The ectomycorrhizal contribution to tree nutrition

Trees can be associated with dozens of fungi helping them to acquire resources from forest soils. The most widespread mutualistic association in boreal and temperate forests is the ectomycorrhizal symbiosis. This symbiosis involves mushroom-forming fungi of basidiomycota, ascomycota, and some zygomycota clades and the roots of woody plant species, including oaks, poplars or pines. Although the impact of this association on ecosystem production and tree nutrition is investigated for about a century, our understanding on the molecular mechanisms that control water and nutrient fluxes between plant and fungal partners is still limited. Here, we review the recent knowledge on the ectomycorrhizal contribution to tree nutrition. We specifically highlight the molecular mechanisms driving the acquisition, translocation and release of water and nutrients in ectomycorrhizal systems. We particularly focus on the transport of macronutrients, including nitrogen, phosphorus, potassium, sulphur and calcium, micronutrients, and water by the symbiotic partner. We also provide background on the evolution, diversity, and importance of this symbiosis, identify knowledge gaps, and propose future research directions.