Summary• Soil humidity and bulk water transport are essential for nutrient mobilization. Ectomycorrhizal fungi, bridging soil and fine roots of woody plants, are capable of modulating both by being integrated into water movement driven by plant transpiration and the nocturnal hydraulic lift.• Aquaporins are integral membrane proteins that function as gradient-driven water and ⁄ or solute channels. Seven aquaporins were identified in the genome of the ectomycorrhizal basidiomycete Laccaria bicolor and their role in fungal transfer processes was analyzed.• Heterologous expression in Xenopus laevis oocytes revealed relevant water permeabilities for three aquaporins. In fungal mycelia, expression of the corresponding genes was high compared with other members of the gene family, indicating the significance of the respective proteins for plasma membrane water permeability.• As growth temperature and ectomycorrhiza formation modified gene expression profiles of these water-conducting aquaporins, specific roles in those aspects of fungal physiology are suggested. Two aquaporins, which were highly expressed in ectomycorrhizas, conferred plasma membrane ammonia permeability in yeast. This indicates that these proteins are an integral part of ectomycorrhizal fungusbased plant nitrogen nutrition in symbiosis.

Ectomycorrhizal (ECM) symbiosis is a mutualistic interaction between certain soil fungi and fine roots of perennial plants, mainly forest trees, by which both partners become capable of efficiently colonising nutrient-limited environments. The success of this interaction is reflected in the dominance of ECM forest ecosystems in the Northern hemisphere. Apart from their economic importance (wood production), forest ecosystems are essential for large-scale carbon sequestration, leading to substantial reductions in anthropogenic CO(2) release. The biological function of ECM symbiosis is the exchange of fungus-derived mineral nutrients for plant-derived carbohydrates. Improved plant nutrition as a result of this interaction, however, has a price. Together with their fungal partner, root systems of ECM plants can receive about half of the photosynthetically fixed carbon. To enable such a strong carbohydrate sink, the monosaccharide uptake capacity and carbohydrate flux through glycolysis and intermediate carbohydrate storage pools (trehalose and/or mannitol) of mycorrhizal fungi is strongly increased at the plant-fungus interface. Apart from their function as a carbohydrate store, trehalose/mannitol are additionally considered to be involved in carbon allocation within the fungal colony. Dependent on the fungal species involved in the symbiosis, regulation and fine-tuning of fungal carbohydrate uptake and metabolism seems to be controlled either by developmental mechanisms or by the apoplastic sugar content. As a consequence of the increased carbohydrate demand in symbiosis, trees increase their photosynthetic capacity. In addition, host plants control and restrict carbohydrate flux towards their partner to avoid fungal parasitism. The mechanisms behind this phenomenon are still largely unknown but rates of local sucrose hydrolysis and hexose uptake by rhizodermal cells are thought to restrict fungal carbohydrate nutrition under certain conditions (e.g., reduced fungal nutrient export).

Three aspects have to be taken into consideration when discussing cellular water and solute permeability of fungal cells: cell wall properties, membrane permeability, and transport through proteinaceous pores (the main focus of this review). Yet, characterized major intrinsic proteins (MIPs) can be grouped into three functional categories: (mainly) water transporting aquaporins, aquaglyceroporins that confer preferentially solute permeability (e.g., glycerol and ammonia), and bifunctional aquaglyceroporins that can facilitate efficient water and solute transfer. Two ancestor proteins, a water (orthodox aquaporin) and a solute facilitator (aquaglyceroporin), are supposed to give rise to today's MIPs. Based on primary sequences of fungal MIPs, orthodox aquaporins/X-intrinsic proteins (XIPs) and FPS1-like/Yfl054-like/other aquaglyceroporins are supposed to be respective sister groups. However, at least within the fungal kingdom, no easy functional conclusion can be drawn from the phylogenetic position of a given protein within the MIP pedigree. In consequence, ecophysiological prediction of MIP relevance is not feasible without detailed functional analysis of the respective protein and expression studies. To illuminate the diverse MIP implications in fungal lifestyle, our current knowledge about protein function in two organisms, baker's yeast and the Basidiomycotic Laccaria bicolor, an ectomycorrhizal model fungus, was exemplarily summarized in this review. MIP function has been investigated in such a depth in Saccharomyces cerevisiae that a system-wide view is possible. Yeast lifestyle, however, is special in many circumstances. Therefore, L. bicolor as filamentous Basidiomycete was added and allows insight into a very different way of life. Special emphasis was laid in this review onto ecophysiological interpretation of MIP function.

Summary Formation of ectomycorrhizas, a symbiosis with fine roots of woody plants, is one way for soil fungi to overcome carbohydrate limitation in forest ecosystems. Fifteen potential hexose transporter proteins, of which 10 group within three clusters, are encoded in the genome of the ectomycorrhizal model fungus Laccaria bicolor. For 14 of them, transcripts were detectable. When grown in liquid culture, carbon starvation resulted in at least twofold higher transcript abundances for seven genes. Temporarily elevated transcript abundance after sugar addition was observed for three genes. Compared with the extraradical mycelium, ectomycorrhiza formation resulted in a strongly enhanced expression of six genes, of which four revealed their highest observed transcript abundances in symbiosis. A function as hexose importer was proven for three of them. Only three genes, of which just one was expressed at a considerable level, revealed a reduced transcript content in mycorrhizas. From gene expression patterns and import kinetics, the L. bicolor hexose transporters could be divided into two groups: those responsible for uptake of carbohydrates by soil‐growing hyphae, for improved carbon nutrition, and to reduce nutrient uptake competition by other soil microorganisms; and those responsible for efficient hexose uptake at the plant–fungus interface.