Arbuscular mycorrhizal fungusRhizophagus irregularisexpresses an outwardly Shaker-like channel involved in potassium nutrition of rice (Oryza sativaL.)

Potassium (K+) plays crucial roles in many physiological, molecular and cellular processes in plants. Direct uptake of this nutrient by root cells has been extensively investigated, however, indirect uptake of K+mediated by the interactions of the roots with fungi in the frame of a mutualistic symbiosis, also called mycorrhizal nutrient uptake pathway, is much less known. We identified an ion channel in the arbuscular mycorrhizal (AM) fungusRhizophagus irregularis. This channel exhibits the canonical features of Shaker-like channel shared in other living kingdoms and is named RiSKC3. Transcriptionally expressed in hyphae and in arbuscules of colonized rice roots, RiSKC3 has been shown to be located in the plasma membrane. Voltage-clamp functional characterization inXenopusoocytes revealed that RiSKC3 is endowed with outwardly-rectifying voltage-gated activity with a high selectivity for K+over sodium ions. RiSKC3 may have a role in the AM K+pathway for rice nutrition in normal and salt stress conditions. The current working model proposes that K+ions taken up by peripheral hyphae ofR. irregularisare secreted towards the host root into periarbuscular space by RiSKC3.Significance StatementMutualistic symbiosis with arbuscular mycorrhizal (AM) fungi are beneficial for about 80% of land plants thanks to an exchange of nutrients. The AM pathway responsible for potassium (K+) nutrition of the plant is not known. Here we uncovered a key step of this phenomenon, by functionally characterizing the first transport system in the AM fungusRhizophagus irregularis, and we univocally demonstrated that RiSKC3 is an K+outwardly-rectifying voltage-gated Shaker-like channel.