OsRAM2 function in lipid biosynthesis is required for arbuscular mycorrhizal symbiosis in rice.

Abstract

Arbuscular mycorrhiza (AM) is a mutualistic symbiosis formed between most land plants and Glomeromycotina fungi. During the symbiosis, plants provide organic carbon to fungi in exchange for mineral nutrients. Previous legume studies showed that the Required for Arbuscular Mycorrhization2 (RAM2) gene is necessary for transferring lipids from plants to AM fungi (AMF) and is also likely to play a signaling role at the root surface. To further explore RAM2 functions in other plant lineages, in this study, two rice (Oryza sativa) genes, OsRAM2 and OsRAM2L, were identified as orthologs of legume RAM2. Examining their expression patterns during symbiosis revealed that only OsRAM2 was strongly upregulated upon AMF inoculation. CRISPR/Cas9 mutagenesis was then performed to obtain three Osram2 mutant lines (-1, -2, and -3). After inoculation by AMF Rhizophagus irregularis or Funneliformis mosseae, all the mutant lines showed extremely low colonization rates and the rarely observed arbuscules were all defective, thus supporting a conserved nutritional role of RAM2 between monocot and dicot lineages. As for the signaling role, although the hyphopodia numbers formed by both AMF on Osram2 mutants were indeed reduced, their morphology showed no abnormality, with fungal hyphae invading roots successfully. Promoter activities further indicated OsRAM2 was not expressed in epidermal cells below hyphopodia or outer cortical cells enclosing fungal hyphae, but expressed exclusively in cortical cells containing arbuscules. It therefore suggested an indirect role of RAM2 rather than a direct involvement in determining the symbiosis signals at the root surface.