Ecological aspects and relationships of the emblematic Vachellia spp. exposed to anthropic pressures and parasitism in natural hyper-arid ecosystems: ethnobotanical elements, morphology, and biological nitrogen fixation.

MAIN CONCLUSION: Emblematic Vachellia spp. naturally exposed to hyper-arid conditions, intensive grazing, and parasitism maintain a high nitrogen content and functional mutualistic nitrogen-fixing symbioses. AlUla region in Saudi Arabia has a rich history regarding mankind, local wildlife, and fertility islands suitable for leguminous species, such as the emblematic Vachellia spp. desert trees. In this region, we investigated the characteristics of desert legumes in two nature reserves (Sharaan and Madakhil), at one archaeological site (Hegra), and in open public domains et al. Ward and Jabal Abu Oud. Biological nitrogen fixation (BNF), isotopes, and N and C contents were investigated through multiple lenses, including parasitism, plant tissues, species identification, plant maturity, health status, and plant growth. The average BNF rates of 19 Vachellia gerrardii and 21 Vachellia tortilis trees were respectively 39 and 67%, with low signs of inner N content fluctuations (2.10-2.63% N) compared to other co-occurring plants. The BNF of 23 R. raetam was just as high, with an average of 65% and steady inner N contents of 2.25 ± 0.30%. Regarding parasitism, infected Vachellia trees were unfazed compared to uninfected trees, thereby challenging the commonly accepted detrimental role of parasites. Overall, these results suggest that Vachellia trees and R. raetam shrubs exploit BNF in hyper-arid environments to maintain a high N content when exposed to parasitism and grazing. These findings underline the pivotal role of plant-bacteria mutualistic symbioses in desert environments. All ecological traits and relationships mentioned are further arguments in favor of these legumes serving as keystone species for ecological restoration and agro-silvo-pastoralism in the AlUla region.

A leguminous species exploiting alpha- and beta-rhizobia for adaptation to ultramafic and volcano-sedimentary soils: an endemic Acacia spirorbis model from New Caledonia.

Acacia spirorbis subsp. spirorbis Labill. is a widespread tree legume endemic to New Caledonia that grows in ultramafic (UF) and volcano-sedimentary (VS) soils. The aim of this study was to assess the symbiotic promiscuity of A. spirorbis with nodulating and nitrogen-fixing rhizobia in harsh edaphic conditions. Forty bacterial strains were isolated from root nodules and characterized through (i) multilocus sequence analyses, (ii) symbiotic efficiency and (iii) tolerance to metals. Notably, 32.5% of the rhizobia belonged to the Paraburkholderia genus and were only found in UF soils. The remaining 67.5%, isolated from both UF and VS soils, belonged to the Bradyrhizobium genus. Strains of the Paraburkholderia genus showed significantly higher nitrogen-fixing capacities than those of Bradyrhizobium genus. Strains of the two genera isolated from UF soils showed high metal tolerance and the respective genes occurred in 50% of strains. This is the first report of both alpha- and beta-rhizobia strains associated to an Acacia species adapted to UF and VS soils. Our findings suggest that A. spirorbis is an adaptive plant that establishes symbioses with whatever rhizobia is present in the soil, thus enabling the colonization of contrasted ecosystems.

Ectomycorrhizal Communities Associated with the Legume Acacia spirorbis Growing on Contrasted Edaphic Constraints in New Caledonia.

This study aims to characterize the ectomycorrhizal (ECM) communities associated with Acacia spirorbis, a legume tree widely spread in New Caledonia that spontaneously grows on contrasted edaphic constraints, i.e. calcareous, ferralitic and volcano-sedimentary soils. Soil geochemical parameters and diversity of ECM communities were assessed in 12 sites representative of the three mains categories of soils. The ectomycorrhizal status of Acacia spirorbis was confirmed in all studied soils, with a fungal community dominated at 92% by Basidiomycota, mostly represented by/tomentella-thelephora (27.6%), /boletus (15.8%), /sebacina (10.5%), /russula-lactarius (10.5%) and /pisolithus-scleroderma (7.9%) lineages. The diversity and the proportion of the ECM lineages were similar for the ferralitic and volcano-sedimentary soils but significantly different for the calcareous soils. These differences in the distribution of the ECM communities were statistically correlated with pH, Ca, P and Al in the calcareous soils and with Co in the ferralitic soils. Altogether, these data suggest a high capacity of A. spirorbis to form ECM symbioses with a large spectrum of fungi regardless the soil categories with contrasted edaphic parameters.

Molecular Characterization of Arbuscular Mycorrhizal Fungi in an Agroforestry System Reveals the Predominance of Funneliformis spp. Associated with Colocasia esculenta and Pterocarpus officinalis Adult Trees and Seedlings.

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.

Ectomycorrhizal fungal diversity associated with endemic Tristaniopsis spp. (Myrtaceae) in ultramafic and volcano-sedimentary soils in New Caledonia.

New Caledonian serpentine (ultramafic) soils contain high levels of toxic heavy metals, in particular nickel, (up to 20 g kg-1) and are deficient in essential elements like carbon, nitrogen and phosphorus while having a high magnesium/calcium ratio. Although previous studies showed that ectomycorrhizal symbioses could play an important role in the adaptation of the endemic plants to ultramafic soils (FEMS Microbiol Ecol 72:238-49, 2010), none of them have compared the diversity of microbial communities from ultramafic vs non-ultramafic soils in New Caledonia. We explored the impact of edaphic characteristics on the diversity of ectomycorrhizal (ECM) fungi associated with different endemic species of Tristaniopsis (Myrtaceae) growing under contrasting soil conditions in the natural ecosystems of New Caledonia. ECM root tips were thus sampled from two different ultramafic sites (Koniambo massif and Desmazures forest) vs two volcano-sedimentary ones (Arama and Mont Ninndo). The molecular characterization of the ECM fungi through partial sequencing of the ITS rRNA gene revealed the presence of different dominant fungal genera including, both soil types combined, Cortinarius (36.1%), Pisolithus (18.5%), Russula (13.4%), Heliotales (8.2%) and Boletellus (7.2%). A high diversity of ECM taxa associated with Tristaniopsis species was found in both ultramafic and volcano-sedimentary soils but no significant differences in ECM genera distribution were observed between both soil types. No link could be established between the phylogenetic clustering of ECM taxa and their soil type origin, thus suggesting a possible functional-rather than taxonomical-adaptation of ECM fungal communities to ultramafic soils.

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

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

Phylogeny of nodulation genes and symbiotic diversity of Acacia senegal (L.) Willd. and A. seyal (Del.) Mesorhizobium strains from different regions of Senegal.

Acacia senegal and Acacia seyal are small, deciduous legume trees, most highly valued for nitrogen fixation and for the production of gum arabic, a commodity of international trade since ancient times. Symbiotic nitrogen fixation by legumes represents the main natural input of atmospheric N2 into ecosystems which may ultimately benefit all organisms. We analyzed the nod and nif symbiotic genes and symbiotic properties of root-nodulating bacteria isolated from A. senegal and A. seyal in Senegal. The symbiotic genes of rhizobial strains from the two Acacia species were closed to those of Mesorhizobium plurifarium and grouped separately in the phylogenetic trees. Phylogeny of rhizobial nitrogen fixation gene nifH was similar to those of nodulation genes (nodA and nodC). All A. senegal rhizobial strains showed identical nodA, nodC, and nifH gene sequences. By contrast, A. seyal rhizobial strains exhibited different symbiotic gene sequences. Efficiency tests demonstrated that inoculation of both Acacia species significantly affected nodulation, total dry weight, acetylene reduction activity (ARA), and specific acetylene reduction activity (SARA) of plants. However, these cross-inoculation tests did not show any specificity of Mesorhizobium strains toward a given Acacia host species in terms of infectivity and efficiency as stated by principal component analysis (PCA). This study demonstrates that large-scale inoculation of A. senegal and A. seyal in the framework of reafforestation programs requires a preliminary step of rhizobial strain selection for both Acacia species.

Genetic diversity patterns and functional traits of Bradyrhizobium strains associated with Pterocarpus officinalis Jacq. in Caribbean islands and Amazonian forest (French Guiana).

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

[Native plant resources to optimize the performances of forest rehabilitation in Mediterranean and tropical environment: some examples of nursing plant species that improve the soil mycorrhizal potential]. / Des ressources végétales endémiques pour optimiser durablement les opérations de réhabilitation du couvert forestier en milieu méditerranéen et tropical : exemple des plantes facilitatrices vectrices de propagation des champignons mycorhiziens.

The overexploitation of natural resources, resulting in an increased need for arable lands by local populations, causes a serious dysfunction in the soil's biological functioning (mineral deficiency, salt stress, etc.). This dysfunction, worsened by the climatic conditions (drought), requires the implementation of ecological engineering strategies allowing the rehabilitation of degraded areas through the restoration of essential ecological services. The first symptoms of weathering processes of soil quality in tropical and Mediterranean environments result in an alteration of the plant cover structure with, in particular, the pauperization of plant species diversity and abundance. This degradation is accompanied by a weakening of soils and an increase of the impact of erosion on the surface layer resulting in reduced fertility of soils in terms of their physicochemical characteristics as well as their biological ones (e.g., soil microbes). Among the microbial components particularly sensitive to erosion, symbiotic microorganisms (rhizobia, Frankia, mycorrhizal fungi) are known to be key components in the main terrestrial biogeochemical cycles (C, N and P). Many studies have shown the importance of the management of these symbiotic microorganisms in rehabilitation and revegetation strategies of degraded environments, but also in improving the productivity of agrosystems. In particular, the selection of symbionts and their inoculation into the soil were strongly encouraged in recent decades. These inoculants were selected not only for their impact on the plant, but also for their ability to persist in the soil at the expense of the residual native microflora. The performance of this technique was thus evaluated on the plant cover, but its impact on soil microbial characteristics was totally ignored. The role of microbial diversity on productivity and stability (resistance, resilience, etc.) of eco- and agrosystems has been identified relatively recently and has led to a questioning of the conceptual bases of controlled inoculation in sustainable land management. It has been suggested that the environmental characteristics of the area to rehabilitate should be taken into account, and more particularly its degradation level in relation to the threshold of ecological resilience. This consideration should lead to the optimization of the cultural practices to either (i) restore the original properties of an ecosystem in case of slightly degraded environments or (ii) transform an ecosystem in case of highly degraded soils (e.g., mine soils). In this chapter, we discuss, through various examples of experiments conducted in tropical and Mediterranean areas, the performance of different strategies to manage the microbial potential in soils (inoculation of exotic vs. native species, inoculation or controlled management potential microbial stratum via aboveground vegetation, etc.) based on the level of environmental degradation.

In vitro propagation of Acacia mangium and A. mangium × A. auriculiformis.

Acacia mangium and A. mangium × A. auriculiformis hybrids have gained an increasing interest in reafforestation programs under the humid tropical conditions, mainly for pulpwood production. This is due to their impressive growth on acid and degraded soils, as well as their capability to restore soil fertility thanks to their natural nitrogen-fixing ability. It is crucial to develop efficient methods for improving the genetic quality and the mass production of the planting stocks of these species. In this regard, in vitro micropropagation is well suited to overcome the limitations of more conventional techniques for mass propagating vegetatively selected juvenile, mature, or even transgenic genotypes. Micropropagation of A. mangium either from seeds or from explants collected from outdoors is initiated on Murashige and Skoog (MS) basal medium supplemented with 4.4 µM BA. Microshoot cultures produced by axillary budding are further developed and maintained by regular subcultures every 60 days onto fresh MS culture medium added with 2.2 µM BA + 0.1 µM NAA. This procedure enhances the organogenic capacity for shoot multiplication by axillary budding, with average multiplication rates of 3-5 every 2 months, as well as for adventitious rooting. The rooting is initiated on Schenk and Hildebrandt culture medium containing 4 µM IAA. The maintenance of shoot cultures in total darkness for 3 weeks increases the rooting rates reaching more than 70%. The hybrid A. mangium × A. auriculiformis genotypes are subcultured at 2-month intervals with an average multiplication rate of 3 and rooting rates of 95-100% on a half-strength MS basal medium containing 1.1 µM NAA. The rooted microshoots are transferred to ex vitro controlled conditions for acclimatization and further growth, prior to transfer to the field, or use as stock plants for cost-effective and true-to-type mass production by rooted cuttings.

Symbiotic characterization and diversity of rhizobia associated with native and introduced acacias in arid and semi-arid regions in Algeria.

The diversity of rhizobia associated with introduced and native Acacia species in Algeria was investigated from soil samples collected across seven districts distributed in arid and semi-arid zones. The in vitro tolerances of rhizobial strains to NaCl and high temperature in pure culture varied greatly regardless of their geographical and host plant origins but were not correlated with the corresponding edaphoclimatic characteristics of the sampling sites, as clearly demonstrated by principal component analysis. Based on 16S rRNA gene sequence comparisons, the 48 new strains isolated were ranked into 10 phylogenetic groups representing five bacterial genera, namely, Ensifer, Mesorhizobium, Rhizobium, Bradyrhizobium, and Ochrobactrum. Acacia saligna, an introduced species, appeared as the most promiscuous host because it was efficiently nodulated with the widest diversity of rhizobia taxa including both fast-growing ones, Rhizobium, Ensifer, and Mesorhizobium, and slow-growing Bradyrhizobium. The five other Acacia species studied were associated with fast-growing bacterial taxa exclusively. No difference in efficiency was found between bacterial taxa isolated from a given Acacia species. The tolerances of strains to salinity and temperature remains to be tested in symbiosis with their host plants to select the most adapted Acacia sp.-LNB taxa associations for further revegetation programs.

Response of native soil microbial functions to the controlled mycorrhization of an exotic tree legume, Acacia holosericea in a Sahelian ecosystem.

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.

Evaluating the nodulation status of leguminous species from the Amazonian forest of Brazil.

Numerous leguminous species are used or have potential uses for timber production, pharmacological products, or land reclamation. Through N(2)-fixation, many leguminous trees contribute to the N-balance of tropical wetlands and rainforests. Therefore, studies of the N(2)-fixation ability of leguminous species appear to be crucial for the better use and conservation of these resources. The global nodulation inventory in the Leguminosae family is constantly being enriched with new records, suggesting the existence of undiscovered nodulated species, especially in tropical natural ecosystems and other hot spots of biodiversity. In this respect, the nodulation of leguminous species from the Amazonian forest of Porto Trombetas (Brazil) was surveyed. Overall, 199 leguminous species from flooded and non-flooded areas, were examined for their nodulation status by combining field observations, seedling inoculations, and screening of N(2)-fixing bacterial strains from the collected nodules. The results revealed a tendency for a higher relative frequency of nodulation in the species from the flooded areas (74%) compared with those from the non-flooded areas (67%). Nodulation was observed in the Caesalpinioideae, Mimosoideae, and Papilionoideae, with 25, 88, and 84% of the examined species in each subfamily, respectively. Of the 137 nodulated leguminous species, 32 including three Caesalpinoideae, 19 Mimosoideae, and 10 Papilionoideae are new records. One new nodulated genus (Cymbosema) was found in the Papilionoideae. Twelve non-nodulating leguminous species were also observed for the first time. The results are discussed based on the systematics of the Leguminosae family and the influence of available nutrients to the legume-bacteria symbiosis.

Multi-host ectomycorrhizal fungi are predominant in a Guinean tropical rainforest and shared between canopy trees and seedlings.

The diversity of ectomycorrhizal (ECM) fungi on adult trees and seedlings of five species, Anthonotha fragrans, Anthonotha macrophylla, Cryptosepalum tetraphyllum, Paramacrolobium coeruleum and Uapaca esculenta, was determined in a tropical rain forest of Guinea. Ectomycorrhizae were sampled within a surface area of 1600 m(2), and fungal taxa were identified by sequencing the rDNA Internal Transcribed Spacer region. Thirty-nine ECM fungal taxa were determined, of which 19 multi-hosts, 9 single-hosts and 11 singletons. The multi-host fungi represented 92% (89% when including the singletons in the analysis) of the total abundance. Except for A. fragrans, the adults of the host species displayed significant differentiation for their fungal communities, but their seedlings harboured a similar fungal community. These findings suggest that there was a potential for the formation of common mycorrhizal networks in close vicinity. However, no significant difference was detected for the δ(13)C and δ(15)N values between seedlings and adults of each ECM plant, and no ECM species exhibited signatures of mixotrophy. Our results revealed (i) variation in ECM fungal diversity according to the seedling versus adult development stage of trees and (ii) low host specificity of ECM fungi, and indicated that multi-host fungi are more abundant than single-host fungi in this forest stand.

Bradyrhizobia nodulating the Acacia mangium x A. auriculiformis interspecific hybrid are specific and differ from those associated with both parental species.

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

Arbuscular mycorrhizal symbiosis can counterbalance the negative influence of the exotic tree species Eucalyptus camaldulensis on the structure and functioning of soil microbial communities in a sahelian soil.

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.

Arbuscular mycorrhizas and ectomycorrhizas of Uapaca bojeri L. (Euphorbiaceae): sporophore diversity, patterns of root colonization, and effects on seedling growth and soil microbial catabolic diversity.

The main objectives of this study were (1) to describe the diversity of mycorrhizal fungal communities associated with Uapaca bojeri, an endemic Euphorbiaceae of Madagascar, and (2) to determine the potential benefits of inoculation with mycorrhizal fungi [ectomycorrhizal and/or arbuscular mycorrhizal (AM) fungi] on the growth of this tree species and on the functional diversity of soil microflora. Ninety-four sporophores were collected from three survey sites. They were identified as belonging to the ectomycorrhizal genera Afroboletus, Amanita, Boletus, Cantharellus, Lactarius, Leccinum, Rubinoboletus, Scleroderma, Tricholoma, and Xerocomus. Russula was the most frequent ectomycorrhizal genus recorded under U. bojeri. AM structures (vesicles and hyphae) were detected from the roots in all surveyed sites. In addition, this study showed that this tree species is highly dependent on both types of mycorrhiza, and controlled ectomycorrhization of this Uapaca species strongly influences soil microbial catabolic diversity. These results showed that the complex symbiotic status of U. bojeri could be managed to optimize its development in degraded areas. The use of selected mycorrhizal fungi such the Scleroderma Sc1 isolate in nursery conditions could be of great interest as (1) this fungal strain is very competitive against native symbiotic microflora, and (2) the fungal inoculation improves the catabolic potentialities of the soil microflora.

Role of methylotrophy during symbiosis between Methylobacterium nodulans and Crotalaria podocarpa.

Some rare leguminous plants of the genus Crotalaria are specifically nodulated by the methylotrophic bacterium Methylobacterium nodulans. In this study, the expression and role of bacterial methylotrophy were investigated during symbiosis between M. nodulans, strain ORS 2060T, and its host legume, Crotalaria podocarpa. Using lacZ fusion to the mxaF gene, we showed that the methylotroph genes are expressed in the root nodules, suggesting methylotrophic activity during symbiosis. In addition, loss of the bacterial methylotrophic function significantly affected plant development. Indeed, inoculation of M. nodulans nonmethylotroph mutants in C. podocarpa decreased the total root nodule number per plant up to 60%, decreased the whole-plant nitrogen fixation capacity up to 42%, and reduced the total dry plant biomass up to 46% compared with the wild-type strain. In contrast, inoculation of the legume C. podocarpa with nonmethylotrophic mutants complemented with functional mxa genes restored the symbiotic wild phenotype. These results demonstrate the key role of methylotrophy during symbiosis between M. nodulans and C. podocarpa.

Occurrence of nodulation in unexplored leguminous trees native to the West African tropical rainforest and inoculation response of native species useful in reforestation.

Despite the abundance and diversity of timber tree legumes in the West African rainforest, their ability to form nitrogen-fixing nodules in symbiosis with rhizobia, and their response to rhizobial inoculation, remain poorly documented. In the first part of this study the occurrence of nodulation was determined in 156 leguminous species growing in six natural forest areas in Guinea, mostly mature trees. In the second part, an in situ experiment of rhizobial inoculation was performed on eight selected tree species belonging to three genera: Albizia, Erythrophleum and Millettia. Of the 97 plant species and 14 genera that had never been examined before this study, 31 species and four genera were reported to be nodulated. After 4 months of growing in a nursery and a further 11 months after transplantation of plants to the field, we observed a highly significant (P < 0.001) and positive effect of inoculation with Bradyrhizobium sp. strains on the growth of the eight tree species tested. The importance of determining the nodulation ability of unexplored local trees and subsequently using this information for inoculation in reforestation programmes was demonstrated.