Enhanced extracellular ammonium release in the plant endophyte Gluconacetobacter diazotrophicus through genome editing

Abstract

The plant growth-promoting bacterium Gluconacetobacter diazotrophicus was originally discovered in association with sugarcane plants as an endophyte. As a member of the small class of organisms defined as diazotrophs, G. diazotrophicus is capable of fixing nitrogen from the atmosphere and could serve an important role in minimizing the requirements for nitrogen from industrial-derived fertilizers. In addition to sugarcane, G. diazotrophicus is capable of forming endophyte associations with a variety of other important crops. It has been reported that this microbe requires micro-aerobic conditions to effectively fix nitrogen gas from the atmosphere through the enzyme nitrogenase, making it slightly more difficult to study the diazotrophic lifestyle in the laboratory. The ability of the strain to reside within the plant during growth means that any extracellular nitrogen released by this microbe would immediately become available to the plant host. For this reason, it is an ideal target for development as an improved biofertilizer strain. In this work, we constructed strains of G. diazotrophicus that result in enhanced ammonium release, as measured by growing with a closely associated algal strain under micro-aerobic conditions, and by further quantifying ammonium concentrations accumulated under micro-aerobic and aerobic growth. IMPORTANCE Our results demonstrate increased extracellular ammonium release in the endophyte plant growth-promoting bacterium Gluconacetobacter diazotrophicus . Strains were constructed in a manner that leaves no antibiotic markers behind, such that these strains contain no transgenes. Levels of ammonium achieved by cultures of modified G. diazotrophicus strains reached concentrations of approximately 18 mM ammonium, while wild-type G. diazotrophicus remained much lower (below 50 µM). These findings demonstrate a strong potential for further improving the biofertilizer potential of this important microbe.