To improve access to limiting nutrients, the vast majority of land plants forms arbuscular mycorrhizal (AM) symbioses with Glomeromycota fungi. We show here that AM-related GRAS transcription factors from different subgroups are upregulated during a time course of mycorrhization. Based on expression studies in mutants defective in arbuscule branching ( ram1-1 , with a deleted MtRam1 GRAS transcription factor gene) or in the formation of functional arbuscules ( pt4-2 , mutated in the phosphate transporter gene MtPt4 ), we demonstrate that the five AM-related GRAS transcription factor genes MtGras1 , MtGras4 , MtGras6 , MtGras7 , and MtRad1 can be differentiated by their dependency on MtRAM1 and MtPT4, indicating that the network of AM-related GRAS transcription factors consists of at least two regulatory modules. One module involves the MtRAM1- and MtPT4-independent transcription factor MtGRAS4 that activates MtGras7 . Another module is controlled by the MtRAM1- and MtPT4-dependent transcription factor MtGRAS1. Genome-wide expression profiles of mycorrhized MtGras1 knockdown and ram1-1 roots differ substantially, indicating different targets. Although an MtGras1 knockdown reduces transcription of AM-related GRAS transcription factor genes including MtRam1 and MtGras7 , MtGras1 overexpression alone is not sufficient to activate MtGras genes. MtGras1 knockdown roots display normal fungal colonization, with a trend towards the formation of smaller arbuscules.