Ectomycorrhizal fungi play an essential role in the ecology of boreal and temperate forests through the improvement of tree mineral nutrition. Potassium (K ) is an essential nutrient for plants and is needed in high amounts. We recently demonstrated that the ectomycorrhizal fungus Hebeloma cylindrosporum improves the K nutrition of Pinus pinaster under shortage conditions. Part of the transport systems involved in K uptake by the fungus has been deciphered, while the molecular players responsible for the transfer of this cation towards the plant remain totally unknown. Analysis of the genome of H. cylindrosporum revealed the presence of three putative tandem-pore outward-rectifying K (TOK) channels that could contribute to this transfer. Here, we report the functional characterization of these three channels through two-electrode voltage-clamp experiments in oocytes and yeast complementation assays. The expression pattern and physiological role of these channels were analysed in symbiotic interaction with P. pinaster. Pine seedlings colonized by fungal transformants overexpressing two of them displayed a larger accumulation of K in shoots. This study revealed that TOK channels have distinctive properties and functions in axenic and symbiotic conditions and suggested that HcTOK2.2 is implicated in the symbiotic transfer of K from the fungus towards the plant.
Most land plants rely on root symbioses to complement or improve their mineral nutrition. Recent researches have put forward that mycorrhizal fungi efficiently absorb and transfer potassium (K) from the soil to host plant roots, but the molecular mechanisms involved are not completely elucidated yet. We have recently revealed that K is likely released from the fungal Hartig net to the plant by TOK channels in the ectomycorrhizal model Hebeloma cylindrosporum - Pinus pinaster. H. cylindrosporum harbours three TOK members. Herein, we report that one of them, HcTOK1, has similar features than the yeast ScTOK1. Moreover, we propose a role for this channel in the transport of K from the medium to ectomycorrhizal roots under K starvation.
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