Mutation of M. truncatula SOBIR1 affects rhizobial specificity and arbuscular mycorrhizal colonization

Legume plants form symbiotic interactions with bacteria (rhizobia sp.) to obtain fixed nitrogen and arbuscular mycorrhizal fungi to obtain phosphorus and other nutrients. These interactions require the plant to distinguish beneficial from pathogenic organisms and trigger the plants innate immune system. We identified mutants in the Medicago truncatula SOBIR1 gene, known to be involved in innate immune response in multiple plants. We examined nodulation with multiple strains and species of Sinorhizobium and examined mycorrhizal interactions with the arbuscular mycorrhizal fungus Glomus versiforme (recently renamed Diversispora epigaea). Plants containing mutations in SOBIR1 exhibit normal nodulation with strains of S. meliloti, but fewer nodules when inoculated with S. medicae strains. The S. medicae nodules show evidence of accumulation of polyphenolic compounds, and abnormal arrangement of the rhizobia within the nodules. In contrast, when inoculated with Glomus versiforme. M. truncatula sobir1 mutant plants have increased mycorrhizal colonization and arbuscule number compared to the wild type. We localized a tagged SOBIR1 protein to the periarbuscular membrane interface. Together the data suggest the immune kinase SOBIR1 is involved in both nodulation and mycorrhizal interactions, but the effects of mutation differ.