Abdala G. Diédhiou

Towards global patterns in the diversity and community structure of ectomycorrhizal fungi

Global species richness patterns of soil micro-organisms remain poorly understood compared to macro-organisms. We use a global analysis to disentangle the global determinants of diversity and community composition for ectomycorrhizal (EcM) fungi-microbial symbionts that play key roles in plant nutrition in most temperate and many tropical forest ecosystems. Host plant family has the strongest effect on the phylogenetic community composition of fungi, whereas temperature and precipitation mostly affect EcM fungal richness that peaks in the temperate and boreal forest biomes, contrasting with latitudinal patterns of macro-organisms. Tropical ecosystems experience rapid turnover of organic material and have weak soil stratification, suggesting that poor habitat conditions may contribute to the relatively low richness of EcM fungi, and perhaps other soil biota, in most tropical ecosystems. For EcM fungi, greater evolutionary age and larger total area of EcM host vegetation may also contribute to the higher diversity in temperate ecosystems. Our results provide useful biogeographic and ecological hypotheses for explaining the distribution of fungi that remain to be tested by involving next-generation sequencing techniques and relevant soil metadata.

Global assessment of arbuscular mycorrhizal fungus diversity reveals very low endemism

Cosmopolitan plant root symbionts The aboveground lives of plants are only sustainable because of the symbiotic soil fungi that encase their roots. These fungi swap nutrients with plants, defend them from attack, and help them withstand abrupt environmental changes. Out of necessity, fungal symbionts in the soil would appear to be restricted and local to certain plant species. Davison et al., however, discovered that some taxa are globally distributed. How these underground fungi have dispersed so widely remains a mystery; perhaps human farmers have had something to do with it. Science, this issue p. 970 The wide distribution of plant-root fungal symbionts seems to be driven by recent dispersal rather than ancient tectonics. The global biogeography of microorganisms remains largely unknown, in contrast to the well-studied diversity patterns of macroorganisms. We used arbuscular mycorrhizal (AM) fungus DNA from 1014 plant-root samples collected worldwide to determine the global distribution of these plant symbionts. We found that AM fungal communities reflected local environmental conditions and the spatial distance between sites. However, despite AM fungi apparently possessing limited dispersal ability, we found 93% of taxa on multiple continents and 34% on all six continents surveyed. This contrasts with the high spatial turnover of other fungal taxa and with the endemism displayed by plants at the global scale. We suggest that the biogeography of AM fungi is driven by unexpectedly efficient dispersal, probably via both abiotic and biotic vectors, including humans.

The distance decay of similarity in communities of ectomycorrhizal fungi in different ecosystems and scales

Summary 1. Despite recent advances in understanding community ecology of ectomycorrhizal fungi, little is known about their spatial patterning and the underlying mechanisms driving these patterns across different ecosystems. 2. This meta-study aimed to elucidate the scale, rate and causes of spatial structure of ectomycorrhizal fungal communities in different ecosystems by analysing 16 and 55 sites at the local and global scales, respectively. We examined the distance decay of similarity relationship in species- and phylogenetic lineage-based communities in relation to sampling and environmental variables. 3. Tropical ectomycorrhizal fungal communities exhibited stronger distance-decay patterns compared to non-tropical communities. Distance from the equator and sampling area were the main determinants of the extent of distance decay in fungal communities. The rate of distance decay was negatively related to host density at the local scale. At the global scale, lineage-level community similarity decayed faster with latitude than with longitude. 4. Synthesis. Spatial processes play a stronger role and over a greater scale in structuring local communities of ectomycorrhizal fungi than previously anticipated, particularly in ecosystems with greater vegetation age and closer to the equator. Greater rate of distance decay occurs in ecosystems with lower host density that may stem from increasing dispersal and establishment limitation. The relatively strong latitude effect on distance decay of lineage-level community similarity suggests that climate affects large-scale spatial processes and may cause phylogenetic clustering of ectomycorrhizal fungi at the global scale.

Ectomycorrhizal symbiosis of tropical African trees

The diversity, ecology and function of ectomycorrhizal (EM) fungi and ectomycorrhizas (ECMs) on tropical African tree species are reviewed here. While ECMs are the most frequent mycorrhizal type in temperate and boreal forests, they concern an economically and ecologically important minority of plants in African tropical forests. In these African tropical forests, ECMs are found mainly on caesalpionioid legumes, Sarcolaenaceae, Dipterocarpaceae, Asterpeiaceae, Phyllantaceae, Sapotaceae, Papilionoideae, Gnetaceae and Proteaceae, and distributed in open, gallery and rainforests of the Guineo-Congolian basin, Zambezian Miombo woodlands of East and South-Central Africa and Sudanian savannah woodlands of the sub-sahara. Overall, EM status was confirmed in 93 (26%) among 354 tree species belonging to EM genera. In addition, 195 fungal taxa were identified using morphological descriptions and sequencing of the ML5/ML6 fragment of sporocarps and ECMs from West Africa. Analyses of the belowground EM fungal communities mostly based on fungal internal transcribed spacer sequences of ECMs from Continental Africa, Madagascar and the Seychelles also revealed more than 350 putative species of EM fungi belonging mainly to 18 phylogenetic lineages. As in temperate forests, the /russula–lactarius and /tomentella–thelephora lineages dominated EM fungal flora in tropical Africa. A low level of host preference and dominance of multi-host fungal taxa on different African adult tree species and their seedlings were revealed, suggesting a potential for the formation of common ectomycorrhizal networks. Moreover, the EM inoculum potential in terms of types and density of propagules (spores, sclerotia, EM root fragments and fragments of mycelia strands) in the soil allowed opportunistic root colonisation as well as long-term survival in the soil during the dry season. These are important characteristics when choosing an EM fungus for field application. In this respect, Thelephoroid fungal sp. XM002, an efficient and competitive broad host range EM fungus, possessed these characteristics and appeared to be a good candidate for artificial inoculation of Caesalps and Phyllanthaceae seedlings in nurseries. However, further efforts should be made to assess the genetic and functional diversity of African EM fungi as well as the EM status of unstudied plant species and to strengthen the use of efficient and competitive EM fungi to improve production of ecologically and economically important African multipurpose trees in plantations.

Pairwise Transcriptomic Analysis of the Interactions Between the Ectomycorrhizal Fungus Laccaria bicolor S238N and Three Beneficial, Neutral and Antagonistic Soil Bacteria

Ectomycorrhizal fungi are surrounded by bacterial communities with which they interact physically and metabolically during their life cycle. These bacteria can have positive or negative effects on the formation and the functioning of ectomycorrhizae. However, relatively little is known about the mechanisms by which ectomycorrhizal fungi and associated bacteria interact. To understand how ectomycorrhizal fungi perceive their biotic environment and the mechanisms supporting interactions between ectomycorrhizal fungi and soil bacteria, we analysed the pairwise transcriptomic responses of the ectomycorrhizal fungus Laccaria bicolor (Basidiomycota: Agaricales) when confronted with beneficial, neutral or detrimental soil bacteria. Comparative analyses of the three transcriptomes indicated that the fungus reacted differently to each bacterial strain. Similarly, each bacterial strain produced a specific and distinct response to the presence of the fungus. Despite these differences in responses observed at the gene level, we found common classes of genes linked to cell–cell interaction, stress response and metabolic processes to be involved in the interaction of the four microorganisms.

Field Trials Reveal Ecotype-Specific Responses to Mycorrhizal Inoculation in Rice.

The overuse of agricultural chemicals such as fertilizer and pesticides aimed at increasing crop yield results in environmental damage, particularly in the Sahelian zone where soils are fragile. Crop inoculation with beneficial soil microbes appears as a good alternative for reducing agricultural chemical needs, especially for small farmers. This, however, requires selecting optimal combinations of crop varieties and beneficial microbes tested in field conditions. In this study, we investigated the response of rice plants to inoculation with arbuscular mycorrhizal fungi (AMF) and plant growth promoting bacteria (PGPB) under screenhouse and field conditions in two consecutive seasons in Senegal. Evaluation of single and mixed inoculations with AMF and PGPB was conducted on rice (Oryza sativa) variety Sahel 202, on sterile soil under screenhouse conditions. We observed that inoculated plants, especially plants treated with AMF, grew taller, matured earlier and had higher grain yield than the non-inoculated plants. Mixed inoculation trials with two AMF strains were then conducted under irrigated field conditions with four O. sativa varieties, two O. glaberrima varieties and two interspecific NERICA varieties, belonging to 3 ecotypes (upland, irrigated, and rainfed lowland). We observed that the upland varieties had the best responses to inoculation, especially with regards to grain yield, harvest index and spikelet fertility. These results show the potential of using AMF to improve rice production with less chemical fertilizers and present new opportunities for the genetic improvement in rice to transfer the ability of forming beneficial rice-microbe associations into high yielding varieties in order to increase further rice yield potentials.

A pantropically introduced tree is followed by specific ectomycorrhizal symbionts due to pseudo-vertical transmission

Global trade increases plant introductions, but joint introduction of associated microbes is overlooked. We analyzed the ectomycorrhizal fungi of a Caribbean beach tree, seagrape (Coccoloba uvifera, Polygonacaeae), introduced pantropically to stabilize coastal soils and produce edible fruits. Seagrape displays a limited symbiont diversity in the Caribbean. In five regions of introduction (Brazil, Japan, Malaysia, Réunion and Senegal), molecular barcoding showed that seagrape mostly or exclusively associates with Scleroderma species (Basidiomycota) that were hitherto only known from Caribbean seagrape stands. An unknown Scleroderma species dominates in Brazil, Japan and Malaysia, while Scleroderma bermudense exclusively occurs in Réunion and Senegal. Population genetics analysis of S. bermudense did not detect any demographic bottleneck associated with a possible founder effect, but fungal populations from regions where seagrape is introduced are little differentiated from the Caribbean ones, separated by thousands of kilometers, consistently with relatively recent introduction. Moreover, dry seagrape fruits carry Scleroderma spores, probably because, when drying on beach sand, they aggregate spores from the spore bank accumulated by semi-hypogeous Scleroderma sporocarps. Aggregated spores inoculate seedlings, and their abundance may limit the founder effect after seagrape introduction. This rare pseudo-vertical transmission of mycorrhizal fungi likely contributed to efficient and repeated seagrape/Scleroderma co-introductions.

International Workshop of the African Network on Mycorrhiza (AFRINOM 1): advancing plant–microbe interactions in crop nutrition, integration of mycorrhiza into agroecosystems

The African Network on Mycorrhiza (AFRINOM, http:// senegal.ird.fr/afrinom) was created following an international workshop on “Mycorrhiza: a biological tool for sustainable development in Africa,” held in Dakar, Senegal, in 2011 (Bâ et al. Mycorrhiza 2011). The general objectives of AFRINOM are to strengthen research, encourage cooperation, and promote exchange of information on mycorrhiza in Africa. The network, which consists of 135 members from 31 countries, encourages the development of joint research activities between researchers in Africa and Europe, America, Oceania, and Asia. It also gathers data and published information on mycorrhiza in Africa and provides information about laboratories working on mycorrhiza in Africa and a database on cultures of ectomycorrhizal and arbuscular mycorrhizal fungi from Africa and abroad. The International Workshop of AFRINOM 1 on “Advancing plant–microbe interactions in crop nutrition, integration of mycorrhiza into agroecosystems” was organized in Nairobi, Kenya, October 22–26, 2012, as part of the ISFMAfrica 2012 conference on “Integrated Soil Fertility Management in Africa: from microbes to markets.” Thirty-one participants from 13 countries attended the workshop, which included oral presentations, poster sessions, and excursion. Abstract of all talks and posters can be found on the AFRINOM website. The workshop objectives were to analyze innovative mycorrhizal interventions (e.g., production of mycorrhizal inoculants) and their impact in land management strategies benefiting smallscale agriculture in Africa. The contents addressed included the following: integration of mycorrhiza into agrosystems, optimization of mycorrhizal interventions and adaptation, mycorrhiza in ecosystems management, alternative interventions and bioprospecting tools, and strain selection and delivery. In the plenary session, Prof. Joanna Dames (South Africa) introduced challenges and future prospects of mycorrhizal agrotechnology for Africa and raised a fundamental question: why have we not embraced mycorrhizal technology and adopted it as standard agricultural practice in Africa? In her talk, she covered the many constraints linked to production of mycorrhizal inoculants, i.e., mass production, formulation, quality control, maintenance of culture collections, mycorrhizal fungal species/plant host matching, persistence among native strains, cost of the product for small-scale farmers, and farmer perception. In a session on integration ofmycorrhiza into agroecosystems, H. Dupre de Boulois (Belgium) presented the potentials and advantages of monoxenic AM fungal cultures, and concluded that present knowledge is inadequate to proposemass production of mycorrhizal inoculants by this method. Amadou Bâ (Senegal) presented results on the domestication of different provenances J. M. Jefwa CIAT, Nairobi, Kenya

An international workshop: mycorrhiza, a biological tool for sustainable development in Africa

Althoughmycorrhizal symbiosis are well known on European, North American, South American, Southeast Asian, and Australian plant species from natural or anthropogenic ecosystems, data from Africa are still scarce and neglected. The international workshop on “Mycorrhiza: a biological tool for sustainable development in Africa” held in Dakar, Senegal, February 21–23, 2011, was therefore particularly timely. Eighty-seven participants from 23 countries attended theworkshop, which included oral presentations, poster sessions, excursion and many free-ranging discussions. Abstracts of all the talks and posters, and details of the participants can be found on the website http://www.ird.sn/web-mycorhizes2011/. The aim of the workshop was to present state of the knowledge and perspectives in mycorrhizal research on tropical and Mediterranean plant species in Africa. The contents addressed diversity and function of mycorrhiza associated with forest and agricultural plants, in Mediterranean and tropical, wet and seasonally dry ecosystems. Impacts of mycorrhiza on plant diversity and composition, regeneration and dynamics of ecosystems, and biomass production in forestry and agriculture, were topics for oral presentations and poster sessions. The workshop was also a pretext to put in concrete form the idea of creating of a scientific network that connects researchers working with mycorrhiza in Africa. In the opening lecture, A. Fortin (Canada) introduced mycorrhiza and their role in the evolution and functioning of plants and terrestrial ecosystems. Although symbiotic systems have provided a major impetus to plant speciation and to the development of soils and therefore to terrestrial ecosystems, all concepts at the base of modern agriculture have been developed and applied as if mycorrhiza did not exist. A. Fortin invited scientists working in the field ofmycorrhiza in tropical and Mediterranean ecosystems, political deciders, and partners for development to exchange experience and to set up new strategies on mycorrhizal research in Africa. There was a large discussion about the use of mycorrhiza in agriculture and forestry in Africa. A. Bâ (Senegal) opened a session on diversity and function of ectomycorrhizal (ECM) symbioses, illustrating that Russulales and Thelephorales dominate aboveand belowground ECM fungal communities in open and rain forests in tropical Africa. Following presentations covered the role of ECM in the regeneration and functioning of tropical African forests, their potential use for ecological restoration of sites contaminated by heavy metals and the rich occurrence of edible ECM fungi in these regions (N. Onguene Awana, Cameroon; G. Kudzo Atsu, Togo; K. Sanon, Burkina Faso; M. Ducousso, France). There followed much debate about the importance of ECM fungi in forest diversity, productivity, resilience, and restoration. V. Gianinazzi-Pearson (France) introduced a session on diversity and function of arbuscular mycorrhiza (AM) by emphasizing the ecosystem services they provide to plant productivity and survival, and the necessity, for their efficient exploitation in plant production systems, to reduce chemical inputs, evaluate in situ fungal potential and plant responsiveness A. Bâ (*) IRD/LCM/LSTM, BP. 1386, Dakar, Senegal e-mail: amadou.ba@ird.fr