Rhizobium gallicum as an efficient symbiont for bean cultivation

Rhizobia are soil bacteria that fix atmospheric nitrogen in symbiosis with legumes in specialized organs called nodules. The legumes thus acquire the autonomy to grow in nitrogen-deficient soils. When nitrogen fixation by indigenous rhizobia is limited, field inoculation with efficient and competitive strains is an economically feasible way to increase production. When the inoculant is made from native strains of rhizobia the success of inoculation should be increased, since local strains are better adapted than commercial inoculants. Here, a Rhizobium gallicum strain, 8a3, previously selected for its competitiveness and symbiotic effectiveness with common bean under laboratory conditions, was tested in field trials in Tunisia. The experiments were conducted in six fields using three common bean cultivars. The majority of the fields showed a low density of the native rhizobia and inefficient nodulation by Sinorhizobium meliloti, a known symbiont of Medicago. Our results show that inoculation with R. gallicum strain 8a3 induced an increase in nodule numbers accompanied by a more than twofold increase in shoot dry yield. Monitoring of the nodulation occupancy through the fingerprinting of the repetitive extragenic palindromic sequences (REP-PCR) showed that strain 8a3 was competitive even in the soil showing a high population density of indigenous R. gallicum, and occupied more than 40% of the nodules. Moreover, in vitro antibiosis assays indicated that strain 8a3 produces antimicrobial activity on agar medium against indigenous common bean rhizobia, including the inefficient strains of S. meliloti. These results point out the benefits that could be achieved by selecting efficient and competitive strains among natural populations of rhizobia

Agrobacterium strains isolated from root nodules of common bean specifically reduce nodulation by Rhizobium gallicum

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699International audienceIn a previous work, we showed that non-nodulating agrobacteria strains were able to colonize root nodules of common bean. Both rhizobia and agrobacteria co-existed in the infected nodules. No impact on symbiosis was found in laboratory conditions when using sterile gravel as a support for growth. In this study, soil samples originating from different geographic and agronomic regions in Tunisia were inoculated with a mixture of agrobacteria strains isolated previously from root nodules of common bean. A significant effect on nodulation and vegetal growth of common bean was observed. Characterization of nodulating rhizobia and comparison with non-inoculated controls showed a biased genetic structure. It seemed that Rhizobium gallicum was highly inhibited, whereas nodulation by Sinorhizobium medicae was favored. Co-inoculation of non-sterile soils with R. gallicum and agrobacteria confirmed these findings. In vitro antibiosis assays indicated that agrobacteria exercised a significant antagonism against R. gallicum

Isolation of a Chitinolytic Bacillus licheniformis S213 Strain Exerting a Biological Control Against Phoma medicaginis Infection

Among nine chitinase-producing strains isolated from Tunisian soil, one isolate called S213 exhibited a potent chitinolytic activity. S213 strain was identified as Bacillus licheniformis by API 50CH system and sequence analysis of its partial 16S ribosomal DNA. Chitinolytic activity was induced either by colloidal chitin or fungal cell walls, and the highest chitinase activity reached at the late stationary phase exhibiting optimal temperature and pH of 50-60°C and pH6.0, respectively. SDS-PAGE analysis of the secreted colloidal chitin-induced proteins showed a major protein of about 65kDa. This protein was identified as chitinase on the basis of its peptide sequences which displayed high homology with chitinase sequence of B. licheniformis ATCC 14580. Moreover, chitinolytic activity containing supernatant inhibited the growth of several phytopathogenic fungi including Phoma medicaginis. Interestingly, S213 strain reduced efficiently the damping-off disease caused by P. medicaginis in Medicago truncatula and should be envisaged in enzyme-based biopesticides against phytopathogen application

Diversité des champignons mycorhiziens à arbuscules en association avec Acacia saligna dans différentes régions de la Tunisie

National audienceAcacia saligna, appelée également Acacia cyanophylla, est une fabacée arbustive originaire de l'Australie. Cette plante a une grande résilience et s’adapte facilement à de nombreux écosystèmes différents, ce qui lui confère une importance économique forte. De plus, elle forme plusieurs symbioses racinaires avec des microorganismes qui favorisent son développement et améliorent sa résistance vis à vis de différents stress biotiques et abiotiques :(i) les rhizobiums qui fixent l’azote atmosphérique et (ii) les champignons mycorhiziens à arbuscules (CMA) En Tunisie, aucune étude ciblant des microorganismes formant des symbioses racinaires associées à A. saligna n’a été effectuée. Ce travail s’intéresse à évaluer une partie de la diversité de ces microorganismes, les CMA. Des échantillons de racines et de sols rhizosphériques ont été prélevés dans six régions de Tunisie, couvrant ainsi différents étages bioclimatiques. Suite à l’extraction d’ADN et au séquençage du marqueur LSU par la technologie MiSeq Illumina, 18 espèces de CMA ont été identifiées appartenant aux genres Scutellospora, Septoglomus, Diversispora, Rhizophagus, Funneliformis, Sclerocystis, Claroideoglomus, Ambispora, Kamienskia, Dominikia et Nanoglomus. Cette étude révèle l’existence d’une riche diversité de CMA et la comparaison des diversités microbiennes racinaires et telluriques est en cours. D’un point de vue appliqué, cette communauté de CMA pourrait servir d’inoculum afin d’améliorer la croissance des plantes et d’améliorer la tolérance des plantes vis à vis de différents stress biotiques et abiotiques

Genetic diversity, phenotypic traits, and symbiotic efficiency of native Bradyrhizobium strains of Lupinus luteus in Morocco

International audienceBackground and aims In Morocco’s semi-arid and sub-humid climates, the fodder legume Lupinus luteus is cultivated for its high economic and ecological value. In this work, we characterized some microsymbionts of L. luteus isolated by trapping from plants grown in soils of the agricultural area of Zaer, Morocco.Methods The phenotypic and genotypic diversity, the plant growth-promoting abilities, and the symbiotic efficiency of rhizobia isolated from rootnodules of L. luteus were analyzed.Results Based on their REP-PCR fingerprinting results, eighteen strains were selected for Multilocus sequence analysis (MLSA) using rrs, glnII, gyrB, recA, and rpoB housekeeping genes, which revealed that all the strains belong to the genus Bradyrhizobium. Some strains were close to B. lupini and B. canariense. However, the remaining strains grouped apart from all described Bradyrhizobium species.Phylogenetic analysis of the nodA and nodC symbiotic genes showed that all the strains are members of the symbiovar genistearum. Quantitative evaluation of selected plant growth-promoting activities showed that the strains solubilize phosphate, and produce auxins and siderophores. All the strains used as inoculum in greenhouse experiments significantly improved the growth of L. luteus under nitrogen-free conditions.Conclusions Bradyrhizobium lupini and B. canariense are the main rhizobia nodulating L. luteus in the Zaer region. In addition to their high nitrogen fixation efficiency, these isolates also exhibit plant growthpromoting activities. These results highlighted one of the major reasons for the success of yellow lupine in this area without nitrogen fertilizers and pointed to the possibility of formulating these rhizobia into aneffective inoculum for L. luteu

Genotypic and symbiotic diversity studies of rhizobia nodulating Acacia saligna in Tunisia reveal two novel symbiovars within the Rhizobium leguminosarum complex and Bradyrhizobium

International audienceAcacia saligna is an invasive alien species that has the ability to establish symbiotic relationships with rhizobia. In the present study, genotypic and symbiotic diversity of native rhizobia associated with A. saligna in Tunisia were studied. A total of 100 bacterial strains were selected and three different ribotypes were identified based on rrs PCR-RFLP analysis. Sequence analyses of rrs and four housekeeping genes (recA, atpD, gyrB and glnII) assigned 30 isolates to four putative new lineages and a single strain to Sinorhizobium meliloti. Thirteen slow-growing isolates representing the most dominant IGS (intergenic spacer) profile clustered distinctly from known rhizobia species within Bradyrhizobium with the closest related species being Bradyrhizobium shewense and Bradyrhizobium niftali, which had 95.17% and 95.1% sequence identity, respectively. Two slow-growing isolates, 1AS28L and 5AS6L, had B. frederekii as their closest species with a sequence identity of 95.2%, an indication that these strains could constitute a new lineage. Strains 1AS14I, 1AS12I and 6AS6 clustered distinctly from known rhizobia species but within the Rhizobium leguminosarum complex (Rlc) with the most closely related species being Rhizobium indicum with 96.3% sequence identity. Similarly, the remaining 11 strains showed 96.9 % and 97.2% similarity values with R. changzhiense and R. indicum, respectively. Based on nodC and nodA phylogenies and cross inoculation tests, these 14 strains of Rlc species clearly diverged from strains of Sinorhizobium and Rlc symbiovars, and formed a new symbiovar for which the name sv. “salignae” is proposed. Bacterial strains isolated in this study that were taxonomically assigned to Bradyrhizobium harbored different symbiotic genes and the data suggested a new symbiovar, for which sv. “cyanophyllae” is proposed. Isolates formed effective nodules on A. saligna

Rhizobium acaciae sp. nov., a new nitrogen-fixing symbiovar isolated from root nodules of Acacia saligna in Tunisia

International audienceThe draft genome sequences of 1AS11 T , 1AS12 and 1AS13 were deposited under the accession numbers JAPCZM000000000, JAPCZN000000000 and JAPCZO000000000, respectively.The Illumina NovaSeq 6000 raw reads are deposited in the Sequence Read Archive under bioproject number PRJNA893020