The expression of an exogenous ACC deaminase by the endophyte Serratia grimesii BXF1 promotes the early nodulation and growth of common bean

Ethylene acts as an inhibitor of the nodulation process of leguminous plants. However, some bacteria can decrease deleterious ethylene levels by the action of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase which degrades ACC, the ethylene precursor in all higher plants. Co-inoculation of rhizobia with endophytes enhances the rhizobial symbiotic efficiency with legumes, improving both nodulation and nitrogen fixation. However, not much is understood about the mechanisms employed by these endophytic bacteria. In this regard, the role of ACC deaminase from endophytic strains in assisting rhizobia in this process has yet to be confirmed. In this study, the role of ACC deaminase in an endophyte's ability to increase Rhizobium tropici nodulation of common bean was evaluated. To assess the effect of ACC deaminase in an endophyte's ability to promote rhizobial nodulation, the endophyte Serratia grimesii BXF1, which does not encode ACC deaminase, was transformed with an exogenous acdS gene. The results obtained indicate that the ACC deaminase-overexpressing transformant strain increased common bean growth, and enhanced the nodulation abilities of R. tropici CIAT899, in both cases compared to the wild-type non-transformed strain. Furthermore, plant inoculation with the ACC deaminase-overproducing strain led to an increased level of plant protection against a seed-borne pathogen. Significance and Impact of the StudyIn this work, we studied the effect of ACC deaminase production by the bacterial endophyte Serratia grimesi BXF1, and its impact on the nodulation process of common bean. The results obtained indicate that ACC deaminase is an asset to the synergetic interaction between rhizobia and the endophyte, positively contributing to the overall legume-rhizobia symbiosis by regulating inhibitory ethylene levels that might otherwise inhibit nodulation and overall plant growth. The use of rhizobia together with an ACC deaminase-producing endophyte is, therefore, an important strategy for the development of new bacterial inoculants with increased performance.