Dual requirement of the ljsym4 gene for mycorrhizal development in epidermal and cortical cells of Lotus japonicus roots

* The LjSym4 mutation leads to Lotus japonicus plants that are defective in arbuscular mycorrhiza (AM) development. * Two alleles of LjSym4 with different phenotypic strength were compared. AM development was assessed by considering five parameters related to fungal structures present in root segments from wild type and mutant plants. The distribution of intercellular hyphae was determined using semithin sections from resin-embedded roots. Cellular interactions were investigated at ultrastructural level, while cell wall components from the host plant were identified by using immunogold labelling. * In roots of Ljsym4-1 fungal hyphae are mostly restricted to the intercellular spaces of the cortex, indicating a block of infection of mutant cortical cells, resulting in a very low number of arbuscules. * This observation suggests the presence of an additional genetically defined checkpoint for mycorrhizal development, located at the wall of cortical cells. The LjSym4 gene is therefore required for infection of both epidermal and cortical cells by AM fungi.