Tuber melanosporum is an ectomycorrhizal (ECM) fungus from Mediterranean transitory ecosystems where ECM trees start to dominate among arbuscular-mycorrhizal (AM) shrubs and herbs (companion plants). Its presence entails the development of 'brûlés', where 2 vegetation is scarce for unknown reasons. Current T. melanosporum production comes from plantations where management often suppresses the understory vegetation, although empirical knowledge advocates a positive role of some companion plants in truffle production. This study aimed at (i) experimentally testing the reciprocal interaction between T. melanosporum and companion plants and (ii) examining T. melanosporum-mediated soil feedback involved in the dynamics of truffle ground vegetation. Methods A three-year experiment was set up with Quercus ilex associated with T. melanosporum (or not, as control), grown in association (or not, as control) with a companion plant. Six companion plant species were chosen based on different empirical criteria including those indicated by local truffle growers' knowledge. A trait-based approach was applied to plants and associated fungi (abundance of T. melanosporum and AM fungi mycelium). Results-Conclusion Companion plants promoted the development of truffle mycelium. In the presence of T. melanosporum, companion plant growth and nutrition and AM fungi abundance decreased, while the nutrition status of its host increased. The truffle inhibited germination of weed seeds. These results highlight the role of T. melanosporum in mediating plant-plant interactions, possible mechanisms underlying brûlé formation and a potential successional role for T. melanosporum.

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The nomenclatural type material of Rhizophagus intraradices (basionym Glomus intraradices) was originally described from a trap pot culture established with root fragments, subcultures of which later became registered in the INVAM culture collection as FL 208. Subcultures of FL 208 (designated as strain ATT 4) and a new strain, independently isolated from the type location (ATT 1102), were established as both pot cultures with soil-like substrate and in vitro root organ culture. Long-term sampling of these cultures shows spores of the species to have considerable morphological plasticity, not described in the original description. Size, shape and other features of the spores were much more variable than indicated in the protologue. Phylogenetic analyses confirmed earlier published evidence that sequences from all R. intraradices cultures formed a monophyletic clade, well separated from, and not representing a sister clade to, R. irregularis. Moreover, new phylogenetic analyses show that Rhizoglomus venetianum and R. irregularis are synonymous. The morphological characters used to separate these species exemplify the difficulties in species recognition due to the high phenotypic plasticity in the genus Rhizophagus. Rhizophagus intraradices is morphologically re-described, an epitype is designated from a single-spore isolate derived from ATT 4, and R. venetianum is synonymised with R. irregularis.

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.

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.

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We investigated the suitability of Acacia spirorbis Labill., a tropical ectomycorrhizal (ECM) tree, as a nurse plant to improve the growth of Tristaniopsis calobuxus Brongn. & Gris seedlings for the restoration of nickel mines in New Caledonia. Rehabilitation of nickel mines in New Caledonia is a major concern. In such harsh soil conditions, ectomycorrhizal (ECM) symbiosis is important for tree growth, survival, and resistance. To improve ecological restoration in New Caledonia, new technical itineraries have undergone experimentation using ECM as a plant nurse, allowing ECM saplings to rapidly acquire a wide range of ECM fungi. We transplanted ECM seedlings of Tristaniopsis calobuxus from the nursery to bare ferralitic soils harbouring some scattered 12-year-old Acacia spirorbis to be used as ECM nurse plants. Using molecular characterisation of ITS rDNA, we characterised ECM fungal communities of A. spirorbis and of T. calobuxus saplings at transplanting time and 13 months later. We observed changes in the composition of fungal communities of T. calobuxus with an increase in diversity, notably the appearance of operational taxonomic units (OTUs) affiliated with /russulaceae, /boletus and /pisolithus-scleroderma and a decrease in ubiquitous nursery orders such as /sebacinales and /helotiales. We also observed a higher number of shared OTUs between T. calobuxus and A. spirorbis. The vicinity of A. spirorbis enabled diversification and adaptation of the T. calobuxus ECM fungal community. These results led us to recommend A. spirorbis as a good nurse tree candidate in the framework of ecological restoration of mine sites.