Endophytic N-Fixation: Controversy and a Path Forward

Abstract

Biological nitrogen fixation (BNF), the conversion of atmospheric dinitrogen gas into organic forms of N, is achieved by the microbial nitrogenase enzyme complex. Symbiosis with N-fixing (diazotrophic) bacteria can provide plants with sufficient levels of this essential macronutrient for growth. While the symbioses between rhizobia and legumes as well as between Frankia and actinorhizal plants are well-known, BNF in root nodules by these bacteria is not the only way in which plants can procure fixed N. Diazotrophic endophytes, microorganisms within the plant host, can also provide significant levels of organic N and have a much broader host range than nodulating bacteria. While there is great potential for improving the sustainability of agriculture using diazotrophic endophytes, research has been limited due to the strong belief that only nodulating bacteria can alleviate host plant N-deficiency. This review describes the arguments and summarizes the data in support of the significance of endophytic N-fixation. It then proposes research directions with the aim of optimizing endophyte-host interactions for reduced reliance on chemical fertilizers.