Hyphal Branching during Arbuscule Development RequiresReduced Arbuscular Mycorrhiza1

During arbuscular mycorrhizal symbiosis, arbuscule development in the root cortical cell and simultaneous deposition of the plant periarbuscular membrane generate the interface for symbiotic nutrient exchange. The transcriptional changes that accompany arbuscule development are extensive and well documented. By contrast, the transcriptional regulators that control these programs are largely unknown. Here, we provide a detailed characterization of an insertion allele ofMedicago truncatulaReduced Arbuscular Mycorrhiza1 (RAM1),ram1-3, which reveals that RAM1 is not necessary to enable hyphopodium formation or hyphal entry into the root but is essential to support arbuscule branching. Inram1-3, arbuscules consist only of the arbuscule trunk and in some cases, a few initial thick hyphal branches.ram1-3is also insensitive to phosphate-mediated regulation of the symbiosis. Transcript analysis ofram1-3and ectopic expression ofRAM1indicate that RAM1 regulates expression ofEXO70IandStunted Arbuscule, two genes whose loss of function impacts arbuscule branching. Furthermore, RAM1 regulates expression of a transcription factorRequired for Arbuscule Development(RAD1). RAD1 is also required for arbuscular mycorrhizal symbiosis, andrad1mutants show reduced colonization.RAM1itself is induced in colonized root cortical cells, and expression ofRAM1andRAD1is modulated by DELLAs. Thus, the data suggest that DELLAs regulate arbuscule development through modulation of RAM1 and RAD1 and that the precise transcriptional control essential to place proteins in the periarbuscular membrane is controlled, at least in part, by RAM1.