Christine Le Roux

A phosphorus magnetic resonance spectroscopy and a differential scanning calorimetry study of the physical properties of N-acylphosphatidylethanolamines in aqueous dispersions

The physical properties of aqueous dispersions of N-acyldipalmitoylphosphatidylethanolamine (N-acyl-DPPE) and N-acyldioleylphosphatidylethanolamine (N-acyl-DOPE) have been investigated by differential scanning calorimetry (DSC), freeze-fracture electron microscopy and 31P-NMR spectroscopy. N-Acylation of DPPE by the shortest fatty chains causes a significant decrease in the gel-to-liquid crystalline phase transition, Tm. However, an increase of the N-acyl chain length induces a regular increase of Tm. 31P-NMR and electron microscopy show that N-acyl-DPPE and N-palmitoyl-DOPE are organized in lamellar phases while N-oleyl-DOPE exhibits a hexagonal phase at room temperature. Above Tm, all phospholipids investigated exhibit an asymmetric line shape with a residual chemical shift anisotropy around 15 ppm. Twenty degrees below Tm, a powder type spectrum governed by the static chemical tensor appears. The shielding components are about 40% lower than those of DPPE. In the case of N-oleyl-DPPE, the longitudinal relaxation time exhibits a minimum around 45°C. These results indicate that the N-acyl chain is embedded in the bilayer leading to a strong perturbation of the head group rotational mobility.

Nodulation and dinitrogen fixation of legume trees in a tropical freshwater swamp forest in French Guiana

Nodulated legume trees comprised 43% of the stand basal area in the low, most frequently flooded microsites, and 23% in higher, drier microsites in a tropical freshwater swamp forest in French Guiana. Dinitrogen fixation in Pterocarpus officinalis , Hydrochorea corymbosa and Inga pilosula was confirmed by acetylene reduction assay (ARA), presence of leghaemoglobin in nodules and the 15 N natural abundance method. The results for Zygia cataractae were inconclusive but suggested N 2 fixation in drier microsites. Nodulated Inga disticha had a 15 N-to- 14 N ratio similar to non-N 2 -fixing trees, but ARA indicated nitrogenase activity and leghaemoglobin was present in nodules. All bacterial strains were identified as Bradyrhizobium spp. according to the partial 16S rDNA sequences, and they were infective in vitro in the model species Macroptilium atropurpureum . About 35-50% of N in the leaves of P. officinalis , H. corymbosa and I. pilosula was fixed from the atmosphere. Dinitrogen fixation was estimated to contribute at least 8-13% and 17-28% to whole-canopy N in high and low microsites, respectively. Symbiotic N 2 fixation appears to provide both a competitive advantage to legume trees under N-limited, flooded conditions and an important N input to neotropical freshwater swamp forests.

Lipid-protein interactions in wheat gluten: a phosphorus nuclear magnetic resonance spectroscopy and freeze-fracture electron microscopy study

Phosphorus magnetic resonance spectroscopy and freeze-fracture electron microscopy of wheat gluten showed that its lipids are organised in small vesicles (60 to 300 nm in diameter), in which polar lipids exhibit a lamellar liquid crystalline phase. Interactions between phospholipids and proteins are sensitive to heating and to mechanical work. From 50 to 70°C and during cooling from 70 to 25°C these interactions are disrupted due to the expulsion into the aqueous phase of lipid vesicles that are embedded in the protein network. This expulsion can be induced by extensive mechanical work and is prevented in the presence of a reducing agent. Residual starch present in gluten does not play a role. Protein rearrangements during heating and mixing may be responsible for the expulsion of lipid vesicles. These results suggest that the formation of complexes between wheat proteins and lipids such as is found in membranes, does not occur in gluten. It appears that lipid vesicles are embedded in the protein matrix due to the particular properties of wheat proteins, so that gluten may be regarded as a system containing stabilised microemulsions.

Neotropical Legume Tree Inga edulis Forms N2-fixing Symbiosis with Fast-growing Bradyrhizobium Strains

Inga edulis Mart. is a tropical legume tree used for shade in coffee and cacao plantations and as a hedgerow in alley-cropping practices. Little information can be found concerning N2 fixation in this species. This study was conducted to characterize the rhizobia of I. edulis and determine if it is capable of fixing substantial amounts of N2. Four strains of fast-growing, Gram-negative rhizobia-type bacteria were isolated from I. edulis nodules. The strains were identified by sequencing of partial 16S–23S rDNA internal spacer region. Nitrogenase activity was determined using acetylene reduction assay (ARA). Dinitrogen fixation was measured under controlled conditions by the 15N isotope dilution technique using two non-N2-fixing reference species, Vochysia guatemalensis Donn. Sm, and Gmelina arborea Roxb. ex. Sm. Seedlings were grown in three growth media: native soil and naturally N-depleted sand amended to a low and high N level. The four strains of symbiotic bacteria were closely related to Bradyrhizobium japonicum and to Bradyrhizobium liaoningense. Nodules demonstrated nitrogenase activity as measured by ARA. Vochysia guatemalensis was a better non-N2-fixing reference than G. arborea. When V. guatemalensis was used as the non-N2-fixing reference, the estimate of the percentage of N fixed from atmosphere out of total N in I. edulis seedlings was ca. 40 in the two sand media treatments and 10 in the native soil.

Bradyrhizobia Nodulating the Acacia mangium × A. auriculiformis Interspecific Hybrid Are Specific and Differ from Those Associated with Both Parental Species

ABSTRACT In the context of an increasing utilization of the interspecific hybrid Acacia mangium × A. auriculiformis as a plantation tree in the tropical humid zone, its symbiotic characterization was carried out in comparison with that of its two parental species. Rhizobium strains of diverse geographical origins were isolated from root nodules of the hybrid and its parents. Almost all Acacia hybrid isolates were fast growing on yeast extract-mannitol medium, in contrast to those isolated from both parental species, which were mostly slow growing. The rhizobium strains were characterized through partial sequencing of the rRNA operon. In the phylogenetic tree, almost all strains isolated from the hybrid were grouped together in a clade close to Bradyrhizobium japonicum, while all strains isolated from both parental species were close to Bradyrhizobium elkanii. Inoculation experiments performed under in vitro or greenhouse conditions showed that all strains were infective with their original hosts but exhibited very variable degrees of effectivity according to the host plant tested. Thus, homologous strain-host associations were more effective than heterologous ones. This shows that there is still a high potential for isolating and testing new strains from hybrids to be used as inoculants in the context of large-scale afforestation programs.

Impact of Wheat/Faba Bean Mixed Cropping or Rotation Systems on Soil Microbial Functionalities

Cropping systems based on carefully designed species mixtures reveal many potential advantages in terms of enhancing crop productivity, reducing pest and diseases, and enhancing ecological services. Associating cereals and legume production either through intercropping or rotations might be a relevant strategy of producing both type of culture, while benefiting from combined nitrogen fixed by the legume through its symbiotic association with nitrogen-fixing bacteria, and from a better use of P and water through mycorrhizal associations. These practices also participate to the diversification of agricultural productions, enabling to secure the regularity of income returns across the seasonal and climatic uncertainties. In this context, we designed a field experiment aiming to estimate the 2 years impact of these practices on wheat yield and on soil microbial activities as estimated through Substrate Induced Respiration method and mycorrhizal soil infectivity (MSI) measurement. It is expected that understanding soil microbial functionalities in response to these agricultural practices might allows to target the best type of combination, in regard to crop productivity. We found that the tested cropping systems largely impacted soil microbial functionalities and MSI. Intercropping gave better results in terms of crop productivity than the rotation practice after two cropping seasons. Benefits resulting from intercrop should be highly linked with changes recorded on soil microbial functionalities.

Genetic diversity patterns and functional traits of bradyrhizobium strains associated with #Pterocarpus officinalis# Jacq. in Caribbean Islands and Amazonian Forest (French Guiana)

Pterocarpus officinalis Jacq. is a legume tree native to the Caribbean islands and South America growing as a dominant species in swamp forests. To analyze (i) the genetic diversity and (ii) the symbiotic properties of its associated nitrogen-fixing soil bacteria, root nodules were collected from P. officinalis distributed in 16 forest sites of the Caribbean islands and French Guiana. The sequencing of the 16S-23S ribosomal RNA intergenic spacer region (ITS) showed that all bacteria belonged to the Bradyrhizobium genus. Bacteria isolated from insular zones showed very close sequence homologies with Bradyrhizobium genospecies V belonging to the Bradyrhizobium japonicum super-clade. By contrast, bacteria isolated from continental region displayed a larger genetic diversity and belonged to B. elkanii super-clade. Two strains from Puerto Rico and one from French Guiana were not related to any known sequence and could be defined as a new genospecies. Inoculation experiments did not show any host specificity of the Bradyrhizobium strains tested in terms of infectivity. However, homologous Bradyrhizobium sp. strain-P. officinalis provenance associations were more efficient in terms of nodule production, N acquisition, and growth than heterologous ones. The dominant status of P. officinalis in the islands may explain the lower bacterial diversity compared to that found in the continent where P. officinalis is associated with other leguminous tree species. The specificity in efficiency found between Bradyrhizobium strains and host tree provenances could be due to a coevolution process between both partners and needs to be taken in consideration in the framework of rehabilitation plantation programs.

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.

Bacteria of the genus Rhodopseudomonas (Bradyrhizobiaceae) : obligate symbionts in mycelial cultures of the black truffles Tuber melanosporum and Tuber brumale

BackgroundThis work aimed at characterizing 12 isolates of the genus Tuber including Tuber melanosporum (11 isolates) and Tuber brumale (one isolate). This was done using internal transcribed spacer (ITS) sequences, confirming their origin.ResultsAnalysis of their mating type revealed that both MAT1-1 and MAT1-2 exist within these isolates (with 3 and 8 of each, respectively). We observed that each of these cultures was consistently associated with one bacterium that was intimately linked to fungal growth. These bacterial associates failed to grow in the absence of fungus. We extracted DNA from bacterial colonies in the margin of mycelium and sequenced a nearly complete 16S rDNA gene and a partial ITS fragment. We found they all belonged to the genus Rhodopseudomonas, fitting within different phylogenetic clusters. No relationships were evidenced between bacterial and fungal strains or mating types. Rhodopseudomonas being a sister genus to Bradyrhizobium, we tested the nodulation ability of these bacteria on a promiscuously nodulating legume (Acacia mangium), without success. We failed to identify any nifH genes among these isolates, using two different sets of primers.ConclusionsWhile the mechanisms of interaction between Tuber and Rhodopseudomonas remain to be elucidated, their interdependency for in vitro growth seems a novel feature of this fungus.

Rhizobial characterization in revegetated areas after bauxite mining

Little is known regarding how the increased diversity of nitrogen-fixing bacteria contributes to the productivity and diversity of plants in complex communities. However, some authors have shown that the presence of a diverse group of nodulating bacteria is required for different plant species to coexist. A better understanding of the plant symbiotic organism diversity role in natural ecosystems can be extremely useful to define recovery strategies of environments that were degraded by human activities. This study used ARDRA, BOX-PCR fingerprinting and sequencing of the 16S rDNA gene to assess the diversity of root nodule nitrogen-fixing bacteria in former bauxite mining areas that were replanted in 1981, 1985, 1993, 1998, 2004 and 2006 and in a native forest. Among the 12 isolates for which the 16S rDNA gene was partially sequenced, eight, three and one isolate(s) presented similarity with sequences of the genera Bradyrhizobium, Rhizobium and Mesorhizobium, respectively. The richness, Shannon and evenness indices were the highest in the area that was replanted the earliest (1981) and the lowest in the area that was replanted most recently (2006).