OsIDD7 integrates signaling networks for arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) symbiosis is a widespread mutualism between plant roots and Glomeromycotina fungi that enables nutrient exchange through arbuscules. Although many transcriptional regulators of AM symbiosis have been identified, the role of the INDETERMINATE DOMAIN (IDD) proteins remain unknown. We show that rice IDD7 expression is strongly induced in arbuscule-containing cells, and its loss markedly reduces fungal colonization and disrupts arbuscule development. IDD7 is required for induction of genes associated with fatty acid biosynthesis, nutrient transport, and symbiotic signaling during AM symbiosis. IDD7 interacts with the transcription factors SLENDER RICE 1 (SLR1) and PHOSPHATE STARVATION RESPONSE 2 (PHR2) through its conserved TQDFLG domain. Electrophoretic mobility shift assays show that IDD7 binds multiple promoter motifs. Together with SLR1 and PHR2, IDD7 synergistically activates the promoters of PHOSPHATE TRANSPORTER 11 (PT11) and the AM-associated transcription factors PHR2, REQUIRED FOR ARBUSCULAR MYCORRHIZATION 1 (RAM1), WRINKLED 5a (WRI5a), and CYCLOPS. Moreover, IDD7 is required for PHR2 expression in arbuscule-containing cells. Here, we show that IDD7 is a central transcriptional regulator in arbuscule-containing cells.