Physiology and Transcriptomics Reveal 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 (P. x canescens) colonised by Paxillus involutus (Pi) or Cenococcum geophilum (Cg) under control conditions, drought stress and recovery. Both fungal isolates primed distinct local (root) and systemic (leaf) defences compared to non-inoculated (Ni) plants. Cg-colonised poplars exhibited constitutively elevated transcripts of heat shock proteins, galactinol synthase and aquaporins in roots and leaves, irrespective of drought. Pi colonisation enhanced growth and nitrogen-use-efficiency, along with transcriptional increases of the TOR/RAPTOR complex in leaves. Under severe soil moisture decline, Pi and Ni poplars showed reduced water potential, photosynthesis, growth and leaf shedding, whereas Cg-colonised 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 acclimation and recovery; Cg confers constitutive tolerance and suppresses growth. Ectomycorrhizal fungi thus occupy different positions on the growth-defence trade-off spectrum. Such genotypic effects have important ecological and applied implications, enabling targeted use of EM fungi in forestry and agriculture, depending on whether maximising productivity or enhancing stress resilience is the primary goal.