Soil microbiology is entering into a new golden age led by the development of cheap sequencing methods, improved metagenomic analyses and the renewed interest in studying an organism microbiome (East, 2013). Mirroring development in human and animal interactions with symbiotic microorganisms, new efforts are directed to identifying and studying the microorganisms associated with plants (Bulgarelli et al., 2012). Arguably, the most important components of a plant microbiome are mycorrhizal fungi. Mycorrhiza is the symbiosis with beneficial soil fungi established in the roots of > 90% of plants. Mycorrhizas have been a key element in plant colonization of terrestrial environments given the crucial role that they play in plant nutrition and stress resistance. In the case of ectomycorrhizas, the fungal partner ensheaths the finer roots of the host plant and takes over the supply of water and mineral nutrients to the host in exchange for photosynthates. It is estimated that over 90% of the phosphorus (P) requirements of the host plant are satisfied by the mycorrhizal fungus, c. 50% of the nitrogen (N) demand and similar amounts of other micronutrients. Interestingly, the mycorrhiza also protects against toxic concentrations of some of these elements, to the point that at least some mycorrhizal fungi act as living buffers, providing essential nutrients when available at low concentrations and detoxifying them when they are at higher levels. In spite of the wealth of knowledge available on P or N nutrition in mycorrhizal plants, very little is known about how potassium (K+) homeostasis is affected by the symbiosis (Smith & Read, 2008). This is despite the important role of K+ in different metabolic processes and physiological functions (such as osmorregulation, neutralization of negative charges and growth) and being the third critical component in most crop fertilizers (after N and P). In this issue of New Phytologist, Garcia et al. (pp. 951–960) present, for the first time, a study of the impact of an ectomycorrhizal fungus on the K+ nutrition of its host and the involvement of a fungal K+ transporter. It has been carried out using the basidiomycete Hebeloma cylindrosporum and Pinus pinaster as the two partners of the symbiotic association.
‘Arguably, the most important components of a plant microbiome are mycorrhizal fungi.’
HcTrk1 and potassium transport in the Hebeloma cylindrosporum–Pinus pinaster symbiosis
Trk transporters are membrane proteins involved in K+ uptake that are widespread among fungi (Benito et al., 2011). The first member of this family was initially identified in Saccharomyces cerevisiae (Ramos et al., 1985) as essential for hydrogen (H+)-dependent high-affinity K+ uptake. HcTrk1 is the first ectomycorrhizal fungal member of this family to be characterized (Corratge et al., 2007). Electrophysiological studies have shown that it is a Na+/K+ transporter, although no information on its role in the symbiosis was provided. The report by Garcia et al. indicates that mycorrhization with Hebeloma plays an important role in plant K+ uptake under deficient conditions. Although not enough to significantly improve plant growth, K+ levels in both the mycorrhizal root and shoot were increased under deficiency conditions. The buffering effect on K+ uptake was maintained even when HcTrk1 was overexpressed. Potassium ion levels remained the same in overexpressing and non-overexpressing mycorrhizal roots in low-K+ conditions. Interestingly, overexpression of this transporter resulted in retention of K+ in the mycorrhized root; this is in contrast to the putative localization of HcTrk1. GFP fusions indicated that these proteins were present in the root-surrounding and soil-exploring hyphae of pine suggesting a role in K+ and/or sodium (Na+) acquisition from the soil. However, in situ hybridization showed that HcTrk1 mRNA was present in all hyphae. This apparent contradiction suggests a putative post-translational regulation of HcTrk1 levels, that somehow is averted by the overexpression. As a result, K+ would be retained in the overexpressing mycorrhizal roots, being less available for the shoots. This result should act as a warning in those cases where strategies to develop overexpressing mycorrhizal fungal lines to solve environmental and nutritional problems are considered. Further research on fungal gene regulation and post-translational regulatory processes in the context of the mycorrhizal symbiosis will help to elucidate these issues.
HcTrk1 has been associated with Na+ transport using biochemical analyses (Corratge et al., 2007); however, its physiological role remains to be shown. The results reported by Garcia et al. seem to indicate that it is very minor or masked by other transporters expressed at the same time, since no increased accumulation of this element was observed in mycorrhizal plants regardless of overexpression of HcTrk1.
Potassium transport and the vacuolar highway
Mycorrhizal networks can extend over huge areas, since the same fungal community can mycorrhize multiple plants, in what has come to be known as the ‘wood wide network’. This poses the question as to how nutrients are delivered over these long distances. In a typical eukaryotic cell cytosol, just several microns wide, K+ typically reaches efflux transporters or organelles by diffusion in a gradient from higher to lower concentrations. However, to maintain these gradients over the extensive lengths of the hyphal network is an extremely complicated and costly process, and which, in addition, would affect bidirectional transport. Other mechanisms must be in place.
The long-distance transport of nutrients along hyphae has been thoroughly explored for P nutrition. Works by Ashford, Bücking, and others have shown that P is stored as polyphosphate granules within tubular vacuoles (Ashford et al., 1999; Bücking, 2004). These vacuoles are motile and common to most fungal phyla (Rees et al., 1994). In the case of basidiomycetes, such as Hebeloma, it has been observed that these vacuoles can pass through the dolipore from cell to cell (Shepherd et al., 2003). As a result, following the cytoskeleton, vacuole-stored polyphosphate packages can be actively directed in favour or against any gradient.
Interestingly, the high negative charge density of the polyphosphate chains can be neutralized by catonic substances of interest, such as amino acids, proteins, transition elements, or K+. In this way, K+ has been frequently detected in elemental analyses of polyphosphate granules in ectomycorrhizal fungi (Bücking & Heyser, 1999). These positive charges would not only serve to neutralize the polyphosphate chains, but in association with them, it could be delivered to the host. This mechanism has been predicted to be used by arbuscular mycorrhizal fungi to translocate N compounds and transition metals (Jin et al., 2005; González-Guerrero et al., 2008). The results by Garcia et al. strongly indicate a correlation in P and K+ levels: deficiency in one of them induces the uptake of the other. Moreover, overexpression of HcTrk1 results in diminished P translocation to the shoot. This could be the result of K+ accumulation in the hyphae that would preclude polyphosphate hydrolysis and/or transfer to the host plant.
The Hebeloma cylindrosporum–Pinus pinaster potassium transportome
Recent progress in completed genome sequencing projects of fungi and plants (Martin et al., 2011) is providing valuable information towards an overview of the membrane transporters that are involved in nutrition during symbiotic interactions between ectomycorrhizal fungi and their plant hosts. The BLAST search in the recently available Hebeloma genome (http://genome.jgi-psf.org/Hebcy2/Hebcy2.home.html) with protein sequences of K+ transporters and channels already described in fungi has allowed the identification of homologous putative K+ transporters that operate in Hebeloma (one HAK and two TRK putative K+ transporters and one SKC and three TOK putative K+ channels). A preliminary proposal of the inventory of K+ transporters and their putative function and localization in the ectomycorrhizal fungal is summarized in Fig. 1. As shown in Garcia et al., the K+ absorption from the soil would take place in the root-surrounding and soil-exploring hyphae by HcTrk1 but also by other transporters like HcHAK and/or HcTrk2 that could operate in the extra-radical mycelium, being effective K+ transporters even where K+ limiting zones may occur since TRK and HAK homologous proteins characterized in other fungi function as high-affinity K+ transporters (Haro et al., 1999; Bañuelos et al., 2000). The movement of K+ from the extra-radical mycelium to the host plant could be dependent on the motile vacuolar network that can pass though perforated septa along the hyphae. The K+ supply of the host plant would depend on K+ within the fungal symplast of the Hartig net and the translocation rate of these ions across the fungal plasma membrane into the apoplastic interface. This involves K+ replenishment of the interfacial apoplast by fungal K+ efflux systems that can be mediated by TOK channels or also by a K+ specific ENA ATPase (Benito et al., 2002) that may also operate in Hebeloma (Lambilliotte et al., 2004; protein ID in Hebeloma genome: fgenesh1_pm.4_#_356). The genome of the host tree P. pinaster is still awaiting completion but a BLAST search in the genome of the Norway spruce Picea abies, that has recently been released (Nystedt et al., 2013), indicates that the host plant is equipped with several K+ transporters (HAK and HKT) and numerous K+ channels (inward AKT-type and outward SKOR-type K+ rectifier channels) homologous to existing plant transporters involved in supplying all the K+ needed for the plant. The Hebeloma–Pinus ectomycorrhizal association emerges now as a very interesting model to decipher the molecular actors involved in K+ homeostasis during symbiotic associations.