Inter- and intra-specific metal tolerance variation in ectomycorrhizal fungal Suillus species

Soil metal contamination can affect growth, metabolism, and reproduction of organisms, and can lead to death. However, some fungi have evolved metal tolerance and are able to live in contaminated soils. Species in the ectomycorrhizal genus Suillus from Europe and Asia display variation in metal tolerance, yet it is unknown whether this is a widespread trait in the genus and whether it occurs in North America. Here we investigate cadmium (Cd) and zinc (Zn) tolerance in S. brevipes and S. tomentosus isolates collected from sites in the Rocky Mountains of Colorado displaying different metal content. In line with previous findings for other Suillus species, we hypothesized (1) S. brevipes and S. tomentosus to display intra-specific metal tolerance variation, (2) Zn and Cd tolerance to be correlated to soil metal content, and (3) tolerant isolates to show lower metal tissue content compared to sensitive isolates (due to increased metal exclusion). We found ample intra- and inter-specific Zn and Cd tolerance variation in both S. brevipes and S. tomentosus, but no correlation between soil metal content and tolerance. There was a negative correlation between tolerance level and Zn uptake, indicating an exclusion-based Zn tolerance strategy. Sensitive and tolerant isolates showed no difference in Cd accumulation, indicating that Cd tolerance in these species is likely not dependent on exclusion. Our study sets the groundwork for further investigation into the genetic basis of Suillus metal tolerance and whether and how it impacts pine mycorrhizal partners.