Homocitrate Synthase Genes of Two Wide-Host-Range Bradyrhizobium Strains are Differently Required for Symbiosis Depending on Host Plants

摘要

The gene encodes homocitrate synthase, the enzyme that catalyzes the formation of homocitrate, which is essential for arranging the FeMo-cofactor in the catalytic center of nitrogenase. Some host plants, such as , supply homocitrate to their symbionts, in this case, , which lacks . In contrast, ORS285, a symbiont of cross-inoculation (CI) groups 2 and 3, requires for symbiosis with species that belong to CI group 3, and some species belonging to CI group 2. However, it currently remains unclear whether rhizobial is required for symbiosis with species belonging to CI group 1 or with other legumes. We generated -disruption (Δ ) mutants of two wide-host-range rhizobia, SUTN9-2 and DOA9, to investigate whether they require for symbiosis. Both Δ mutant strains showed significantly less nitrogenase activity in a free-living state than the respective wild-type strains. The symbiotic phenotypes of SUTN9-2, DOA9, and their Δ mutants were examined with four legumes, , , , and . was required for the efficient symbiosis of SUTN9-2 with (CI group 1), but not for that of DOA9. SUTN9-2 established symbiosis with all three other legumes; was required for symbiosis with and . These results suggest that, in addition to CI groups 2 and 3, CI group 1 and several other legumes require the rhizobial for symbiosis.