The Lotus japonicus LjSym4 gene is required for the successful symbiotic infection of root epidermal cells

The role of the Lotus japonicus LjSym4 gene during the symbiotic interactio n with Mesorhizobium loti and arbuscular mycorrhizal (AM) fungi was analyze d with two mutant alleles conferring phenotypes of different strength. Ljsy m4-1 and Ljsym4-2 mutants do not form nodules with M. loti. Normal root hai r curling and infection threads are not observed, while a nodC-dependent de formation of root hair tips indicates that nodulation factors are still per ceived by Ljsym4 mutants. Fungal infection attempts on the mutants generall y abort within the epidermis, but Ljsym4-1 mutants allow rare, successful, infection events, leading to delayed arbuscule formation. On roots of mutan ts homozygous for the Ljsym4-2 allele, arbuscule formation was never observ ed upon inoculation with either of the two AM fungi, Glomus intraradices or Gigaspora margarita. The strategy of epidermal penetration by G. margarita was identical for Ljsym4-2 mutants and the parental line, with appressoria , hyphae growing between two epidermal cells, penetration of epidermal cell s through their anticlinal wall. These observations define a novel, genetic ally controlled step in AM colonization. Although rhizobia penetrate the ti p of root hairs and AM fungi access an entry site near the base of epiderma l cells, the LjSym4 gene is necessary for the appropriate response of this cell type to both microsymbionts. We propose that LjSym4 is required for th e initiation or coordinated expression of the host plant cell's accommodati on program, allowing the passage of both microsymbionts through the epiderm is layer.