Arbuscular mycorrhizal fungal colonization improves growth, photosynthesis, and ROS regulation of split-root poplar under drought stress

Arbuscular mycorrhizal (AM) fungi form ubiquitous symbioses with terrestrial plants in different ecosystems and provide a variety of benefits including improved drought tolerance of host plants. However, the difference and contribution of colonized and un-colonized root-system parts within mycorrhizal plants against drought stress is uncertain. A split-root system was used and the root compartments were either non-inoculated or inoculated with Rhizophagus irregularis, and were subjected to either well-watered or drought-stressed conditions. The growth, photosynthesis, reactive oxygen species (ROS) scavenging, and relative gene expression of aquaporins and phosphate transporters of hybrid poplar (Populus x canadensis 'Neva') were evaluated. Our results indicated that the inoculation by R. irregularis in either one or both compartments of split-root systems increased poplar biomass accumulation, photosynthesis, and ROS regulation under well-watered and drought-stressed conditions. When inoculum was applied in both compartments of split-root systems, the beneficial effect of R. irregularis was greater than that in treatment where only one compartment received inoculum. The effect of R. irregularis may attribute to improved phosphorus uptake via upregulation of relative expressions of PcPT3, PcPT4, PcPT5, and a possible improvement of water uptake via modulation of aquaporins (PcPIP2-3, PcPIP2-5, PcTIP1-1, and PcTIP1-2) in colonized root-system parts. Our results demonstrated that the benefits of the AM symbiosis depend on the extent of root colonization through which AM fungus may modulate plant phosphate and water uptake to improve tolerance of poplar against drought stress.