Mycorrhizal competition release and microbial dynamics in native and non-native Tuber melanosporum habitats

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Abstract
Truffles in the genus Tuber (Pezizales) are among the few ectomycorrhizal taxa successfully cultivated worldwide. Australia has recently become one of the top producers of Tuber melanosporum , a high-valued black truffle native to Europe. Truffle co-introductions in the Southern Hemisphere with their Northern Hemisphere plant symbionts are hypothesized to benefit from reduced ectomycorrhizal competition and natural enemies. In this study, we tested whether T. melanosporum in Australia experiences competition release by sampling soils and truffles across France, Spain, Italy, and Australia. Fungal and prokaryotic communities in truffle plantations were assessed in native (European) and non-native (Australian) habitats through ITS and 16S rDNA amplicon sequencing from soil and truffles. Community composition was primarily structured by site of origin and secondarily by presence of brûlé, vegetation-free area induced by truffle production of plant-growth inhibiting compounds, with significant interactions indicating site-dependent brûlé effects. European soils showed higher fungal richness outside the brûlé, with higher evenness overall and for ectomycorrhizal fungi only outside brûlé. T. melanosporum showed higher abundance in Australia, with significant differences restricted to outside the brûlé. Overall, ectomycorrhizal fungi in European soils had more than four times the taxa and higher diversity compared to Australian soils. Among the main competitors, Tomentella , Inocybe , and Trichophaea co-dominated in Europe, versus Scleroderma , Hebeloma , and Tarzetta in Australia. Despite differences in soil microbiomes, bacterial communities within T. melanosporum truffle ascocarps were strikingly similar across sites and continents and were dominated by Bradyrhizobium . Despite high site-level variation, our results support the competition release hypothesis, with reduced enemies benefiting T. melanosporum colonization outside the brûlé in Australia. IMPORTANCE: This study provides the first cross-hemisphere analysis of the truffle microbiome, comparing native and non-native soils and truffles from Europe and Australia. We demonstrate that the remarkable success of Tuber melanosporum cultivation in Australia is compatible with ecological release from competitors, which favors its development outside the brûlé. At the same time, we reveal striking cross-hemispheric similarities in truffle-associated bacterial communities, consistently dominated by Bradyrhizobium . These findings highlight both the novelty and transcontinental relevance of our work, offering new perspectives on fungal ecology and truffle cultivation.

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