Azoarcus sp. strain BH72 as a model for nitrogen-fixing grass endophytes

The availability of nitrogen often limits plant growth in terrestrial ecosystems. The only biological reaction counterbalancing the loss of N from soils or ecosystems is biological nitrogen fixation, the enzymatic reduction of N2 to ammonia. Some gramineous crops such as certain Brazilian sugar cane cultivars or Kallar grass can derive a substantial part of the plant nitrogen from biological nitrogen fixation. Our research on grass-associated diazotrophs focuses on endophytic bacteria, microorganisms that multiply and spread inside plants without causing damage of the host plants or conferring an ecological threat to the plant. This review summarizes the current knowledge on the diazotrophic endophyte Azoarcus sp. BH72, which is capable of colonizing the interior of rice roots, one of the globally most important crops.