AMF significantly improved growth and mineral nutrition of Ziziphus spp., particularly P nutrition. All the Ziziphus spp. (except for Z. lotus) and provenances of jujube seedlings showed the highest mycorrhizal dependency (MD) values when inoculated with R. irregularis IR27. The differences of MD among the tested Ziziphus spp. seem to be due to the differences in the development of hyphal length in the soil and in P uptake by the external hyphae. Rhizophagus irregularis IR27 constitutes a promising biological tool for the production of higher-quality nursery stock with expected improved performance of Ziziphus spp. and Z. mauritiana provenances in agroforestry systems.

Rhizobial Inoculation Increases Soil Microbial Functioning and Gum Arabic Production of 13-Year-Old Senegalia senegal (L.) Britton, Trees in the North Part of Senegal

Rhizobial inoculation has been widely used in controlled conditions as a substitute for chemical fertilizers to increase plants growth and productivity. However, very little is known about such effects on mature trees in natural habitats. In this study, we investigated the effect of rhizobial inoculation on soil total microbial biomass, mineral nitrogen content, potential CO2 respiration, fluorescein diacetate (FDA), acid phosphatase activities, and gum arabic production by 13-year-old Senegalia senegal (synonym: Acacia senegal) under natural conditions in the north part of Senegal during two consecutive years. Rhizobial inoculation was performed at the beginning of the rainy season (July) for both years with a cocktail of four strains (CIRADF 300, CIRADF 301, CIRADF 302, and CIRADF 303). Rhizospheric soils were collected in both dry and rainy seasons to a depth of 0–25 cm under uninoculated and inoculated trees. Trees were tapped in November (beginning of dry season) using traditional tools. Gum arabic was harvested every 15 days from December to March. The results obtained from both years demonstrated that rhizobial inoculation increased significantly the percentage of trees producing gum arabic, gum arabic production per tree, soil microbial biomass, FDA, and acid phosphatase activities. However, there was no significant effect on C mineralization and mineral nitrogen (N) content. Gum arabic production was positively correlated to rainfall, soil microbial biomass, and mineral nitrogen content. Our results showed a positive effect of rhizobial inoculation on soil microbial functioning and gum arabic production by mature S. senegal trees. These important findings deserve to be conducted in several contrasting sites in order to improve gum arabic production and contribute to increase rural population incomes.

The Linkages between Plant Species Composition and Soil Microbial Communities: What about Symbiotic Microorganisms within Man-Made Tree Plantations?

The Linkages between Plant Species Composition and Soil Microbial Communities: What about Symbiotic Microorganisms within Man-Made Tree Plantations? Man-made forest systems are usually focused upon the trees and are defined mainly in relation to their capacity to produce timber and prevent catastrophic events such as damage by wind. In recent years, however, there has been a growing awareness amongst plant ecologists and soil microbial ecologists that understanding the connectivity between their study organisms is of utmost importance. The interactions between plants and soil microorganisms are particularly important because plants represent the main pathway through which carbon, the element that severely limits microbial growth, enters into soil. From a reciprocal viewpoint, microbial associations have been pointed as an important strategy to guarantee plant survival under semiarid conditions. However, there are several recent studies that have been carried out on the devastating ecological impact resulting from anthropogenic dispersal of exotic plants.

DIVERSITY AND COMPETITIVENESS OF RHIZOBIA OF THE LEGUME TREE ACACIA NILOTICA IN SENEGAL

Acacia nilotica (var. adansonii and var. tomentosa) is a tree species belonging to the family Leguminosae, subfamily Fabaceae (Allen and Allen, 1981). They are endemic to the sahelian arid and Sudanean semi-arid tropical regions, respectively. They have high socio-economic and ecological importance as sources of forage and wood for local populations and might improve the fixed-N status of soil due to their nitrogen-fixing