The homologous GRAS transcription factors RAM1 and RAD1 coordinate shared and separate gene networks to regulate arbuscular mycorrhiza symbiosis

Arbuscular mycorrhiza (AM), a symbiosis between plants and Glomeromycotan fungi is based on the exchange of nutrients, which majorly occurs between fungal tree-shaped arbuscules and root cortex cells. Two homologous plant GRAS transcription factors, RAM1 and RAD1, were previously reported to interact, and both were implicated in regulating arbuscule branching in Lotus japonicus. We revisited the role of L. japonicus RAM1 and RAD1 and asked whether they act together in mediating arbuscule development and gene expression, or whether they act additively. We performed comprehensive genetic, phenotypic, and transcriptome analyses of two allelic Lotus ram1, rad1, and ram1 rad1 mutants; and of roots ectopically expressing RAM1 and/or RAD1. We found that RAD1 is required for full root colonization in a RAM1-dependent manner, but in our growth conditions, RAD1 is not required for arbuscule branching. In addition, RAM1 and RAD1 are required for activation of partially common and partially distinct sets of downstream genes. Thus, in contrast to RAM1, Lotus RAD1 is not essential for arbuscule branching. Furthermore, RAM1 and RAD1 seem to act partially synergistically but seem to participate in additional alternative transcription factor complexes to regulate AM-induced gene expression and AM development.