Aquaporins: for more than water at the plant-fungus interface?

International audienceAquaporins are abundant membrane proteins that channel water, but also small uncharged molecules, across the membranes of plants, animals and microbes (Maurel et al., 2008). In this issue of New Phytologist, Dietz et al. (pp. 927–940) report on a thorough molecular and functional analysis of the whole aquaporin family of an ectomycorrhizal (ECM) fungus, the basidiomycete Laccaria bicolor. This study is novel as it describes, from an evolutionary perspective, the diversity of aquaporins in a fungus, which, with the exception of the yeast Saccharomyces cerevisiae, had not been extensively studied at a genome scale in these organisms. This study is also of special importance as it addresses the role of aquaporins in both nutrient and water exchanges during symbiosis between plants and fungi, with a potentially great impact on water and nutrient economy in forest ecosystems. In brief, Dietz et al. screened the full genomic sequence of Laccaria bicolor to identify seven open reading frames (ORFs) encoding aquaporin homologues. They used heterologous expression in yeast and Xenopus oocytes to characterize the ability of these homologues to transport water but also various solutes such as glycerol, urea andammonium (NH4+)/ammonia (NH3). It is of note that nitrogen (N) compounds, some of which are putative aquaporin substrates, are, besides water, crucial during ECM symbiosis. Similar to observations of aquaporins in other organisms, various selectivity profiles were identified for the L. bicolor aquaporins; some isoforms specifically transported water whereas others were also permeable to urea, glycerol and/or NH3