Routes to Roots: Direct Evidence of Water Transport by Arbuscular Mycorrhizal Fungi to Host Plants

Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with 80% of surveyed land plant species and are well-recognized for accessing and transferring nutrients to plants 1 . Yet AMF also perform other essential functions, notably improving plant-water relations 2 . Some research attributes the role of AMF in plant-water relations solely to enhancing plant nutrition and osmoregulation for plants partnered with AMF 3,4,5 , while indirect evidence suggests AMF may transport water to plants 1,6,7 . Here, we used isotopically-labeled water and a fluorescent dye to directly track and quantify water transport by AMF to plants in a greenhouse experiment. We specifically assessed whether AMF can access water in soil unavailable to plants and transport it across an air gap to host plants. Plants grown with AMF that had access to a physically separated 18 O-labeled water source transpired twice as much, and this transpired water contained three times as much label compared to plants with AMF with no access to the separated labeled water source. We estimated that water transported by AMF could explain 46.2% of the water transpired. In addition, a fluorescent dye indicated that water was transported via an extracytoplasmic hyphal pathway.