Rhizobial Strategies to Enhance Symbiotic Interactions: Rhizobitoxine and 1-Aminocyclopropane-1-Carboxylate Deaminase

Nitrogen-fixing nodules are formed as a result of a series of interactions between rhizobia and leguminous plants. Bradyrhizobium elkanii produces rhizobitoxine, an enol-ether amino acid, which has been regarded as a phytotoxin because it causes chlorosis in soybeans. However, recent studies have revealed that rhizobitoxine plays a positive role in establishing symbiosis between B. elkanii and host legumes: rhizobitoxine enhances the nodulation process and nodulation competitiveness by inhibiting ethylene biosynthesis in host roots. In addition, the gene for 1-aminocyclopropane-1-carboxylate (ACC) deaminase was recently found in some rhizobia, such as Mesorhizobium loti, Bradyrhizobium japonicum and Rhizobium sp. ACC deaminase also facilitates symbiosis by decreasing ethylene levels in host roots. The cumulative evidence reveals general strategies by which rhizobia produce an inhibitor and an enzyme to decrease ethylene levels in host roots and thereby enhance nodulation. In this review, we compare these strategies and discuss how they function and have evolved in terms of genetics, biochemistry, and ecology. These rhizobial strategies might be utilized as tools in agriculture and biotechnology.