Transporter gene family evolution in ectomycorrhizal fungi in relation to mineral weathering capabilities

Abstract. The role of ectomycorrhizal (ECM) fungi in biological mineral weathering is increasingly recognised, although the quantitative significance of microbially mediated mineral dissolution for plant growth is debated. Species within the ECM genus Suillus are found to preferentially inhabit mineral soils and are frequently reported to possess mineral weathering capabilities. Though studies growing ECM fungi with minerals have shown heightened mycelial nutrient content compared to growth without minerals, the mechanistic understanding of nutrient mobilisation and uptake associated with weathering remains largely unknown. We examined copy numbers of 173 transporter gene families present in 108 Agaricomycetes species, and analysed evolutionary expansions and contractions of base cation transporter gene families across the phylogeny. We also quantified mycelial base cation uptake by ECM species in the genera Suillus and Piloderma, and two saprotrophic fungi, when grown in pure culture with and without minerals. We hypothesised that 1) greater base cation uptake is dependent on evolutionary expansions in copy-numbers of base cation transporter genes, 2) mineral weathering results in base cation uptake by Suillus growing in pure culture with mineral additions, 3) base cation uptake by Suillus growing in pure culture with mineral additions will be greater compared to other fungal species, and 4) base cation uptake will correlate with base cation transporter gene family copy numbers. We showed that 25 transporter gene families are significantly expanding and contracting in the genus Suillus, 10 of which correspond to base cation transporter families and two of which are accessories to base cation transport, highlighting the importance of base cation uptake and transport in the life strategy of Suillus species. Additionally, there are significant expansions in the Fungal Mating-type Pheromone Receptor (MAT-PR) Family in Suillus species, suggesting that these species are adapting to their environment. For all elements there were examples of higher mycelial concentrations after growth in pure culture with mineral additions as compared to the nutrient limited treatment for Suillus species but also for some Piloderma species, confirming that mineral weathering resulted in base cation uptake in pure culture. For 40 % of the significantly expanding and/or contracting base cation gene transporters families, copy numbers were significantly correlated with uptake of mineral elements, and most of the significant correlations were positive. This suggested that members of the genus Suillus are rapidly evolving, and that the expansions and/or contractions in these transporter families are likely to be related to the requirement of base cation uptake, underpinned by the finding that mycelial uptake of elements in the presence of minerals often increased with base cation transporter family copy numbers. Further work into other levels of regulation, e.g. through transcriptional regulation of transporter proteins, would be useful to gain a deeper understanding of base cation uptake regulation and its role in mineral weathering by Suillus species, and ECM fungi in general.