Divergent Strategies of Mycorrhiza-Mediated Drought Adaptation in Poplar

Mycorrhizal symbiosis shapes plant growth and stress resilience. Here, we compared physiological and molecular responses of poplars colonized by Paxillus involutus (Pi) or Cenococcum geophilum (Cg) under control conditions, drought stress, and recovery. Both fungal species primed distinct local (root) and systemic (leaf) defenses compared to non-inoculated (Ni) plants. Cg-colonized poplars exhibited constitutively elevated transcripts of heat shock proteins (HSPs), galactinol synthase, and aquaporins in roots and leaves, irrespective of drought. Pi colonization enhanced growth and nitrogen-use-efficiency along with transcriptional increases of TOR/RAPTOR complex. Under severe soil moisture decline, Pi and Ni poplars showed reduced water potential, photosynthesis, growth, and leaf shedding, whereas Cg-colonized plants maintained water status, sustained photosynthesis, and retained foliage. These results reveal two contrasting mycorrhiza-mediated drought strategies in poplar: Pi fosters stress acclimation via drought-induced leaf abscission, enabling rapid recovery; Cg suppresses growth even without stress, conferring constitutive tolerance. Ectomycorrhizal species thus occupy different positions on the growth- defense trade-off spectrum. Such species-specific effects have important ecological and applied implications, enabling targeted use of EM fungi in forestry and agriculture depending on whether maximizing productivity or enhancing stress resilience is the primary goal.