La diversité des microorganismes symbiotiques : une clef pour la réussite écologique et la protection des plantes

International audiencePlant-associated microbial diversity encompasses symbionts, protecting their host against various aggressions. Mycorrhizal and rhizospheric microorganisms buffer effects of soil toxic compounds and soil-borne pathogens. Endophytic bacteria and fungi, some of which are vertically inherited through seeds, take part in plant protection by acting directly on aggressive factors (mainly pathogens and herbivores) or by enhancing plant responses. Plant protective microbial symbionts determine the ecological success of plants; they drastically modify plant communities and related trophic webs. This review suggests approaches to improve the inventory of diversity and functions of in situ plant-associated microorganisms.Parmi les microorganismes associés aux plantes se trouvent des espèces bénéfiques, qui protègent leurs hôtes contre les agressions. Ainsi, les microorganismes mycorhiziens et rhizosphériques protègent des composés toxiques et des pathogènes du sol. Certains endophytes, bactéries et champignons, parfois transmis par les graines, protègent la plante en agissant directement sur les facteurs d'agression (pathogènes et herbivores surtout) ou en stimulant les défenses végétales. Ces symbioses protectrices déterminent le succès écologique des plantes ; elles modifient communautés végétales et réseaux trophiques. Cette synthèse aborde la nécessité et les moyens d'étudier la diversité fonctionnelle des microorganismes associés aux plantes in situ

Shifts in size, genetic structure and activity of the soil denitrifier community by nematode grazing

International audienceBacterial-feeding nematodes represent an important driver of the soil microbial activity and diversity. This study aimed at characterizing the impact of nematode grazing on a model functional bacterial guild involved in N-cycling, the denitrifiers. Bacterial-feeding nematodes (Cephalobus pseudoparvus) were inoculated into soil microcosms whose indigenous nematofauna had previously been removed. The size, genetic structure and activity of the soil denitrifier community were characterized 15 and 45 days after nematodes inoculation using quantitative PCR of the nirK, nirS and nosZ denitrification genes, fingerprinting of the nirK and nirS genes and denitrification enzyme activity measurements, respectively. A significant impact of C. pseudoparvus was observed on genetic structure of the nirK community, mainly due to shifts in the relative abundance of the dominant populations, but not on the nirS community. The grazing pressure also tended to decrease the density of all denitrification genes as well as that of 16S rRNA genes. Despite being non-significant, the extent of this decline in gene copy numbers ranged between 60 and 80% of the control microcosm genes densities. Finally, compared to non-inoculated microcosms, denitrification activity significantly decreased by 8% in response to the nematodes inoculation. The herewith data showed that predation by a single species of bacterial-feeding nematode can affect the soil denitrifier communit

Arbuscular mycorrhizal fungi, partners of the rhizostabilization process of an orphan mining site

International audienc

Impact of inoculation with the phytostimulatory PGPR Azospirillum lipoferum CRT1 on the genetic structure of the rhizobacterial community of field-grown maize

International audienceThe phytostimulatory PGPR Azospirillum lipoferum CRT1 was inoculated to maize seeds and the impact on the genetic structure of the rhizobacterial community in the field was determined during maize growth by Automated Ribosomal Intergenic Spacer Analysis (ARISA) of rhizosphere DNA extracts. ARISA fingerprints could differ from one plant to the next as well as from one sampling to the next. Inoculation with strain CRT1 enhanced plant-to-plant variability of the ARISA fingerprints and caused a statistically significant shift in the composition of the indigenous rhizobacterial community at the first two samplings. This is the first study on the ecological impact of Azospirillum inoculation on resident bacteria done in the field and showing that this impact can last at least one mont

Arbuscular mycorrhizal fungi, partners of the rhizostabilization proccess of an orphan mining site

International audienc

Raisonner l'utilisation des légumineuses ligneuses pour réhabiliter des sols dégradés en milieu méditerranéen et tropical

Ce numéro est constitué d’articles issus des « 1ères rencontres francophones sur les légumineuses » (RFL1), organisées les 31 mai et 1er juin 2016 à Dijon.International audienceThe process of desertification that has been worsening in recent decades requires the identification ofurgent measures to rehabilitate degraded soils and render the expected ecosystem services. Amongthe biological tools that can be used to optimize the performance of these reforestation operations arethe plant / microorganism symbiosis (Rhizobia, mycorrhizal symbionts) and also the use of fast-growthwoody species (eg Australian Acacias, Pins Exotic species, Eucalyptus spp.). Despite convincing resultsin afforestation of these environments, the use of exotic species leads to strong changes in thecomposition of the soil microflora and its functional diversity resulting in an inhibition of the naturalregeneration process of endemic plant species of the environment to be rehabilitated.Le processus de désertification qui s’est aggravé au cours de ces dernières décennies, nécessited’identifier des mesures urgentes afin de réhabiliter les sols dégradés et de rendre à ces milieux lesservices écosystémiques attendus. Parmi les outils biologiques susceptibles d’être valorisés pouroptimiser durablement la performance de ces opérations de reboisement figurent les symbioses plantes/ microorganismes (Rhizobia, symbiotes mycorhiziens) et également l’utilisation d’essences ligneuses àcroissance rapide (ex : Acacias australiens, Pins exotiques, Eucalyptus spp.). Malgré des résultatsprobants obtenus en matière de révégétalisation de ces milieux, le recours à des essences exotiquesentraîne de profondes modifications dans la composition de la microflore du sol et sa diversitéfonctionnelle résultant en une inhibition du processus de régénération naturelle des espèces végétalesendémiques de l’environnement à réhabiliter

Impact of irrigation water quality on soil nitrifying and total bacterial communities

Disturbance induced by two contrasting irrigation regimes (groundwater versus urban wastewater) was evaluated on a sandy agricultural soil through chemical and microbial analyses. Contrary to wastewater, groundwater displayed very high nitrate contents but small amounts of ammonium and organic matter. Despite these strong compositional shifts, soil organic carbon and nitrogen, nitrate and ammonium contents were not significantly different in both types of irrigated plot. Moreover, neither microbial biomass nor its activity, determined as fluorescein diacetate hydrolysis activity, was influenced by irrigation regimes. Bacterial community structure, assessed by denaturing gradient gel electrophoresis (DGGE) of 16S ribosomal DNA fragments, was also weakly impacted as molecular fingerprints shared an overall similarity of 85%. Ammonia-oxidizing bacterial community (AOB) was monitored by DGGE of the functional molecular marker amoA gene (alpha subunit of the ammonia monooxygenase). Surprisingly, no amoA signals were obtained from plots irrigated with groundwater, whereas signal intensities were high in all plots under wastewater. Among the last, compositional shifts of the AOB community were weak. Overall, impact of irrigation water quality on soil chemistry could not be evidenced, whereas effects were low on the total bacterial compartment but marked on the AOB community

Rhizobacterial Pseudomonas spp. strains harbouring acdS gene could enhance metallicolous legume nodulation in Zn/Pb/Cd mine tailings

Phytostabilisation can benefit from phytostimulatory rhizobacteria. Forty-three bacterial strains were isolated from the roots of the metallicolous legume Anthyllis vulneraria ssp. carpatica grown in a highly contaminated mine tailing (total Cd, Pb and Zn were up to 1200; 34,000; and 170,000 mg kg(-1), respectively). We aimed at evaluating their phytostimulatory effects on the development of leguminous metallophytes. Strains were screened for fluorescent siderophores and auxin synthesis, inorganic P solubilisation and 1-amino-cyclopropane-1-carboxylate deaminase (ACCd) activity to define a subset of 11 strains that were inoculated on the leguminous metallophytes A. vulneraria and Lotus corniculatus grown in diluted mine spoil (Zn 34,653; Pb 6842; and Cd 242, all in mg kg-1). All strains were affiliated to Pseudomonas spp. (except two), synthetised auxins and siderophores and solubilised P (except three), and seven of them were ACCd positive. The inoculation effects (shoot-root-nodule biomass, chlorophyll content) depended on legume species and bacterial strain genotype. Phytostimulation scores were unrelated to siderophore/auxin synthesis and P solubilisation rates. Inoculations of the strain nos. 17-43 triggered a 1.2-fold significant increase in the chlorophyll content of A. vulneraria. Chlorophyll content and root biomass of L. corniculatus were significantly increased following the inoculations of the strain nos. 17-22 (1.5-1.4-fold, respectively). The strongest positive effects were related to increases in the nodule biomass of L. corniculatus in the presence of three ACCd-positive strains (1.8-fold), one of which was the highest auxin producer. These data suggest to focus on interactions between ACCd activity and auxin synthesis to enhance nodulation of metallicolous legumes

Australian acacia introduction to rehabilitate degraded ecosystems is it free of environmental risks?

International audienceThe use of exotic tree species and especially fast-growing trees (acacias, pines or eucalypts) has been frequently recommended to rehabilitate and restore in short term degraded areas resulting from natural events or human activities. The consequences on the environment of the introduction of these species, as the case invasive, are generally evaluated on their impact on plant biodiversity and soil physico-chemical characteristics but rarely on the composition of microbial communities. The soil microbiota, especially mycorhizal fungi, plays a key role vis-a-vis biological mechanisms governing the chemical soil fertility, productivity and stability of plant terrestrial ecosystems. Chosen approach was to describe the impact of exotic species on soil biological characteristics and the consequences of the recovering of vegetation cover composed of native species from the previous stand. After pointing out importance of the use of acacias worldwide, two case studies implemented in Senegal and Algeria, have shown that two Australian acacias, Acacia holosericea and Acacia mearnsii, induce deep changes in the functional diversity of soil microorganisms and in the structure of symbiotic microorganisms communities (mycorhizal fungi and rhizobia). These acacias inhibit the growth of two native tree species, Quercus suber and Faidherbia albida. These results confirm the need to identify the biological processes related to the actions of introducing exotic species in order to modulate their use. And this knowledge will prevent risks and ensure the performance of afforestation including rehabilitation of degraded land.Es necesario conocer la durabilidad natural de la madera de especies de zonas aridas y semiaridas para determinar su futuro aprovechamiento. Esto permitiria reducir el uso de productos protectores. El objetivo de esta investigacion es contribuir a la mejora cualitativa de la madera de una especie lenosa comun en Argelia pero muy poco estudiada. Se trata del almacigo, Pistacia atlantica Desf., una planta vivaz, resistente y endemica capaz de vivir varios siglos adaptandose perfectamente a las severas condiciones edaficas y climaticas de su habitat. La durabilidad natural se evaluo mediante probetas normalizadas de albura y duramen de arboles procedentes de dos regiones bioclimaticas distintas del noroeste de Argelia. Las pruebas se efectuaron segun las normas europeas NE 113 (1996) y NE 350-1 (1994) que permiten evaluar la durabilidad de la madera frente a un ataque fungico. De los resultados de perdida de masa obtenidos se desprenden dos hechos particularmente interesantes: el duramen y la albura de los arboles de las dos procedencias muestran la misma durabilidad y no se observa ninguna diferencia significativa en el mismo tipo de madera entre ambas procedencias. Esta madera alcanza la categoria de altamente resistente.L'utilisation d'essences forestières exotiques et plus particulièrement des arbres à croissance rapide (acacias, pins ou eucalyptus) a été fréquemment recommandée pour réhabiliter et restaurer à brève échéance des milieux dégradés suite à des événements naturels ou à des activités anthropiques. L'incidence sur l'environnement de l'introduction de ces espèces, parfois envahissantes, est surtout évaluée pour leur impact sur la biodiversité végétale et les caractéristiques physico-chimiques des sols, mais rarement en ce qui concerne la composition de la microflore. Les micro-organismes, et plus particulièrement les champignons mycorhiziens, jouent un rôle clé vis-à-vis des mécanismes biologiques régissant la fertilité chimique des sols et leur productivité, facteurs de stabilité des écosystèmes terrestres. L'approche retenue a été de décrire l'incidence de l'introduction d'essences exotiques sur les caractéristiques biologiques des sols, ainsi que les conséquences sur la reconstruction d'un couvert végétal composé par des espèces natives du milieu d'origine. Après avoir rappelé l'importance de l'utilisation des acacias à travers le monde, deux études réalisées au Sénégal et en Algérie ont permis de montrer que deux acacias australiens, Acacia holosericea et Acacia mearnsii, induisent de profondes modifications de la diversité fonctionnelle de la microflore du sol et aussi de la structure des microorganismes symbiotiques (champignons mycorhiziens et rhizobia). Ces acacias entraînent une inhibition de la croissance de deux espèces forestières natives, Faidherbia albida et Quercus suber. Les résultats confirment le besoin de cerner les processus biologiques liés aux actions d'introduction d'essences exotiques afin de moduler leur utilisation. Ainsi, cette connaissance préviendra les risques et assurera les performances des opérations de reboisement, notamment pour la réhabilitation des terrains dégradés

Is soil fauna an important driver of the structure and activities of denitrifier communities in tropical soils?

International audienc