Ectomycorrhizal fungi <i>Suillus</i> accelerating the uptake and transport of Cd in the host plant <i>Pinus thunbergii</i> rather than as a barrier

The effect of ectomycorrhizal (ECM) fungi on the absorption and transport of heavy metals by host plants remains elusive. We experimentally assessed rapid cadmium (Cd) diffusion by two species of Suillus mycelium. Furthermore, we evaluated Cd absorption by ECM Pinus thunbergii Parl. and used transcriptomics to study the gene expression of P. thunbergii under Cd stress. In vitro experiments revealed that Cd2+ was transported through the apoplastic space more rapidly than through the mycelial symplast. The net Cd2+ influx rates in epitaxial hyphae were the highest, followed by those in the mantle of P. thunbergii inoculated with Suillus, whereas the lowest influx rate was found in the ECM-free fine root portions. Under Cd stress, the expression levels of PtZnTs, PtZIPs and PtHMA2 in ECM P. thunbergii roots were significantly higher than those in non-mycorrhized P. thunbergii. The assessment of Cd distribution in P. thunbergii revealed that Cd was transported to the needles of ECM P. thunbergii after 48 h; however, it was not detected in non-mycorrhized P. thunbergii. The essential element Cu exhibited similar results as the non-essential element Cd. Furthermore, two species ECM fungi Suillus accelerates the uptake and transport of Cd in the host plant P. thunbergii.