Responses of L. bicolor to P. tremula x alba at different time points post-inoculation. (A) In vitro co-culture of Laccaria and Poplar. Mycelium within a distance of 0.5 cm of a root was considered as ectomycorrhizal (EcM), mycelium at a longer distance as extramatrical (ExM). (B) External view of representative EcM root tips at different time points. (C) Transversal sections of lateral roots at 0, 3, 7, 14 and 21 dpi. Sections were stained with PI and WGA-oregon green to visualize plant (red) and fungal (green) cells, respectively. (D) Hierarchical clusters of overall transcriptomic responses of 37 biological replicates belonging to different stages of EcM development.