Mycorrhiza-Triggered Transcriptomic and Metabolomic Networks Impinge on Herbivore Fitness

Symbioses between plants and mycorrhizal fungi are ubiquitous in ecosystems and strengthen the plants' defense against aboveground herbivores. Here, we studied the underlying regulatory networks and biochemical mechanisms in leaves induced by ectomycorrhizae that modify herbivore interactions. Feeding damage and oviposition by the widespread poplar leaf beetle Chrysomela populi were reduced on ectomycorrhizal hybrid poplar Populus x canescens. Integration of transcriptomics, metabolomics and volatile emission patterns via mass difference networks demonstrated changes in nitrogen allocation in leaves of mycorrhizal poplars, down-regulation of phenolic pathways and up-regulation of defensive systems, including protease inhibitors, chitinases and aldoxime biosynthesis. Ectomycorrhizae had a systemic influence on jasmonate-related signalling transcripts. Our results suggest that ectomycorrhizae prime wounding responses and shift resources from constitutive phenol-based to specialized protective compounds. Consequently, symbiosis with ectomycorrhizal fungi enabled poplars to respond to leaf beetle feeding with a more effective arsenal of defense mechanisms than non-mycorrhizal poplars, thus demonstrating the importance of belowground plant-microbe associations in mitigating aboveground biotic stress.