The majority of the vascular flowering plants form symbiotic associations with fungi from the phylum Glomeromycota through which both partners gain access to nutrients, either mineral nutrients in the case of the plant, or carbon, in the case of the fungus. 1 The association develops in the roots and requires substantial remodeling of the root cortical cells where branched fungal hyphae, called arbuscules, are housed in a new membrane-bound apoplastic compartment.2 Nutrient exchange between the symbionts occurs over this interface and its development and maintenance is critical for symbiosis. Previously, we showed that DELLA proteins, which are well known as repressors of gibberellic acid signaling, also regulate development of AM symbiosis and are necessary to enable arbuscule development.3 Furthermore, constitutive overexpression of a dominant DELLA protein (della1-D18) is sufficient to induce transcripts of several AM symbiosis-induced genes, even in the absence of the fungal symbiont. 4 Here we further extend this approach and identify AM symbiosis genes that respond transcriptionally to constitutive expression of a dominant DELLA protein and also genes that do respond to this treatment. Additionally, we demonstrate that DELLAs interact with REQUIRED FOR ARBUSCULE DEVELOPMENT 1 (RAD1) which further extends our knowledge of GRAS factor complexes that have the potential to regulate gene expression during AM symbiosis. Development of arbuscular mycorrhizal (AM) symbiosis involves signal exchange between the symbionts and a highly conserved plant symbiosis signaling pathway plays a central role in controlling fungal growth into the root and the development of the symbiotic interface in the cortical cells. 5,6 The gene expression changes that underlie symbiotic development have been documented through detailed transcript profiling 7,8 and some transcriptional regulators have been identified, some of which are downstream components of the symbiosis signaling pathway. 4,[9][10][11] Development of AM symbiosis is also influenced by plant hormones and treatment of roots with gibberellic acid (GA) leads either to alterations in fungal entry into the root or abolishes arbuscule development. The precise outcome depends on the concentration of GA applied.12 Genes encoding enzymes of GA biosynthesis and metabolism are differentially regulated during symbiosis 7,13-17 and associated with the colonized regions of the cortex.18 Furthermore, both bioactive and inactive GAs are present in mycorrhizal roots. All of these data suggest a role for GAs in regulating aspects of symbiotic development. 18,19 DELLA proteins are members of a specific sub-group of the GRAS family of transcriptional regulators that possess a unique domain, the DELLA domain, located toward the N-terminal end of the protein.20 DELLAs function as repressors of GA signaling and their mechanism of action has been elucidated through extensive studies in Arabidopsis. Briefly, in the presence of GA, the GA receptor (GID) interacts with the DELLAs through ...