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Cropping systems with legumes play key roles in farming systems in sub-Saharan Africa. However, how commonly legume associations perform in low input-systems is not well-known. Here, we studied four legume species used in three systems in rotation with upland rice, i.e., groundnut monocropping, sorghum–cowpea intercropping, and velvet bean–crotalaria intercropping, in two fertilization managements on the previous rice, i.e., manure alone or complemented with mineral fertilization. Legume suitability was assessed using rhizobial and mycorrhizal colonization rates, plant biomass production, shoot N and P content, and biological N2 fixation based on their δ15N natural abundance. Shoot and root biomasses varied significantly between legume species and were positively correlated with nodule number (r = 0.49 and 0.74, p-value &lt, 0.05 and &lt, 0.001, respectively) and the amount of fixed N (r = 0.73 and 0.50, p-value &lt, 0.001 and &lt, 0.05, respectively). The proportion of plant N derived from N2 fixation also varied significantly between species, with a higher percentage for velvet bean (66%), compared to the other three species (50 to 60%). Legume roots were weakly colonized by AM fungi, with similar levels between species. Overall, fertilization management did not significantly impact legume biomass, symbioses, or N2 fixation, yet the organo-mineral fertilization significantly increased legume shoot P content. The lack of effect of mineral fertilization on N2 fixation and biomass could be due to other nutrient deficiencies (Ca, Mg, micronutrients), which can hamper symbioses with rhizobia and mycorrhizae.

Arbuscular mycorrhizal (AM) fungi are crucial for plant nutrition and the sustainability of agroforestery systems. However, the contribution of each agroforestry component (i.e. trees, under-tree herbaceous vegetation -UtHV- and crops) in the establishment and maintenance of AM communities is poorly documented particularly in temperate areas. This study investigates the spatio-temporal dynamics of AM fungi colonizing roots of the three agroforestry components in southwest France. Standing fine root length density and production, AM activity (colonization and extraradical hyphal growth) and diversity of walnut trees, UtHV and soft wheat were assessed over one year in two agroforestry systems at different distances from the perennial tree-UtHV alley. Compared to UtHV, trees showed a higher ability to colonize superficial layers far into the cultivated alleys due to their wider root system in summer. However, due to higher root densities and well established AM fungi observed throughout all the year, UtHV appeared to be more ecologically relevant to maintain an active source of AM inoculum for newly developing crop roots in winter. High degree of root proximity and similarity of AM fungal communities among the three agroforestry components provide new perspectives in deciphering the significance of common mycorrhizal networks in plant to plant interactions.

(IF 3.95; Q1); International audience; The rapid expansion of perennial crops is a major threat to biodiversity in Southeast Asia. The biodiversity losses related to the conversion of forest lands to oil palm or rubber plantations (RP) are well documented by recent studies. However, the impact of the conversion from intensively managed annual crops to perennial crops on soil biodiversity has not yet been addressed. This study aims at assessing the impact on soil biodiversity of a) the short-term effect of land use conversion from cassava crop to RP, and b) the long-term effect of RP ageing. Soil biodiversity (bacterial, fungal and macrofaunal), microbial activities and pedoclimatic characteristics were measured over a chronosequence of 1–25 years old of RP compared to cassava fields, the former crop, in Thailand. The conversion from cassava to young RP (1–3 yr) had a significant effect on microbial biomass and activities and fungal composition, but did not impact the bacterial and macrofaunal diversity. This effect of land use conversion could be explained by the change in land management due to the cultivation of pineapple in the inter-row of the young RP. Canopy closure appeared to be the main driver of soil biota shifts, as most of the biotic parameters, composition, abundance and activities were significantly modified after 7 years of RP. The changes of composition in older rubber plantations originated from the dominance of Trichoderma (fungi), Firmicutes (bacteria), and earthworms. Old rubber plantations (23–25 yr) harboured the highest microbial and macrofaunal biomass; however, they were also characterised by a significant decrease in bacterial richness. The change in pedoclimatic conditions across the rubber chronosequence, i.e. increase in soil moisture, litter and organic carbon content, was a stronger driver of soil biota evolution than land use conversion. The macrofaunal composition was more resistant to land use conversion than the bacterial composition, whereas the microbial biomass was sensitive to land use conversion, but showed resilience after 20 years. However, bacterial, fungal and macrofaunal diversity, macrofaunal and microbial biomass and microbial activities were all sensitive to RP ageing.

Pterocarpus officinalis (Jacq.) is a leguminous forestry tree species endemic to Caribbean swamp forests. In Guadeloupe, smallholder farmers traditionally cultivate flooded taro (Colocasia esculenta) cultures under the canopy of P. officinalis stands. The role of arbuscular mycorrhizal (AM) fungi in the sustainability of this traditional agroforestry system has been suggested but the composition and distribution of AM fungi colonizing the leguminous tree and/or taro are poorly characterized. An in-depth characterization of root-associated AM fungal communities from P. officinalis adult trees and seedlings and taro cultures, sampled in two localities of Guadeloupe, was performed by pyrosequencing (GS FLX+) of partial 18S rRNA gene. The AM fungal community was composed of 215 operational taxonomic units (OTUs), belonging to eight fungal families dominated by Glomeraceae, Acaulosporaceae, and Gigasporaceae. Results revealed a low AM fungal community membership between P. officinalis and C. esculenta. However, certain AM fungal community taxa (10% of total community) overlapped between P. officinalis and C. esculenta, notably predominant Funneliformis OTUs. These findings provide new perspectives in deciphering the significance of Funneliformis in nutrient exchange between P. officinalis and C. esculenta by forming a potential mycorrhizal network.

In the arbuscular mycorrhizal (AM) symbiosis, plants satisfy part of their nitrogen (N) requirement through the AM pathway. In sorghum, the ammonium transporters (AMT) AMT3;1, and to a lesser extent AMT4, are induced in cells containing developing arbuscules. Here, we have characterized orthologs of AMT3;1 and AMT4 in four other grasses in addition to sorghum. AMT3;1 and AMT4 orthologous genes are induced in AM roots, suggesting that in the common ancestor of these five plant species, both AMT3;1 and AMT4 were already present and upregulated upon AM colonization. An artificial microRNA approach was successfully used to downregulate either AMT3;1 or AMT4 in rice. Mycorrhizal root colonization and hyphal length density of knockdown plants were not affected at that time, indicating that the manipulation did not modify the establishment of the AM symbiosis and the interaction between both partners. However, expression of the fungal phosphate transporter FmPT was significantly reduced in knockdown plants, indicating a reduction of the nutrient fluxes from the AM fungus to the plant. The AMT3;1 knockdown plants (but not the AMT4 knockdown plants) were significantly less stimulated in growth by AM fungal colonization, and uptake of both 15N and 33P from the AM fungal network was reduced. This confirms that N and phosphorus nutrition through the mycorrhizal pathway are closely linked. But most importantly, it indicates that AMT3;1 is the prime plant transporter involved in the mycorrhizal ammonium transfer and that its function during uptake of N cannot be performed by AMT4.

Arbuscular Mycorrhizal Fungi have a large potential to help increase global food security. They constitute the most important microbial symbiosis for the majority of terrestrial plant species. Their ecological functions in the productivity and stability of agroecosystems have been recognized for many years. Many studies have shown that these symbionts improved plant growth and plant resistance to biotic and abiotic stresses. Despite the proven potential of mycorrhizal symbiosis to sustainably improve the productivity of agroecosystems, this biotechnology is still under exploited. This failure mainly results from technical difficulties to mass-produced fungal inoculum of high quality and a Jack of knowledge about the biological factors regulating the soil receptivity of arbuscular mycorrhizal inoculation. In order to promote mycorrhizal soil infectivity, two main approaches could be considered: (1) the "reductionist" approach that consists to add into the soil, a large quantity of fungal propagules of a specialized AMF and (2) the "holistic" approach that aims to conserve and restore native AMF diversity and abundance. In this chapter, we will examjne the environmental factors that affect the mycorrhizal diversity and abundance and limit both approaches as they can both be of interest, trying to ex plain to what environmental solution they would be more adapted.

In vitro cultivation systems of arbuscular mycorrhizal fungi are useful tools to study the interaction between plants and their fungal symbiont, and also to develop new biotechnologies. Plantlets of the latex-producing species Hevea brasiliensis clone PB 260 were grown in a dense extraradical mycelium network of the arbuscular mycorrhizal fungus Rhizophagus irregularis MUCL 41833 developed from a mycelium donor plant (Medicago truncatula A17). The factors indole-3-butyric acid (IBA), 2-morpholineoethanesulfonic acid monohydrate (MES) buffer, and carbon dioxide (CO2) were tested on root development and colonization by the fungus. No colonization was observed in the presence of plantlets pre-treated with IBA. The highest levels of root colonization were obtained when plantlets were mycorrhized under a high CO2 concentration (1,000 μmol mol−1) with MES (10 mM) added to the growth medium. Widespread root colonization (with presence of arbuscules, intraradical mycelium, and spores/vesicles) was predominantly observed in newly produced roots. Therefore, it appears essential to improve root initiation and growth for improving in vitro mycorrhization of H. brasiliensis. We demonstrated the potential of the “mycelium donor plant” in vitro culture system to produce colonized H. brasiliensis plantlets before their transfer to ex vitro conditions.

Cinq espèces végétales (Withania frutescens Pauquy, Lavandula multifida L., Pistacia atlantica Desf., Olea europaea L. subsp. oleaster (Hoffmanns. & Link) Negodi et Tetraclinis articulata [Vahl] Mast.), rencontrées au niveau de l'écosystème tétraclinaie au Maroc ont été sélectionnées afin d'évaluer leur potentiel mycorhizien et de comparer l'impact de leur sol rhizosphérique sur la croissance et la nutrition des jeunes plants de thuya (Tetraclinis articulata). Les racines des plantes sélectionnées ont été abondamment colonisées par les champignons mycorhiziens arbusculaires (AM). Les racines de L. multifida ont montré le pourcentage le plus élevé de mycorhization (100 %). Nous avons ensuite isolé les spores de champignons AM à partir du sol de ces cinq espèces de plantes et nous avons constaté que le nombre total de spores isolées de la rhizosphère de L. multifida et T. articulata a été sensiblement différent de celui des autres plantes. Le potentiel mycorhizien des sols indigènes a été évalué en utilisant l'approche du nombre le plus probable (MPN). Nous avons constaté que ce nombre était significativement plus élevé dans les sols récoltés près des cinq espèces de plantes, que celui dans le sol sans couvert végétal, soit une moyenne du MPN par 100 g de sol sec de 11,8 (compris entre 5,6 et 25,0) dans le sol nu et 228,5 (compris entre 108,0 et 476,0) dans la rhizosphère de L. multifida. Ce résultat indique que le sol entourant les plantes cibles peut être utilisé comme inoculum mycorhizien. En pépinière forestière, les plants de thuya mis en culture dans le sol sous la lavande ont montré les meilleurs pourcentages de mycorhization et ont pu améliorer leur croissance. Toutefois, la production en arbuscules du thuya se développant dans la rhizosphère de la lavande n'a pas significativement affecté la croissance ou la nutrition des plants par rapport à ceux se développant dans un sol sous le thuya, sauf pour la concentration du potassium. Nos recherches ont montré clairement que le sol sous L. multifida peut être utilisé comme inoculum mycorhizien efficace dans la production de T. articulata en pépinières forestières, et peut participer ainsi au maintien de l'écosystème tétraclinaie. Mots-clés. Inoculum indigene, peuplement de thuya, plantes arbustives, mycorhizes à arbuscules, nutrition.

International audience; The productivity and diversity of plant communities are affected by soil organisms such as arbuscular mycorrhizal fungi (AMF), root herbivores and decomposers. However, it is unknown how interactions between such functionally dissimilar soil organisms affect plant communities and whether the combined effects are additive or interactive. In a greenhouse experiment we investigated the individual and combined effects of AMF (five Glomus species), root herbivores (wireworms and nematodes) and decomposers (collembolans and enchytraeids) on the productivity and nutrient content of a model grassland plant community as well as on soil microbial biomass and community structure. The effects of the soil organisms on productivity (total plant biomass), total root biomass, grass and forb biomass, and nutrient uptake of the plant community were additive. AMF decreased, decomposers increased and root herbivores had no effect on productivity, but in combination the additive effects canceled each other out. AMF reduced total root biomass by 18%, but decomposers increased it by 25%, leading to no net effect on total root biomass in the combined treatments. Total shoot biomass was reduced by 14% by root herbivores and affected by an interaction between AMF and decomposers where decomposers had a positive impact on shoot growth only in presence of AMF. AMF increased the shoot biomass of forbs, but reduced the shoot biomass of grasses, while root herbivores only reduced the shoot biomass of grasses. Interactive effects of the soil organisms were detected on the shoot biomasses of Lotus corniculatus, Plantago lanceolata, and Agrostis capillaris. The C/N ratio of the plant community was affected by AMF. In soil, AMF promoted abundances of bacterial, actinomycete, saprophytic and AMF fatty acid markers. Decomposers alone decreased bacterial and actinomycete fatty acids abundances but when decomposers were interacting with herbivores those abundances were increased. Our results suggests that at higher resolutions, i.e. on the levels of individual plant species and the microbial community, interactive effects are common but do not affect the overall productivity and nutrient uptake of a grassland plant community, which is mainly affected by additive effects of functionally dissimilar soil organisms. (c) 2010 Elsevier Ltd. All rights reserved.

The main goal of this work was to describe the mycorrhizal status, AM or ECM or dual AM and ECM or nonmycorrhizal (NM), of different trees species in five coastal humid forests located along the eastern coast of Madagascar. Identification of the plants was carried out by carefully tracing the root system in the field. Several field collection surveys were under-taken and the biological material was examined in laboratory conditions to check the mycorrhizal status of roots and to identify sporophores. (Resume d'auteur)

Our study aimed to assess the effect of dual inoculation with the selected Rhizobium strain KWN35 and the arbuscular mycorrhizal isolate BEG 176 (Glomus etunicatum) on the growth of Calliandra calothyrsus cultivated under irrigation in the field in Senegal (Dakar) over a period of 24 months. Although plants inoculated with both microsymbionts grew better (height and root collar diameter) than plants from three other inoculation treatments (control, single inoculation with KWN35 or BEG 176), these results were not statistically different except at 1 and 5 months after field transplantation. KWN35 was present in a relatively high percentage of nodules harvested from plants inoculated with this rhizobial strain either alone or with mycorrhiza (approximately 60% and 40% 1 and 2 years after field transplantation respectively). The percentage of mycorrhizal root infections was around 60% in the inoculated plants after 12 months with significantly higher N, P and K foliar contents of trees compared with the non-inoculated controls. However, as for nodulation, by 24 months after transplantation, there were no significant differences between treatments. We conclude that field inoculation of C. calothyrsus with Rhizobium strain KWN35 and arbuscular mycorrhizal isolate BEG 176 did not have a long-lasting effect on the growth of trees, even when a majority of nodules were occupied by the inoculated rhizobia and the roots infected by the mycorrhiza. Several reasons can be postulated for these results, such as a possible effect of soil fertility on the efficiency of the nodules.

This chapter discusses the advances in biological control of cacao diseases over the last 15 years. Most attention has been focused on biological control of frosty pod rot (Moniliophthora roreri), witches’ broom (Moniliophthora perniciosa) and black pod disease (Phytophthora spp.). Research on biocontrol of other diseases in the cacao phyllosphere or rhizosphere is scarce or in its infancy. There is, however, a steady increase in information regarding the factors influencing and the mechanisms underlying biological control of cacao diseases as well as practical aspects such as inoculum production, formulation and application. There has been a clear shift away from inundative approaches using epiphytic BCAs towards more classical biocontrol approaches using bacterial and fungal endophytes as well as vesicular arbuscular mycorrhiza. These have the advantage that they can permanently establish themselves in the cacao tree. Moreover, besides direct competition for space and nutrients, antibiosis and mycoparasitism, through induced resistance and growth promotion, endophytes have a larger arsenal of mechanisms through which they can help protect their host. Endophytic BCAs could thus provide more effective and sustainable disease control. Recent advances in our understanding of the mechanisms through which endophytic biocontrol agents can reduce pest and disease impact provide possibilities for innovative disease control strategies, including combination therapies together with natural or chemical products. Continued work on production, formulation and application is also necessary in order for biocontrol to become economically interesting. However, biological control will not become a stand-alone solution for disease control but should become part of integrated pest management strategies, with cultural management as a central and reinforcing pillar.

In Morocco, the use of seed legumes is limited because significant water deficits and the low availability of soil phosphorus (P) limit nitrogen fixation. However, little is known about the ability of faba bean-rhizobium symbiosis to fix nitrogen in P-deficient soils and to transfer fixed nitrogen (N) to intercropped wheat. Arbuscular mycorrhizal fungi (AM) and their extraradical hyphae networks play an important role in the facilitation process by promoting interconnectivity and the transfer of nutrients, such as N and P, between associated plants. The aim of this study was to analyse the impact of AM inoculation on N-2 fixation and the transfer of fixed N from faba bean to intercropped wheat. Germinated faba bean and wheat seeds were transferred into 1-l pots filled with a P-deficient soil that was collected from the Haouz valley near Marrakech (Morocco). Plants from the two species were grown in pots in either pure or mixed stands under greenhouse conditions, and each cropping system was subjected to three mycorrhizal inoculation treatments with a non-inoculated (AM0) and two concentrations of Rhizophagus irregularis inoculants containing 1000 (AM1) or 2000 (AM2) spores pot(-1). The N-15 isotope dilution method was used to determine the amount and proportion of atmospheric N fixed by faba bean (Ndfa%) and the fixed N that was transferred to wheat. Mycorrhizal inoculation had a significantly positive effect on the shoot dry weights and total shoot N in faba bean, but not in wheat. The cropping system had no significant effect on the plant growth and total shoot N in both faba bean and wheat. The Ndfa percentage was very high in all of the treatments, varying from 86 to 91%. The total N fixed by faba bean was 27% significantly higher in the AM2 treatment compared with the AM1 and AM0 treatments for both cropping systems combined. The estimated proportions of fixed N that were transferred from faba bean to wheat were far higher in AM1 (50%) and AM2 (32%) treatments than in AM0 (15%) treatment as well as for the total transferred fixed N. As corroborated by a parallel observation of root mycorrhizal colonization, these results suggest that the development of mycorrhizal networks stimulates the transfer of fixed N from faba bean to wheat, which could significantly contribute to the facilitation process under intercropping conditions.

Rubber tree (Hevea brasiliensis) is of major economic importance in Southeast Asia and for small land holders in Thailand in particular. Due to the high value of latex, plantations are expanding into unsuitable areas, such as the northeast province of Thailand where soil fertility is very low and therefore appropriate management practices are of primary importance. Arbuscular mycorrhizal fungi (AMF) contribute to plant growth through a range of mechanisms and could play a key role in a more sustainable management of the rubber plantations. We described the diversity of AMF associated with rubber tree roots in Northeast Thailand in relation to tree age and soil parameters along a chronosequence of rubber tree plantations. Cassava fields were included for comparison. Rubber tree and cassava roots harbored high diversity of AMF (111 Virtual Taxa, VT), including 20 novel VT. AMF VT richness per sample was consistently high (per site mean 16 to 21 VT per sample) along the chronosequence and was not related to soil properties. The composition of AMF communities differed between cassava and rubber tree plantations and was influenced by soil texture and nutrient content (sand, K, P, Ca). AMF community composition gradually shifted with the age of the trees. Our results suggest that the high diversity of AMF in this region is potentially significant for maintaining high functionality of AMF communities.

Rubber tree is a very important crop in Thailand, representing an essential source of income for farmers. In the past two decades, rubber tree plantations have been greatly expanding in unfavorable areas, where climate conditions are difficult and soil fertility is very poor. To optimize latex yields, mineral fertilizers have been widely used. A better understanding of the roles of the biological compartment in soil fertility is essential to determine alternative management practices to sustain soil fertility and optimize latex yields. Arbuscular mycorrhizal fungi (AMF) are widely recognized as beneficial for plants, mainly through their role in improving plant nutrient uptake. The objective of this study was to assess the AMF populations in rubber tree plantations and the impact of both soil characteristics and plantation age on these communities. Our results showed that all rubber trees were highly colonized, regardless of the soil structure and nutrient contents. AMF colonization was not affected by the age of the trees, suggesting that maintaining the symbiosis is likely to be beneficial at all stages. A better understanding and management of the microbial communities would contribute to maintaining or restoring soil fertility, leading to a better tree growth and optimized latex yield.

International audience; Although it is usually admitted that arbuscular mycorrhizal (AM) fungi are key components in soil bio-functioning, little is known on the response of microbial functional diversity to AM inoculation. The aims of the present study were to determine the influence of Glomus intraradices inoculum densities on plant growth and soil microflora functional diversity in autoclaved soil or non-disinfected soil. Microbial diversity of soil treatments was assessed by measuring the patterns of in situ catabolic potential of microbial communities. The soil disinfection increased sorghum growth, but lowered catabolic evenness (4.8) compared to that recorded in the non-disinfected soil (6.5). G. intraradices inoculation induced a higher plant growth in the autoclaved soil than in the non-disinfected soil. This AM effect was positively related to inoculum density. Catabolic evenness and richness were positively correlated with the number of inoculated AM propagules in the autoclaved soil, but negatively correlated in the non-disinfected soil. In addition, after soil disinfection and AM inoculation, these microbial functionality indicators had higher values than in the autoclaved or in the non-disinfected soil without AM inoculation. These results are discussed in relation to the ecological influence of AM inoculation, with selected fungal strains and their associated microflora on native soil microbial activity

Introduction. La banane et le plantain occupent une place importante dans l'economie du Cameroun. Ces fruits sont consommes sous diverses formes et ils entrent dans l'alimentation de base des populations. L'utilisation des champignons mycorhiziens a arbuscules peut favoriser une bonne croissance des bananiers. Une etude a ete conduite en pots sous ombriere au Cameroun avec des vitroplants de bananier plantain (cv. Bâtard) pour evaluer leur reaction a la mycorhization sur cinq sols de fertilite differente. Materiel et methodes. Les vitroplants de bananier plantain ont ete inocules pendant la premiere phase de sevrage avec une souche locale de champignons mycorhiziens a arbuscules du genre Glomus sp. Un mois apres inoculation, les plants ont ete transferes sur cinq types de sols differents. Les parametres de croissance (circonference, hauteur du pseudo-tronc, nombre de feuilles et surface foliaire de la derniere feuille emise, ainsi que poids frais et sec de la plante) et de mycorhization (frequence et intensite) ont ete observes huit et douze semaines apres inoculation. Resultats et discussion. Les observations realisees ont montre que les plants inocules ont eu une croissance significativement plus elevee que les plants non inocules et cela quel que soit le type de sol. Le poids frais et le taux de matiere seche ont presente egalement des valeurs plus elevees sur les plants inocules. Le taux de colonisation racinaire par le champignon a ete meilleur sur les sols pauvres. Conclusion. La mycorhization des bananiers plantains a eu un effet benefique non negligeable sur certains sols riches, mais cet effet a ete plus important encore sur les sols pauvres. La mycorhization pourrait donc permettre d'ameliorer la croissance et la production des bananiers plantains sur des sols a faible fertilite; ces resultats obtenus en conditions controlees devront etre confirmes par des essais en grandeur reelle. (Resume d'auteur)

We have isolated 17 strains of rhizobiums from nodules collected on roots of Pterocarpus officinalis in different sites of swamp forest in Guadeloupe and Martinique. The genotypic diversity of these strains were characterized by PCR-RFLP analysis of the IGS (inter-genic spacer) region between 16S and 23S rRNA genes, by using two 4-base restriction endonucleases Hae III and Msp I. Results of IGS PCR-RFLP analysis showed that restriction patterns were similar for DNA extract from root nodules and cultures of rhizobium strains. The diversity of these strains varied from one site to another. They were grouped into 5 ribotypes by IGS PCR-RFLP analysis. Results of IGS sequence analysis showed that the 5 ribotypes are very close to the genus Bradyrhizobium. The symbiotic properties of the 5 ribotypes were evaluated on Acacia seyal , a plant test that is able to form nodules with several Bradyrhizobium species. One month and nineteen days after inoculation, the green colour of the leaves and the best growth of inoculated seedlings contrasting with the yellow colour of the leaves and the lowest growth of uninoculated seedlings, were the parameters indicating the level of nitrogen-fixing activity of the three Bradyrhizobium strains. Concurrently, the vesicular and arbuscular mycorrhizal status of P. officinalis was established. The two types of symbiotic microorganisms could contribute to mobilize available phosphorus and nitrogen for improving the growth of P. officinalis in degraded swamp forests., Nous avons isolé 17 souches de rhizobiums à partir de nodules récoltés sur des racines de Pterocarpus officinalis dans différents sites de forêt marécageuse en Guadeloupe et Martinique. La diversité de ces souches a été caractérisée par PCR-RFLP de l’intergène IGS situé entre les gènes nucléaires codant pour les ARNr 16S et 23S en utilisant deux enzymes de restriction Hae III et Msp I. Des profils RFLP obtenus par digestion de l’IGS d’extraits d’ADN de nodules ou de cultures bactériennes sont comparables. L’analyse de ces profils RFLP a révélé une hétérogénéité génétique des souches qui varie suivant les sites. Les 17 souches de P. officinalis ont été groupées en 5 ribotypes par comparaison de leur profil RFLP. L’analyse phylogénétique de l’IGS a montré que ces ribotypes sont très proches du genre Bradyrhizobium. Les propriétés symbiotiques d’un représentant de chaque groupe de souches ont été testées sur Acacia seyal , espèce qui nodule avec un grand nombre d’espèces de Bradyrhizobium. Un mois et dix neuf jours après inoculation, l’observation de la vigueur et de la couleur verte des parties aériennes des plantes inoculées, comparativement aux plantes témoins peu vigoureuses et de couleur jaune, ont été des paramètres indiquant le caractère fixateur de trois souches de Bradyrhizobium. Parallèlement, la présence de champignons mycorhiziens à vésicules et à arbuscules a été révélée sur des racines de P. officinalis. Ces deux types de symbiose pourraient agir en synergie sur la mobilisation du phosphore et de l’azote pour améliorer la croissance de P. officinalis dans des forêts marécageuses dégradées., Bâ Amadou M.,Samba R.,Sylla S. N.,Le Roux C.,Neyra M.,Rousteau Alain,Imbert D.,Toribio A. Caractérisation de la diversité des microorganismes symbiotiques de pterocarpus officinalis dans des forêts marécageuses de Guadeloupe et Martinique . In: Revue d'Écologie (La Terre et La Vie), tome 59, n°1-2, 2004. pp. 163-170.

Soil microorganisms viz . bacteria, fungi, actinomycetes and arbuscular mycorrhizal (AM) fungi present in different typologies of coffee production systems were compared. In this study, two types of coffee plants, namely Arabica and Robusta, were grown under different agroforestry management such as coffee under one specialized shade species, multi-story coffee systems with 2 shade tree species, and coffee with 3 or more tree species under moist deciduous and evergreen ecological conditions. Samples were collected from 36 points to include different coffee ecosystems. The highest number of infective propagules of AM fungi was encountered in Arabica coffee under evergreen conditions. Population of bacteria, fungi and actinomycetes were higher under evergreen ecosystem compared to that of deciduous conditions. The population of nitrogen fixing bacteria was more than double in evergreen conditions compared to deciduous ecosystem. Number of lignin decomposing bacteria was higher in evergreen compared to deciduous conditions, but starch hydrolyzing bacteria and pectin-utilizing bacteria were more in deciduous ecosystem. Actinomycetes DAT2-1 isolated from deciduous ecosystem showed antagonistic activity against the root pathogen Fusarium chlamydosporum. It can be concluded that evergreen coffee system supports higher population of microorganisms. Of the two species of coffee, Arabica harboured more AM fungi, bacterial population, N fixers, P solubilizers and cellulose decomposing organisms while Robusta harboured higher number of fungi and actinomycetes. Of the three typologies, coffee grown under two shade tree species supported higher population of all microorganisms.

Metal-rich soils, especially heavy metal-polluted soils and ultramafic soils, are generally toxic to non-adapted plants and microorganisms. The role of mycorrhizal fungi in the metal tolerance of adapted plant species has become clear in the last decade. This review aims to synthesize the findings of representative studies of the effects of mycorrhizas on the alleviation of heavy metal toxicity on plants and on the absorption/accumulation of heavy metals in their roots and shoots. The adaptation to heavy metals by mycorrhizal symbionts is associated with their efficiency in metal-rich soils. More than 80 % of the studies have indicated a positive role of mycorrhizal fungi in the adaptation of plants to heavy metals in these soils, but the relationships between plant tolerance to heavy metals and the absorption of metals are complex and depend on a range of biological, physical and chemical factors.

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.

A greenhouse experiment was conducted to investigate the effects of a root-lesion nematode, #Pratylenchus coffeae#, two arbuscular mycorrhizal (AM) fungi, #Acaulospora mellea# and #Glomus clarum#, and timing of inoculation on the growth and nutrition of a nematode-susceptible Arabica coffee cultivar. The late AM inoculation (added simultaneously with nematodes) did not enhance coffee tolerance to #P. coffeae#. In the presence of #P. coffeae#, late-mycorrhizal plants were P deficient during the entire experiment and their foliar P concentration remained as low as that of non-mycorrhizal plants. After 7.5 months, nematodes decreased AM colonization of late-mycorrhizal plants by half and their biomass was only 20-30% that of the controls. In contrast, early AM inoculation (4 months before nematode inoculation) with either AM species improved the tolerance of coffee to #P. coffeae#. Root colonization by AM was not significantly reduced by #P. coffeae#. Despite higher densities of nematodes, root lesions were less numerous and more localized in early AM inoculated plants than in those of non-mycorrhizal plants. In the presence of #P. coffeae#, early AM-inoculated plants remained P sufficient and their biomass was still 75-80% that of their nematode-free controls. This study shows that in soils with low P levels, enhanced tolerance to #P. coffeae# seems limited to mycorrhizal coffee plants with well established AM symbiosis and improved P status.

Les contraintes majeures qui limitent la productivité des agroécosystèmes en milieu arides sont généralement attribuées aux carences minérales [plus particulièrement en phosphore (P) et en azote (N)], à un déficit en eau, aux processus d’érosion hydrique et éolienne, ainsi qu’à des perturbations dans le biofonctionnement du sol (faible diversité génétique et fonctionnelle de la microflore tellurique, etc.). Lorsque ces milieux ne sont pas soumis à une surexploitation, ils présentent une activité biologique élevée. Il a été ainsi suggéré que leur valorisation via des stratégies culturales adéquates pouvait réactiver les processus écologiques impliqués dans le maintien de la fertilité des sols et ainsi assurer une meilleure productivité de l’agro-écosystème. La plupart des plantes de ces régions arides (espèces sauvages ou cultivées) sont associées à des champignons mycorhiziens à arbuscules (CMA). Ces symbiotes fongiques sont reconnus comme étant des composantes microbiennes majeures dans le déroulement des principaux cycles biogéochimiques des sols (C, P et N) et en conséquence dans le développement des plantes en améliorant leur nutrition minérale mais aussi hydrique et leur état sanitaire. Paradoxalement, leur valorisation au sein d’itinéraires culturaux reste encore très limitée, voire inexistante. Ce chapitre a pour objectifs principaux de décrire le rôle des CMA dans le maintien et l’amélioration de la fertilité des sols et sur le développement des plantes tant au niveau de leur nutrition minérale, de leur résistance au stress hydrique que de l’état sanitaire des sols cultivés (ex. : lutte contre Striga spp.). Des stratégies d’ingénierie écologique seront ensuite proposées afin d’optimiser certaines techniques culturales (ex. : agroforesterie) dans le rôle de maintien du potentiel infectieux mycorhizogène des sols en relation avec une pleine expression des propriétés de la symbiose mycorhizienne sur les plantes cultivées, permettant ainsi d’atteindre des rendements culturaux compatibles avec les besoins des populations locales.

In order to improve the use of commercial inoculants, 12 arbuscular mycorrhizal fungi (AMF) inoculants were evaluated in a two-step experiment under greenhouse conditions using maize. First, commercial mycorrhizal inoculants were propagated in a trap pot culture experiment under sterilized sand to evaluate their potential for maize (Zea may L.) root colonization as compared with an indigenous soil inoculum and to survey the AMF species present in the products. Three inoculants significantly increased root colonization levels compared with a soil inoculum. Instead of 12 declared AMF species, 13 fungal strains were extracted from the pot culture survey, including five undeclared species, while four declared species did not produce spores. In a second experiment, commercial products were inoculated into soil to assess their impact on maize growth and yield. Six weeks after planting, seven inoculants increased root colonization levels compared with control soil, while only three inoculants increased slightly the shoot biomass of maize plants. These experiments highlight the need to pre-evaluate commercial mycorrhizal inoculants on a selected crop and regional soil before launching large-scale field use.; Afin d'optimiser l'usage des mycorhizes en agriculture, douze inoculants commerciaux de champignon mycorhizien arbusculaire (CMA) furent évalués en serres sur le maïs. Dans un premier temps, les inoculants furent propagés en pots sur sable stérilisé afin d’évaluer leur potentiel sur la colonisation racinaire du maïs par rapport à celui d'un sol agricole du Kenya et d'inventorier les espèces CMA contenues dans les inoculants. Trois inoculants augmentèrent le taux de colonisation racinaire comparé au sol agricole. Treize espèces AMF furent isolées des inoculants dont 5 non déclarées. Quatre des 12 espèces annoncées n'ont pas sporulé. Dans une seconde expérience, les inoculants furent utilisés en combinaison avec le sol agricole afin d’évaluer leur impact sur le rendement du maïs. Six semaines après le semis, 7 inoculants augmentèrent le taux de colonisation racinaire par rapport au sol témoin alors que 3 inoculants entraînèrent une légère augmentation de la biomasse aérienne. Ces évaluations démontrent la nécessité d'effectuer une pré-évaluation des inoculants commerciaux sur une culture et un sol donnés avant de les implanter à grande échelle.

The main objective was to evaluate the impact of nurse plant species commonly found in Mediterranean areas (Lavandula dentata and Thymus satureoides) on microbial soil functions, on the native inoculum potential of AM fungi involved in the rock phosphate weathering and to measure the potential benefits to the growth of Atlas Cypress (Cupressus atlantica G.), an endemic Cupressacea of Morocco. Soils collected from an old C. atlantica forest and pre-cultivated with each of the target plant species (L. dentata and T. satureoides). After 5 months of cultivation, they were uprooted and the treated substrate was amended or not with Khouribga Rock Phosphate (KRP). Then pots were filled with the soil mixtures and planted with one pre-germinated seed of C. atlantica. The results show that pre-cultivation step with native mycotrophic plant species improves the mycorrhizal soil infectivity, modifies soil microbial functionalities and increases the impact of rock phosphate amendment on the C. atlantica growth. This low cost cultivation practice by improving forest plant development and cultural soil quality constitutes a promising ecological engineering tool to improve the performances of ecosystem restoration.

International audience; Fifty years of overexploitation have disturbed most forests within Sahelian areas. Exotic fast growing trees (i.e., Australian Acacia species) have subsequently been introduced for soil improvement and fuelwood production purposes. Additionally, rhizobial or mycorrhizal symbioses have sometimes been favored by means of controlled inoculations to increase the performance of these exotic trees in such arid and semiarid zones. Large-scale anthropogenic introduction of exotic plants could also threaten the native biodiversity and ecosystem resilience. We carried out an experimental reforestation in Burkina Faso in order to study the effects of Acacia holosericea mycorrhizal inoculation on the soil nutrient content, microbial soil functionalities and mycorrhizal soil potential. Treatments consisted of uninoculated A. holosericea, preplanting fertilizer application and arbuscular mycorrhizal inoculation with Glomus intraradices. Our results showed that (i) arbuscular mycorrhizal (AM) inoculation and prefertilizer application significantly improved A. holosericea growth after 4 years of plantation and (ii) the introduction of A. holosericea trees significantly modified soil microbial functions. The results clearly showed that the use of exotic tree legume species should be directly responsible for important changes in soil microbiota with great disturbances in essential functions driven by microbial communities (e.g., catabolic diversity and C cycling, phosphatase activity and P availability). They also highlighted the importance of AM symbiosis in the functioning of soils and forest plantation performances. The AM effect on soil functions was significantly correlated with the enhanced mycorrhizal soil potential recorded in the AM inoculation treatment.

The responses of the soil microbial community features associated to the legume tree Acacia senegal (L.) Willd. including both arbuscular mycorrhizal fungal (AMF) diversity and soil bacterial functions, were investigated under contrasting environmental conditions. Soil samples were collected during dry and rainy seasons in two contrasting rainfall sites of Senegal (Dahra and Goudiry, in arid and semiarid zone, resp.). Soils were taken from the rhizosphere of A. senegal both in plantation and natural stands in comparison to bulk soil. A multiple analysis revealed positive correlations between soil physicochemical properties, mycorrhizal potential and enzyme activities variables. The positive effects of A. senegal trees on soil mycorrhizal potential and enzyme activities indicates that in sahelian regions, AMF spore density and diversity as well as soil microbial functions can be influenced by land-use systems (plantation versus natural population of A. senegal) and environmental conditions such as moisture and soil nutrient contents. Our study underlines the importance of prior natural AMF screening for better combinations of A. senegal seedlings with AMF species to achieve optimum plant growth improvement, and for restoration and reforestation of degraded lands.

The central structure of the symbiotic association between plants and arbuscular mycorrhizal (AM) fungi is the fungal arbuscule that delivers minerals to the plant. Our earlier transcriptome analyses identified two half-size ABCG transporters that displayed enhanced mRNA levels in mycorrhizal roots. We now show specific transcript accumulation in arbusculated cells of both genes during symbiosis. Presently, arbuscule-relevant factors from monocotyledons have not been reported. Mutation of either of the Oryza sativa (rice) ABCG transporters blocked arbuscule growth of different AM fungi at a small and stunted stage, recapitulating the phenotype of Medicago truncatula stunted arbuscule 1 and 2 (str1 and str2) mutants that are deficient in homologous ABCG genes. This phenotypic resemblance and phylogenetic analysis suggest functional conservation of STR1 and STR2 across the angiosperms. Malnutrition of the fungus underlying limited arbuscular growth was excluded by the absence of complementation of the str1 phenotype by wild-type nurse plants. Furthermore, plant AM signaling was found to be intact, as arbuscule-induced marker transcript accumulation was not affected in str1 mutants. Strigolactones have previously been hypothesized to operate as intracellular hyphal branching signals and possible substrates of STR1 and STR2. However, full arbuscule development in the strigolactone biosynthesis mutants d10 and d17 suggested strigolactones to be unlikely substrates of STR1/STR2. Interestingly, rice STR1 is associated with a cis-natural antisense transcript (antiSTR1). Analogous to STR1 and STR2, at the root cortex level, the antiSTR1 transcript is specifically detected in arbusculated cells, suggesting unexpected modes of STR1 regulation in rice.

Mycorrhizal fungi constitute a key functional group of soil biota that greatly contribute to productivity and sustainability of terrestrial ecosystems. The benefits of mycorrhizal symbiosis to the host plant have usually been considered as a result of closed relationships between the host plant and the fungal symbiont. However, the extramatrical mycelium provides an increased area for interactions with other soil microorganisms by enhancing the development of the host plant root systems. Mycorrhizal fungi act as a bridge connecting the rhizosphere to the bulk soil and, through an active development of extraradical mycelium into the soil, this soil compartment (mycorrhizosphere) extends root–fungal interactions with soil microbial communities. Interactions within the mycorrhizosphere microbial community are of special interest because some microorganisms associated with mycorrhiza may complement mycorrhizal activities. The purpose of this chapter is to outline the mycorrhizosphere interactions between ectomycorrhizal fungi associated with forest tree species and soil microflora of potentially synergistic properties that lead to stimulation of plant growth. By focussing on the ectomycorrhizal symbiosis associated with Tropical and Mediterranean tree species, we will review the global effects of ectomycorrhizal symbiosis on the functional diversity of soil microflora and in particular, the interactions between ectomycorrhizal fungi and some plant-growth-promoting rhizobacteria. This review will focus on the interactions between ectomycorrhizal fungi and soil microflora leading to a sustainable microbial complex ecosystem with high efficiency against phosphorus mobilization and transferring phosphorus from the soil organic matter or from soil minerals to the host plant.

The diversity of arbuscular mycorrhizal fungi (AMF) in roots of two plant species (arar, Tetraclinis articulata and lavender, Lavandula multifida) that exist alone or co-occur in Moroccan Tetraclinis woodlands was characterized using molecular techniques. The interaction between the two plant species on their root AMF communities was also studied. We analysed the large subunit (LSU) ribosomal RNA gene amplified by nested PCR from a root DNA extract using AM fungal-specific primers. A total of 200 cloned fragments from five root bulk samples of the two plant species were analysed by restriction fragment length polymorphism (RFLP) and 30 of them were sequenced. Phylogenetic analyses demonstrated that these RFLP types correspond to eight phylotypes: five belonged to the Gigasporaceae and three to the Glomeraceae. The highest diversity was found in the roots of T. articulata alone, followed by the association T. articulata/L. multifida and, finally, L. multifida alone. Many AMF were shared between the two species, although a few of them had a preference for one or the other. In addition, PCA analysis showed clearly that the AMF community colonizing T. articulata differed significantly from that colonizing L. multifida. Our results reveal an impact of plant interactions on the AMF community.

International audience; The influence of shrubs used as nurse plants was tested on the growth of Cupressus atlantica, on microbial activity and on arbuscular mycorrhizal (AM) soil potential in a Mediterranean environment. An experimental plantation was conducted combining uninoculated, arbuscular mycorrhized Cypress seedlings and an association between Lavandula stoechas planted close to newly planted C. atlantica seedlings. After three years plantation, this association between C. atlantica and L stoechas lead to a higher growth of C atlantica and better soil microbial characteristics compared to the control treatment. AM mycelium network, total microbial activity, dehydrogenase activity, phosphate-solubilizing fluorescent pseudomonads and N. P nutrient uptake by C atlantica, were significantly higher in the presence of L stoechas than those recorded in the other treatments. This pioneer shrubs facilitates the early establishment of Cypress seedlings by improving soil microbial characteristics and AM fungus community development. Given that the facilitative effect of one plant species to another increases with abiotic stress, the benefits of this technique would be useful in reforestation programs undertaken to rehabilitate degraded areas in Mediterranean region. (C) 2011 Elsevier Ltd. All rights reserved.

The study of arbuscular mycorrhiza often requires the staining of fungal structures using specific dyes. Fluorescent dyes such as acid fuchsin and wheat germ agglutinin conjugates give excellent results, but these compounds are either hazardous or very expensive. Here, we show that a safer and inexpensive dye, Uvitex2B, can be efficiently used to stain intraradical fungal structures formed by the arbuscular mycorrhizal fungus Glomus intraradices in three plant species: carrot, Casuarina equisetifolia, and Medicago truncatula. The intensity and stability of Uvitex2B allow the acquisition of high-quality images using not only confocal laser scanning microscopy but also epifluorescence microscopy coupled with image deconvolution. Furthermore, we demonstrate that Uvitex2B and beta-glucuronidase staining are compatible and can thus be used to reveal arbuscular mycorrhizal structures in the context of promoter activation analysis.

International audience; Type III secretion systems (T3SSs) of Gram-negative bacteria mediate direct interactions with eukaryotic cells. Pseudomonas spp. harboring T3SS genes (T3SS+) were previously shown to be more abundant in the rhizosphere than in bulk soil. To discriminate the contribution of roots and associated arbuscular mycorrhizal fungi (AMF) on the enrichment of T3SS+ fluorescent pseudomonads in the rhizosphere of Medicago truncatula, their frequency was assessed among pseudomonads isolated from mycorrhizal and nonmycorrhizal roots and from bulk soil. T3SS genes were identified by PCR targeting a conserved hrcRST DNA fragment. Polymorphism of hrcRST in T3SS+ isolates was assessed by PCR-restriction fragment length polymorphism and sequencing. Genotypic diversity of all pseudomonads isolated, whether or not harboring T3SS, was described by BOX-PCR. T3SS+ pseudomonads were significantly more abundant in mycorrhizal than in nonmycorrhizal roots and in bulk soil, and all were shown to belong to the phylogenetic group of Pseudomonas fluorescens on the basis of 16S rRNA gene identity. Four hrcRST genotypes were described; two only included isolates from mycorrhizal roots. T3SS+ and T3SS− pseudomonads showed different genetic backgrounds as indicated by their different BOX-PCR types. Taken together, these data suggest that T3SSs are implicated in interactions between fluorescent pseudomonads and AM in medic rhizosphere.

International audience

Au Maroc, une pratique culturale traditionnelle, les agroforêts, repose sur l'association d'essences ligneuses telles que l'arganier ou le caroubier à des cultures de production agricole (orge, blé...) ou maraîchère (haricot, fève...) très diversifiées. Toutefois, sous l'effet combiné de pressions anthropiques directes (exploitation, surpâturage,...) et indirectes (surirrigation, baisse des nappes phréatiques, déprise agricole, ...) mais aussi du changement climatique, ces pratiques agroforestières ont tendance à disparaître. Souvent peu gourmandes en eau, elles représentent pourtant une opportunité unique de ressources financières pour les populations rurales, qui se maintiennent ainsi dans les villages. La mise en oeuvre d'un programme visant à créer, restaurer ou maintenir ces systèmes agroforestiers typiques exige la prise en compte des communautés microbiennes associées aux plantes, en particulier les endomycorhizes présents dans pratiquement tous les systèmes racinaires, et l'étude de leur rôle dans le fonctionnement et la régénération de ce système. L'objectif de ce travail est tout d'abord de décrire la diversité des communautés microbiennes, en particulier les mycorhizes arbusculaires, associées aux cultures annuelles (espèces mycorhizées fixatrices d'azote ou non, jachère non cultivée) et d'en évaluer l'impact sur les communautés microbiennes associées aux essences ligneuses. L'échantillonnage est réalisé au Maroc dans les régions d'Agadir et d'Essaouira avec des prélèvements de sol et de racines au pied des arbres et dans les cultures associées. Par la suite, la structure et la diversité des espèces mycorhiziennes arbusculaires associées aux essences ligneuses et aux cultures annuelles sont révélées par PCR-DGGE, par amplification spécifique de la région LSU des ADNr. Les taxons mycorhiziens arbusculaires présents dans les racines sont alors identifiés par clonage-séquençage. La comparaison de la structure et de la diversité des mycorhizes arbusculaires associées aux racines entre espèces ligneuses et cultures annuelles situées dans un même champ permettra de déterminer s'il existe ou non un phénomène de partage. Nous avons aussi recherché la présence éventuelle de taxons mycorhiziens arbusculaires "marqueurs" d'une espèce végétale, en particulier spécifique de l'Arganier et du Caroubier. Ces résultats pourront être confrontés par la suite aux performances de croissance de ces 2 espèces ligneuses et à leur aptitude à se régénérer spontanément.

Au Maroc, une pratique culturale traditionnelle, les agroforêts, repose sur l'association d'essences ligneuses telles que l'arganier ou le caroubier à des cultures de production agricole (orge, blé...) ou maraîchère (haricot, fève...) très diversifiées. Toutefois, sous l'effet combiné de pressions anthropiques directes et indirectes, ces pratiques agroforestières ont tendance à disparaître. Souvent peu gourmandes en eau, elles représentent pourtant une opportunité unique de ressources financières pour les populations rurales. La mise en oeuvre d'un programme visant à créer, restaurer ou maintenir ces agroforêts exige la prise en compte des communautés microbiennes associées aux plantes et l'étude de leur rôle dans le fonctionnement de ce système. L'objectif de ce travail est donc de décrire la diversité des communautés microbiennes associées aux cultures vivrières (espèces mycorhizées fixatrices d'azote ou non, jachère non cultivée) et d'en évaluer l'impact sur les communautés microbiennes associées aux ligneux. La structure et la diversité de la communauté bactérienne et des champignons endomycorhiziens associées aux essences ligneuses et aux cultures vivrières sont révélées par PCR-DGGE par amplification des ADNr 16S et 18S spécifiques. L'arganier et le caroubier n'étant pas des plantes nodulées, nous avons recherché l'existence d'une population bactérienne possédant les gènes nif et associée aux espèces mycorhiziennes détectées. Pour cela nous avons analysé la diversité de ces populations en ciblant le gène nifH par PCR-DGGE (spore individuelle, mélange de spores, sol, racines), en relation avec les différents taxons mycorhiziens identifiés. Ces résultats seront confrontés aux performances de croissance de ces 2 espèces ligneuses et à leur aptitude à se régénérer spontanément. (Texte intégral)

Sur les Hautes Terres de Madagascar, le rendement agricole est faible du fait de la dégradation de la fertilité du sol. La teneur en phosphore soluble est basse. Actuellement, l'agriculture s'oriente vers des systèmes plus durables en tenant compte des interactions biologiques dans le sol. Les champignons mycorhiziens à arbuscules (MA) apparaissent comme les organismes à prendre en considération. En effet les mycorhizes résultent de l'association symbiotique des champignons MA avec les racines des plantes. Ces symbioses sont directement impliquées dans l'amélioration de la nutrition minérale, dont le phosphore, l'absorption de l'eau et la protection contre certains agents pathogènes telluriques. Les pratiques culturales peuvent influencer l'évolution des populations de champignons MA. Les premières investigations sur l'état de mycorhization du sol de la région ont été réalisées en 2002. L'observation des racines d'une vingtaine de plantes, prélevées en sols cultivés et non cultivés, nous a permis de constater que la mycorhization était peu développée. Par ailleurs, nous avons évalué le potentiel infectieux mycorhizogène (PIM) du sol provenant des parcelles du dispositif d'Andranomanelatra de l'ONG TAFA âgé de plus de dix ans. Nous avons étudié l'effet de la rotation des cultures maïs/soja, le semis direct sur résidus de cultures ou couverture végétale morte, la fertilisation et le précédent cultural sur le PIM du sol. La méthode d'évaluation est basée sur un test biologique. Une plante mycotrophe (le soja) est cultivée dans des vases de végétation contenant le sol de différentes parcelles et se développe sous serre. Le sol des parcelles en semis direct sur résidus de cultures présente un PIM élevé que celui du système labouré, ce qui a un effet positif sur la production du soja. Mots-clés : potentiel infectieux, semis direct, hautes terres, champignons mycorhiziens à arbuscules, pratiques culturales, résidus des cultures.

International audience; Soil from two protected forests of Acacia nilotica var. tomentosa, located in the Senegal River Valley, was sampled to assess their rhizobial and mycorrhizal status. Forty-one soil samples (0.5 kg each) were collected at 10-15 cm depth in the rhizosphere of Acacia in the two protected forests of Diarra and Lam Nadie (26 and 15 samples, respectively). Soil extracts were used to test rhizobial nodulation of in-vitro growing seedlings of two varieties of A. nilotica, var. tomentosa and var. adansonii. Arbuscular mycorrhizal (AM) fungi soil infectivity was estimated by the most probable number (MPN) method using Acacia nilotica var. tomentosa as test plant. In the Diarra forest, eight soil samples of the 26 tested contained indigenous rhizobial strains nodulating both A. nilotica var. adansonii and A. nilotica var. tomentosa, whereas only one from Lam Nadie contained rhizobial strains. Fifty-six strains were isolated, purified, and their infectivity and efficiency tested on Acacia nilotica var. tomentosa after 60 days of culture under controlled conditions. Strains ND37 from Diarra and NL2 from Lam Nadie are the more effective in terms of shoot and root dry weight. The estimation of the number of viable propagules using the MPN method showed some significant differences between the populations of AMF of Diarra, which was high, and that of Lam Nadie, which was very low. The percentage of colonization of Acacia nilotica var. tomentosa was correlated with the MPN values. These results are discussed with regards to the state of forests.

International audience; Pterocarpus officinalis (Jacq.) seedlings inoculated with the arbuscular mycorrhizal fungus, Glomus intraradices, and the strain of Bradyrhizobium sp. (UAG 11A) were grown under stem-flooded or nonflooded conditions for 13 weeks after 4 weeks of nonflooded pretreatment under greenhouse conditions. Flooding of P. officinalis seedlings induced several morphological and physiological adaptive mechanisms, including formation of hypertrophied lenticels and aerenchyma tissue and production of adventitious roots on submerged portions of the stem. Flooding also resulted in an increase in collar diameter and leaf, stem, root, and total dry weights, regardless of inoculation. Under flooding, arbuscular mycorrhizas were well developed on root systems and adventitious roots compared with inoculated root systems under nonflooding condition. Arbuscular mycorrhizas made noteworthy contributions to the flood tolerance of P. officinalis seedlings by improving plant growth and P acquisition in leaves. We report in this study the novel occurrence of nodules connected vascularly to the stem and nodule and arbuscular mycorrhizas on adventitious roots of P. officinalis seedlings. Root nodules appeared more efficient fixing N2 than stem nodules were. Beneficial effect of nodulation in terms of total dry weight and N acquisition in leaves was particularly noted in seedlings growing under flooding conditions. There was no additive effect of arbuscular mycorrhizas and nodulation on plant growth and nutrition in either flooding treatment. The results suggest that the development of adventitious roots, aerenchyma tissue, and hypertrophied lenticels may play a major role in flooded tolerance of P. officinalis symbiosis by increasing oxygen diffusion to the submerged part of the stem and root zone, and therefore contribute to plant growth and nutrition.

The aim of this study was to determine the effectiveness of the mycorrhizal inoculation on the early development ofAustralian acacias in a dry tropical environment. Field experiments were carried out with an Australian Acacia species (Acacia holosericea) and four fungal strains were tested: an arbuscularmycorrhizal (AM) fungus (Glomus intraradices) and three ectomycorrhizal isolates belonging to the Pisolithus and Scleroderma genera. Mycorrhizal inoculation has significantly enhanced the growth of A. holosericea seedlings after 4 months of culturing in the greenhouse conditions in a disinfected soil. Mycorrhized and non-mycorrhized (control) plants were then transferred into the field. One of the experiments (Experiment EC5) was used to determine allometric relations based on stem diameter by destructive sampling. Two forms of biomass equations were generated and compared: (i) form 1 equation incorporated only stem diameter as the independent variable and form 2 equation incorporated stem diameter and height (product of square of stem diameter and height, D2H) as the independent variables. As no distinctive advantage of one form over the other was recorded, simplest equation (Eq. (1)) was used for above-ground biomass calculations. Mycorrhizal inoculations with one Pisolithus isolate (P. albus IR100) or G. intraradices have significantly improved the growth of A. holosericea after 18 months aswell as after 30 months of plantation. These results show that mycorrhizal inoculation could be of great relevance to improvement of the reafforestation process of degraded soils with leguminous fast growing trees, more particularly in Sahelian ecozones. © 2006 Elsevier B.V. All rights reserved

The hypothesis of the present study was that bacterial communities would differentiate under Eucalyptus camaldulensis and that an enhancement of arbuscular mycorrhizal (AM) density would minimize this exotic plant species effect. Treatments consisted of control plants, preplanting fertilizer application and AM inoculation. After 4 months of culture in autoclaved soil, E. camaldulensis seedlings were either harvested for growth measurement or transferred into containers filled with the same soil but not sterilized. Other containers were kept without E. camaldulensis seedlings. After 12 months, effects of fertilizer amendment and AM inoculation were measured on the growth of Eucalyptus seedlings and on soil microbial communities. The results clearly show that this plant species significantly modified the soil bacterial community. Both community structure (assessed by denaturing gradient gel electrophoresis profiles) and function (assessed by substrate-induced respiration responses including soil catabolic evenness) were significantly affected. Such changes in the bacterial structure and function were accompanied by disturbances in the composition of the herbaceous plant species layer. These results highlight the role of AM symbiosis in the processes involved in soil bio-functioning and plant coexistence and in afforestation programmes with exotic tree species that target preservation of native plant diversity.

[EN] A survey of arbuscular mycorrhizal (AM) fungi was conducted in seven Tetraclinis woodlands. Microscopic analysis of the mycorrhizal status of T. articulata (Vahl) Masters roots revealed that all samples formed only AM, and no ectomycorrhizal fungi were detected. The mycorrhizal colonisation level was generally high (more than 80%), thus reflecting the mycotrophic nature of T. articulata. A "Paris-type" mycorrhizal structure was noted in all studied samples. The number of AM fungal spores detected in field-collected soils was relatively high. All recovered spores belonged to the Glomineae order, represented by Glomaceae and Acaulosporaceae families. Two groups were dominant: the first one included small (90 mm), hyaline, white to dark-yellow spores, and the second involved large (295 mm), light orange to dark orange-brown spores. The morphological characters indicated that the spore populations consisted of 3-6 morphotypes. The Glomus genus was represented by five species, i.e. Glomus aggregatum, Glomus constrictum, Glomus sp. 1, Glomus sp. 2, and Glomus sp. 3, while the Acaulospora genus was represented by only one unidentified species., [FR] La présence des champignons mycorhiziens arbusculaires (CMA) a été étudiée dans sept tetraclinaies marocaines. Les examens microscopiques des racines de T. articulata (Vahl) Masters ont révélé la présence, dans tous les échantillons, des endomycorhizes arbusculaires ; aucune ectomycorhizes n'a été détectée. Le taux d'infection par les endomycorhizes à arbuscules a été très élevé (plus de 80 %), indiquant le caractère mycotrophique de l'espèce. La structure mycorhizienne observée dans tous les échantillons analysés est de type « Paris ». Le nombre de spores de CMA isolées à partir des différents sols est relativement élevé. Toutes les spores appartiennent à l'ordre des Glomineae, représenté par deux familles : Glomaceae et Acaulosporacea. Deux groupes sont dominants : le premier groupe renferme des spores hyalines, blanches à jaunes foncées et de petites tailles (90 µm en moyenne) et le second correspond à des spores orange-claires à orange foncées et de grandes tailles (295 µm en moyenne). Les caractères morphologiques indiquent que les populations de spore comportent 3 à 6 morphotypes selon le site. Le genre Glomus, le plus dominant, est représenté par cinq espèces - Glomus aggregatum, Glomus constrictum, Glomus sp. 1, Glomus sp. 2, et Glomus sp. 3 - alors que le genre Acaulospora est représenté par une seule espèce non identifiée., This study was carried out within the framework of an Agronomic Research Project for Development (PRAD No. 03/14)

La symbiose Pterocarpus officinalis (Jacq)-Bradyrhizobium spp. est dominante dans les plaines marécageuses côtières des Caraïbes, et sporadiquement présente le long de rivières et dans les montagnes de cette même zone géographique. La coupe et le drainage des forêts côtières, pour l'habitat et l'agriculture, restreignent leurs peuplements à de petites parcelles, posant le problème aigu du maintien de ces populations. Des informations phylogéographiques et de diversité génétique recueillies de ces populations de P. officinalis et de leurs bactéries associées, au sein des peuplements des caraïbes et du continent américain, pourraient alors s'avérer utiles dans la gestion des populations de légumineuses, mais aussi de ses bactéries symbiotiques associées, dans le cadre d'un programme de reforestation. Dans cette optique, nous avons évalué la diversité génétique des deux organismes à différentes échelles géographiques comprenant des populations continentales et insulaires. Sept populations de P. officinalis ont été échantillonnées sur différentes Îles des caraïbes (Guadeloupe, Dominique, Martinique et Porto Rico) et sur le continent en Guyane française. Entre 20 et 30 individus ont été échantillonnés dans chaque population et pour chaque individu, 2 feuilles et 5 nodules ont été prélevés pour les futures analyses moléculaires. La diversité génétique de P. officinalis est étudiée à l'aide de marqueurs microsatellites spécifiquement développés. Concernant les bactéries symbiotiques isolées à partir des nodules de P. officines, la diversité génétique est déterminée par le séquençage de l'espace intergénique 16S-23S de I'ADNr (IGS). Nos premiers résultats laissent apparaître une plus faible diversité génétique des populations de P. officinalis/Bradyrhizobium insulaires par rapport aux populations continentales. Ces résultats suggèrent une évolution parallèle des plantes et de leurs bactéries, conséquence possible de forces évolutives similaires (flux de gènes limités et fortes dérives génétiques attendues en milieu insulaire). Ils demandent cependant à être confirmés par des analyses (moléculaires et statistiques) complémentaires. (Texte intégral)