Marc Ducousso

Ectomycorrhizal Pisolithus albus inoculation of Acacia spirorbis and Eucalyptus globulus grown in ultramafic topsoil enhances plant growth and mineral nutrition while limits metal uptake

Ectomycorrhizal fungi (ECM) isolates of Pisolithus albus (Cooke and Massee) from nickel-rich ultramafic topsoils in New Caledonia were inoculated onto Acacia spirorbis Labill. (an endemic Fabaceae) and Eucalyptus globulus Labill. (used as a Myrtaceae plant host model). The aim of the study was to analyze the growth of symbiotic ECM plants growing on the ultramafic substrate that is characterized by high and toxic metal concentrations i.e. Co, Cr, Fe, Mn and Ni, deficient concentrations of plant essential nutrients such as N, P, K, and that presents an unbalanced Ca/Mg ratio (1/19). ECM inoculation was successful with a plant level of root mycorrhization up to 6.7%. ECM symbiosis enhanced plant growth as indicated by significant increases in shoot and root biomass. Presence of ECM enhanced uptake of major elements that are deficient in ultramafic substrates; in particular P, K and Ca. On the contrary, the ECM symbioses strongly reduced transfer to plants of element in excess in soils; in particular all metals. ECM-inoculated plants released metal complexing molecules as free thiols and oxalic acid mostly at lower concentrations than in controls. Data showed that ECM symbiosis helped plant growth by supplying uptake of deficient elements while acting as a protective barrier to toxic metals, in particular for plants growing on ultramafic substrate with extreme soil conditions. Isolation of indigenous and stress-adapted beneficial ECM fungi could serve as a potential tool for inoculation of ECM endemic plants for the successful restoration of ultramafic ecosystems degraded by mining activities.

Tracking nickel-adaptive biomarkers in Pisolithus albus from New Caledonia using a transcriptomic approach

The fungus Pisolithus albus forms ectomycorrhizal (ECM) associations with plants growing on extreme ultramafic soils, which are naturally rich in heavy metals such as nickel. Both nickel‐tolerant and nickel‐sensitive isolates of P. albus are found in ultramafic soils in New Caledonia, a biodiversity hotspot in the Southwest Pacific. The aim of this work was to monitor the expression of genes involved in the specific molecular response to nickel in a nickel‐tolerant P. albus isolate. We used pyrosequencing and quantitative polymerase chain reaction (qPCR) approaches to investigate and compare the transcriptomes of the nickel‐tolerant isolate MD06‐337 in the presence and absence of nickel. A total of 1u2003071u2003375 sequencing reads were assembled to infer expression patterns of 19u2003518 putative genes. Comparison of expression levels revealed that 30% of the identified genes were modulated by nickel treatment. The genes, for which expression was induced most markedly by nickel, encoded products that were putatively involved in a variety of biological functions, such as the modification of cellular components (53%), regulation of biological processes (27%) and molecular functions (20%). The 10 genes that pyrosequencing analysis indicated were induced the most by nickel were characterized further by qPCR analysis of both nickel‐tolerant and nickel‐sensitive P. albus isolates. Five of these genes were expressed exclusively in nickel‐tolerant isolates as well as in ECM samples in situ, which identified them as potential biomarkers for nickel tolerance in this species. These results clearly suggest a positive transcriptomic response of the fungus to nickel‐rich environments. The presence of both nickel‐tolerant and nickel‐sensitive fungal phenotypes in ultramafic soils might reflect environment‐dependent phenotypic responses to variations in the effective concentrations of nickel in heterogeneous ultramafic habitats.

Distribution patterns of microbial communities in ultramafic landscape: a metagenetic approach highlights the strong relationships between diversity and environmental traits

Microbial species richness and assemblages across ultramafic ecosystems were investigated to assess the relationship between their distributional patterns and environmental traits. The structure of microorganism communities in the Koniambo massif, New Caledonia, was investigated using a metagenetic approach correlated with edaphic and floristic factors. Vegetation cover and soil properties significantly shaped the large phylogenetic distribution of operational taxonomic unit within microbial populations, with a mean per habitat of 3.477 (±317) for bacteria and 712 (±43) for fungi. Using variance partitioning, we showed that the effect of aboveground vegetation was the most significant descriptor for both bacterial and fungal communities. The floristic significant predictors explained 43% of the variation for both the bacterial and fungal community structures, while the edaphic significant predictors explained only 32% and 31% of these variations, respectively. These results confirm the previous hypothesis that the distribution of microorganisms was more structured by the vegetation cover rather than the edaphic characteristics and that microbial diversity is not limited in ultramafic ecosystems.

Evidence of nickel (Ni) efflux in Ni‐tolerant ectomycorhizal Pisolithus albus isolated from ultramafic soil

Nickel (Ni)-tolerant ectomycorrhizal Pisolithus albus was isolated from extreme ultramafic soils that are naturally rich in heavy metals. This study aimed to identify the specific molecular mechanisms associated with the response of P.u2009albus to nickel. In presence of high concentration of nickel, P.u2009albusu2005Ni-tolerant isolate showed a low basal accumulation of nickel in its fungal tissues and was able to perform a metal efflux mechanism. Three genes putatively involved in metal efflux were identified from the P.u2009albus transcriptome, and their overexpression was confirmed in the mycelium that was cultivated in vitro in the presence of nickel and in fungal tissues that were sampled in situ. Cloning these genes in yeast provided significant advantages in terms of nickel tolerance (+u200931% Ni EC50) and growth (+u200983% μ) compared with controls. Furthermore, nickel efflux was also detected in the transformed yeast cells. Protein sequence analysis indicated that the genes encoded a P-type-ATPase, an ABC transporter and a major facilitator superfamily permease (MFS). This study sheds light on a global mechanism of metal efflux by P.u2009albus cells that supports nickel tolerance. These specific responses to nickel might contribute to the fungal adaptation in ultramafic soil.

Rhizosphere bacteria of Costularia spp. from ultramafic soils in New Caledonia: diversity, tolerance to extreme edaphic conditions, and role in plant growth and mineral nutrition

Rhizosphere bacteria were isolated from Costularia spp., pioneer sedges from ultramafic soils in New Caledonia, which is a hotspot of biodiversity in the South Pacific. Genus identification, ability to tolerate edaphic constraints, and plant-growth-promoting (PGP) properties were analysed. We found that 10(5) colony-forming units per gram of root were dominated by Proteobacteria (69%) and comprised 21 genera, including Burkholderia (28%), Curtobacterium (15%), Bradyrhizobium (9%), Sphingomonas (8%), Rhizobium (7%), and Bacillus (5%). High proportions of bacteria tolerated many elements of the extreme edaphic conditions: 82% tolerated 100 μmol·L(-1) chromium, 70% 1 mmol·L(-1) nickel, 63% 10 mmol·L(-1) manganese, 24% 1 mmol·L(-1) cobalt, and 42% an unbalanced calcium/magnesium ratio (1/16). These strains also exhibited multiple PGP properties, including the ability to produce ammonia (65%), indole-3-acetic acid (60%), siderophores (52%), and 1-aminocyclopropane-1-carboxylate (ACC) deaminase (39%); as well as the capacity to solubilize phosphates (19%). The best-performing strains were inoculated with Sorghum sp. grown on ultramafic substrate. Three strains significantly enhanced the shoot biomass by up to 33%. The most successful strains influenced plant nutrition through the mobilization of metals in roots and a reduction of metal transfer to shoots. These results suggest a key role of these bacteria in plant growth, nutrition, and adaptation to the ultramafic constraints.

Molecular phylogeny of Pisolithus species from Moroccan forest woodlands

The phylogenetic relationships among 200 Pisolithus basidiomata collected from pine, oak, and eucalypt forests and rockrose scrubs in Morocco were investigated. Using PCR-RFLP analysis of the internal transcribed spacer (ITS) of ribosomal DNA, this collection could be divided into 5 groups, by using PCR-RFLP analysis of the internal transcribed spacer (ITS) of ribosomal DNA. The ITS of a representative basidioma of each group was sequenced, and a phylogenetic analysis was performed. The dendrogram suggests that at least five Pisolithus species are present in Morocco. Pisolithus basidiomata collected in the Pinus and Quercus forests correspond to Pisolithus arrhizus and P. species 4 as previously described by Martin and colleagues in 2002. Those collected from the eucalyptus forests, under E. gomphocephala and E. camaldulensis, correspond to P. albus and P. microcarpus. Basidiomata collected from the rockrose scrubs, under Cistus crispus, C. monspeliensis or C. salviifolius, are all identified as Pisolithus species 3. Phylogenetic analyses showed that our different Pisolithus grouped well with Pisolithus specimens from other geographical origins.

Microbial island biogeography: isolation shapes the life history characteristics but not diversity of root-symbiotic fungal communities

Island biogeography theory is one of the most influential paradigms in ecology. That island characteristics, including remoteness, can profoundly modulate biological diversity has been borne out by studies of animals and plants. By contrast, the processes influencing microbial diversity in island systems remain largely undetermined. We sequenced arbuscular mycorrhizal (AM) fungal DNA from plant roots collected on 13 islands worldwide and compared AM fungal diversity on islands with existing data from mainland sites. AM fungal communities on islands (even those >6000u2009km from the closest mainland) comprised few endemic taxa and were as diverse as mainland communities. Thus, in contrast to patterns recorded among macro-organisms, efficient dispersal appears to outweigh the effects of taxogenesis and extinction in regulating AM fungal diversity on islands. Nonetheless, AM fungal communities on more distant islands comprised a higher proportion of previously cultured and large-spored taxa, indicating that dispersal may be human-mediated or require tolerance of significant environmental stress, such as exposure to sunlight or high salinity. The processes driving large-scale patterns of microbial diversity are a key consideration for attempts to conserve and restore functioning ecosystems in this era of rapid global change.

1st International congress on mycorrhizal symbiosis: ecosystems and environment of Mediterranean area (MYCOMED).

The congress MYCOMED was held from 11 to 13 October 2010 in Marrakesh, Morocco, and organised by Prof. M. Hafidi (University Cadi Ayyad of Marrakesh) and Dr. R. Duponnois (IRD, LSTM, Montpellier). It brought together 180 participants from 22 countries with the aim to present recent results and concepts concerning the expected benefits of mycorrhizal symbiosis for the conservation of Mediterranean endemic plant diversity and for the rehabilitation of degraded soils (e.g. mining sites). Below, we present a synthesis of the scientific content and perspectives for further development in the research area. Oral presentations (53) and posters (61) were organised into three sessions devoted to: mycorrhizal symbiosis and performance of the ecoand agro systems, mycorrhizal symbiosis and rehabilitation of soils, and biotechnology of mycorrhizal inoculations and their applications. In the opening lecture, Thierry Gauquelin addressed the question What are Mediterranean ecosystems and environments? He presented the complexity, limits, and characteristics of the Mediterranean region, giving the framework of this congress. In Mediterranean countries, performances of ecoand agro systems largely depend on plant-associated microorganisms because of their role in plant adaptation to environmental stresses such as drought, high disturbance, poor soils, etc. The first session on Mycorrhizal symbiosis and performance of the ecoand agro systems was opened by a lecture from Jean Garbaye highlighting long-term consequences of human activities on diversity and functioning of ectomycorrhizal species. This was followed by a presentation illustrating the complexity of Acacia taxonomy with regard to the diversity of their microbial symbioses (nitrogen-fixing nodules, arbuscular mycorrhiza, ectomycorrhiza) and communications showing how the mycorrhizal symbiosis may influence structural and functional diversity of soil microflora, particularly mycorrhizospheric bacterial populations. A full understanding of the impact of complex interactions between soil microorganisms on ecosystem organization is still a long way off, but research is actively contributing. Another 16 talks about diverse ecosystems in Mediterranean and Sahelian regions focused on the possible role of mycorrhizae in positive effects of shrub on high-altitude tree species establishment, the importance of ectomycorrhizal fungi in plant management and adaptation to particular soil conditions, new ways of land management aimed at enhancing edible Boletus and Terfez production, arbuscular mycorrhizal inoculation of crops, mycorrhizal fungal population density D. Diouf Laboratoire Commun de Microbiologie IRD/ISRA/UCAD, Departement de Biologie Vegetale, Centre de Recherche de Bel-Air, University Cheikh Anta Diop, BP 1386, CP 18524 Dakar, Senegal e-mail: diegane.diouf@ird.sn