Phosphorus nutrition of mycorrhizal trees

Globally, phosphorus (P) limits productivity of trees in many forests and plantations especially in highly weathered, acidic or calcareous pro!les. Most trees form mycorrhizal associations which are prevalent in the organic and mineral soil horizons. This review critically examines mechanisms that enhance the acquisition of P by tree roots.Mycorrhizal roots have a greater capacity to take up phosphate (Pi) from the soil solution than non-mycorrhizal root tips. Factors that contribute to this include the extent of extraradical hyphal penetration of soil and the physiology and biochemistry of the fungal/soil and fungal/plant interfaces. Ectomycorrhizal (ECM) trees are likely to bene!t from association with basidiomycetes that possess several high-affinity Pi transporters that are expressed in extraradical hyphae and whose expression is enhanced by P de!ciency. To understand fully the role of these putative transporters in the symbiosis, data regarding their localization, Pi transport capacities and regulation are required. Some ECM fungi are able to effect release of Pi from insoluble mineral P through excretion of lowmolecular-weight organic anions such as oxalate, but the relative contribution of insoluble P dissolution in situ remains to be quanti!ed. How the production of oxalate is regulated by nitrogen remains a key question to be answered. Lastly, phosphatase release from mycorrhizas is likely to play a significant role in the acquisition of Pi from labile organic forms of P (Po). As labile forms of Po can constitute the major fraction of the total P in some tropical and temperate soils, a greater understanding of the forms of Po available to the phosphatases is warranted

Trophic relationships in the rhizosphere (effect of fungal, bacterial and nematode interactions on mineral nutrition of Pinus pinaster seedlings)

Les microorganismes agissent comme un puits et une source de N et Pdisponibles car ils sont responsables des cycles biogéochimiques de N et P. La bouclemicrobienne, basée sur la prédation des bactéries par les microprédateurs tels que lesnématodes bactérivores, est considérée comme un facteur majeur de la minéralisation de Net de P dans les écosystèmes terrestres. Cependant, peu de données sont disponibles surl'impact de la prédation par les nématodes sur la nutrition minérale des plantes ligneusesectomycorhizées. Différentes expérimentations ont été conduites pour quantifier le rôle dela prédation des bactéries par les nématodes sur l'architecture et la croissance racinaire, lanutrition minérale (N et P) d'une espèce ligneuse, Pinus pinaster, associée ou non avec lebasidiomycète ectomycorhizien Hebeloma cylindrosporum. Les plantes ont été cultivéesdans un système expérimental simplifié et stérile, et inoculées ou non avec Bacillus subtiliset des nématodes bactérivores (de la famille des Rhabditidae ou des Cephalobidae) isolés àpartir d'ectomycorhizes et de sol provenant d'une plantation de Pin maritime. L'effet de laprédation sur la croissance des plantes et le devenir du 15N bactérien vers les partiesaériennes dépend très fortement de la disponibilité en P du milieu. De plus, la prédationdes bactéries est indispensable pour permettre à la plante d'utiliser le P du phytate, unesource de P organique très peu disponible pour la plante mais très facilement utilisable parB. subtilis car cette bactérie est capable de libérer de la phytase dans le milieu. Cesrésultats ouvrent de nouvelles perspectives pour améliorer l'utilisation du phytate pour lanutrition phosphatée des plantes.Soil microorganisms act as a sink and a source of available N and P bymediating key processes in the biogeochemical N and P cycling. The microbial loop, basedupon the grazing of bacteria by predators such as bacterial-feeding nematodes, is thoughtto play a major role in the mineralization of nutrients such as nitrogen (N) and phosphorus(P) in terrestrial ecosystems. However, little is known about the impact of grazing bynematodes on mineral nutrition of ectomycorrhizal woody plants. Different studies wereundertaken to quantify the role of nematode grazing on bacteria on the root growth andarchitecture, mineral nutrition (N and P) of a woody species, Pinus pinaster, whether ornot associated with the ectomycorrhizal basidiomycete Hebeloma cylindrosporum. Plantswere grown in a sterile simplified experimental system, whether inoculated or not withBacillus subtilis and bacterial-feeding nematodes (belonging to Rhabditidae orCephalobidae families) that were isolated from ectomycorrhizae and from soil of a P.pinaster plantation. The effect of nematode grazing on plant growth and the fate ofbacterial 15N towards plant shoots was strongly dependent upon medium P availability. Inaddition, nematode grazing was required to enable the plant to access P from phytate, awell-known poorly available P source to plants but that was used by bacterial populationsof B. subtilis due to its ability to release phytase in the medium. These results open analternative route to increase the use of phytate for plant P nutrition.MONTPELLIER-SupAgro La Gaillarde (341722306) / SudocSudocFranceF

Ecological importance of soil bacterivores for ecosystem functions

BackgroundBacterivores, mostly represented by protists and nematodes, are a key component of soil biodiversity involved in soil fertility and plant productivity. In the current context of global change and soil biodiversity erosion, it becomes urgent to suitably recognize and quantify their ecological importance in ecosystem functioning.ScopeUsing meta-analysis tools, we aimed at providing a quantitative synthesis of the ecological importance of soil bacterivores on ecosystem functions. We also intended to produce an overview of the ecological factors that are expected to drive the magnitude of bacterivore effects on ecosystem functions.ConclusionsBacterivores in soil contributed significantly to numerous key ecosystem functions. We propose a new theoretical framework based on ecological stoichiometry stressing the role of C:N:P ratios in soil, microbial and plant biomass as important parameters driving bacterivore-effects on soil N and P availability for plants, immobilization of N and P in the bacterial biomass, and plant responses in nutrition and growth

Enval, Durif, abri

Le siteÀ 20 km au sud de Clermont-Ferrand, le long d’un affluent de l’Allier, l’abri Durif s’ouvre dans le hameau d’Enval (commune de Vic-le-Comte), au pied d’une falaise d’arkose (fig. 1a).Dès 1887, un érudit local y signale des ossements et des outils en silex, mais ce n’est qu’en 1929, après la découverte de nouveaux artefacts lors de travaux de construction sur la propriété de M. Durif, que la société des Amis de l’université de Clermont-Ferrand vient en constater l’existence. En 1969, a..

Modification of a commercial dna extraction kit to simultaneously recover rna, safely and rapidly, and to assess molecular biomass of the total and the active part of microbial communities, from soils with diverse mineralogy and carbon content : S11.04-P -15

We have modified a commercial DNA extraction kit for soil to simultaneously co-extract RNA. In this new procedure RNA and DNA are separated by two selective purifications in cascade without the need of DNAase or RNAse digestion. Consequently DNA and RNA are respectively purified from the whole co-extraction solution. Nucleic acids extraction is based on the action of SDS coupled with an efficient bead-beating step, but it does not require any solvent. Avoiding the use of solvents, which are damaging for human health and environmental quality, was one of our most important motivations to develop this protocol. In a second time, we have optimized this protocol to improve the DNA and RNA yield, but kipping those yields below the saturation limit of the kit to assess and quantify the variations of molecular biomass of the total (DNA) and the active (RNA) part of microbial communities in natural samples. We have also introduced a first step of homogenization of soil sample in liquid nitrogen to improve the reliability of the fungal 18S gene sequence quantification. Finally, we have shown that this protocol can be applied to a wide diversity of soils whatever their mineralogy and metal content (2 Ferralsols, 1 Vertisol, 2 Andosols from Madagascar), texture or biomass content (1 poor sandy soil from Congo and one carbon rich temperate soil sample submitted or not to a 1 month cold stress). * E Tournier, L. Amenc and AL. Pablo contributed equally to this study. (Texte intégral

From soil to plant, the journey of P through trophic relationships and ectomycorrhizal association

Phosphorus (P) is essential for plant growth and productivity. It is one of the most limiting macronutrients in soil because it is mainly present as unavailable, bound P whereas plants can only use unbound, inorganic phosphate (Pi), which is found in very low concentrations in soil solution. Some ectomycorrhizal fungi are able to release organic compounds (organic anions or phosphatases) to mobilize unavailable P. Recent studies suggest that bacteria play a major role in the mineralization of nutrients such as P through trophic relationships as they can produce specific phosphatases such as phytases to degrade phytate, the main form of soil organic P. Bacteria are also more effective than other microorganisms or plants at immobilizing free Pi. Therefore, bacterial grazing by grazers, such as nematodes, could release Pi locked in bacterial biomass. Free Pi may be taken up by ectomycorrhizal fungus by specific phosphate transporters and transferred to the plant by mechanisms that have not yet been identified. This mini-review aims to follow the phosphate pathway to understand the ecological and molecular mechanisms responsible for transfer of phosphate from the soil to the plant, to improve plant P nutrition