The paradigms of legume–rhizobium symbiosis are derived primarily from conserved features of Inverted-Repeat Lacking Clade (IRLC) legumes and closely related species. The Dalbergioids diverged from the IRLC early in legume evolution and possess unique symbiotic features but few genetically tractable models. The small, diploid dalbergioid Aeschynomene americana (American jointvetch) has promise as a genetic model for Dalbergioid-rhizobia symbiosis, yet only a few studies have examined its symbiotic properties. We examined the symbiont range of A. americana from central Florida and characterized a native A. americana nodule isolate, Bradyrhizobium sp. USDA3516. We find that A. americana forms effective symbioses with Bradyrhizobium sp. USDA3516, which is closely related to Thai A. americana symbiont Bradyrhizobium sp DOA9, and with symbionts from the dalbergioids stylo and peanut. Interestingly, several strains that effectively nodulated A. americana exhibited branched bacteroid morphologies, but we found that branching was neither necessary nor sufficient for effective symbiosis. Our study contradicts the prevailing view that bacteroid shape is a major determinant of symbiotic efficiency and presents the A. americana–Bradyrhizobium sp. USDA3516 interaction as an optimal model of A. americana symbiosis.
Carlew et al. (Mon,) studied this question.