A major limitation to plant growth is the restricted access to nutrients in the soil. To improve nutrient acquisition, the majority of land plants enter a beneficial symbiosis with arbuscular mycorrhizal (AM) fungi. The accommodation of fungal hyphae in roots requires the extensive transcriptional reprogramming of host cells. Several GRASdomain proteins, including NSP1 (NODULATION SIGNALLING PATHWAY 1), NSP2, and RAM1 (REQUIRED FOR ARBUSCULAR MYCORRHIZATION 1), have emerged as important transcriptional regulators during mycorrhization. Interaction studies suggest that these proteins form multicomponent complexes, raising the question whether they regulate similar or different mycorrhizal processes. Here, the functions of NSP1, NSP2 and RAM1 during AM development were investigated by detailed phenotypic and transcriptional analyses of the corresponding loss‐of‐function mutants. Global gene expression profiling of nsp1‐1 revealed that NSP1 is required for the expression of a large number of genes involved in strigolactone and gibberellin biosynthesis at the pre‐contact stage of AM development. Strigolactones are known to attract the fungus to the root. In line with this, the quantification of mycorrhizal structures showed a delay in mycorrhization in nsp1‐1. Transcriptional profiling confirmed that the expression of the majority of mycorrhizal‐induced genes was delayed, but not abolished in nsp1‐1, suggesting that NSP1 only has a minor role in the transcriptional regulation once the contact between the fungus and the roots has been established. Unlike NSP1, RAM1 plays a critical role in the transcriptional regulation at later stages of AM symbiosis. Mycorrhization was strongly impaired in ram1‐1, and transcriptional profiling revealed that RAM1 is essential for the expression of several genes involved in arbuscule development and the nutrient exchange between the symbionts. Meanwhile, the exact function of NSP2 remains unclear, as no effect on mycorrhization was observed in nsp2‐2 under the conditions tested here. These findings suggest that NSP1, NSP2 and RAM1 play largely different roles in the transcriptional regulation during AM development.