Ectomycorrhizae (ECM), symbiosis between tree roots and fungal hyphae, are essential for tree health. In exchange for carbohydrates, ECM fungi offer an improved mineral supply. Communication between plant and ECM fungus is essential for the initiation, establishment and maintenance of the symbiosis. In the ECM fungus Laccaria bicolor, the Mycorrhiza induced Small Secreted Protein of 7 kDa (MiSSP7) is a master switch controlling symbiosis development. Transcriptomic data revealed the presence of other MiSSPs with unknown functions and CAZyme (carbohydrate-active-enzymes) up-regulated during ectomycorrhiza development. I will present how the study of such proteins, using L. bicolor-Populus interaction as a model, can help us deciphering the molecular mechanisms driving the development of mycorrhizal symbioses (e.g. control of hormonal-signaling pathway and plant cell wall remodeling).