Molecular and Functional Characterization of a Na+-K+ Transporter from the Trk Family in the Ectomycorrhizal Fungus Hebeloma cylindrosporum

Abstract

Ectomycorrhizal symbiosis between fungi and woody plants strongly improves plant mineral nutrition and constitutes a major biological process in natural ecosystems. Molecular identification and functional characterization of fungal transport systems involved in nutrient uptake are crucial steps toward understanding the improvement of plant nutrition and the symbiotic relationship itself. In the present report a transporter belonging to the Trk family is identified in the model ectomycorrhizal fungus Hebeloma cylindrosporum and named HcTrk1. The Trk family is still poorly characterized, although it plays crucial roles in K ؉ transport in yeasts and filamentous fungi. In Saccharomyces cerevisiae K ؉ uptake is mainly dependent on the activity of Trk transporters thought to mediate H ؉ :K ؉ symport. The ectomycorrhizal HcTrk1 transporter was functional when expressed in Xenopus oocytes, enabling the first electrophysiological characterization of a transporter from the Trk family. HcTrk1 mediates instantaneously activating inwardly rectifying currents, is permeable to both K ؉ and Na ؉ , and displays channel-like functional properties. The whole set of data and particularly a phenomenon reminiscent of the anomalous mole fraction effect suggest that the transport does not occur according to the classical alternating access model. Permeation appears to occur through a single-file pore, where interactions between Na ؉ and K ؉ might result in Na ؉ :K ؉ co-transport activity. HcTrk1 is expressed in external hyphae that explore the soil when the fungus grows in symbiotic condition. Thus, it could play a major role in both the K ؉ and Na ؉ nutrition of the fungus (and of the plant) in nutrient-poor soils.

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

“…This hypothesis reconciles the observations reported by Lan et al (2010) and Horie et al (2011) and the present observations, which are more in line with the functional analyses performed so far in other HKT transporters (for review, see Corratgé-Faillie et al, 2010) as well as with first structural data obtained in the Trk/ Ktr/HKT superfamily (Cao et al, 2011). Indeed, the observation of an instantaneously activating non-timedependent current kinetics is in line with what has been so far observed in HKT/Trk transporters of plant or fungal origin when expressed in Xenopus oocytes (Fairbairn et al, 2000;Corratgé et al, 2007;Jabnoune et al, 2009). Furthermore, the fact that the instantaneously activating conductance observed in our OsHKT2;4-expressing oocytes was not permeable to Ca 2+ is in agreement with the statement of Lan et al (2010) that monovalent cations and not Ca 2+ carried the ohmic component of the current they observed upon OsHKT2;4 expression in oocytes.…”

Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

“…A Trk transporter, HcTrk1, has been identified and characterized in the ectomycorrhizal fungus model Hebeloma cylindrosporum [98,99]. Ectomycorrhizal symbiosis between fungi and woody plants strongly improves plant mineral nutrition, the fungal partner efficiently taking up mineral nutrients, including K ?…”

“…-K ? symport activity [99]. Laccaria bicolor, the first ectomycorrhizal fungus the genome of which has been made publicly available [96], harbors one HAK and two Trk genes, like N. crassa.…”

“…since they all complement S. cerevisiae mutant strains defective for K ? [29,37,42,88,90,99]. Indirect evidence for permeability to Na ?…”

A Single Amino-Acid Substitution in the Sodium Transporter HKT1 Associated with Plant Salt Tolerance

“…Only TsHKT1;2 and AtHKT1 N211D showed improved K + transport relative to Na + . Modeling Asp or Asn into the second pore-loop region based on the yeast TRK1 structure provided further indication for the crucial role of these amino acids (Corratgé et al, 2007;Corratgé-Faillie et al, 2010;Cao et al, 2011). Transgenic Arabidopsis expressing AtHKT1 N211D tolerated salt stress to a higher degree than wild-type AtHKT1;1.…”