Colonization of Brachypodium distachyon by Gluconacetobacter diazotrophicus and its effect on plant growth promotion

Abstract

Gluconacetobacter diazotrophicus is a nitrogen (N) fixing bacterium originally isolated from sugarcane in Brazil. Understanding the interactions between this bacterium and plants is important to exploit the nitrogen-fixing feature of G. diazotrophicus in different crops. Research was conducted to establish, monitor and optimize methods for introducing G. diazotrophicus into Brachypodium distachyon, a model for molecular research involving monocot plants. Colonization was studied in three plant growth media including liquid, semi-solid, and solid media. The effect of mineral nitrogen on colonization was also studied in each of these three media. The colonization of B. distachyon by G. diazotrophicus was analyzed by a histochemical β-glucuronidase (GUS) assay, PCR using G. diazotrophicus specific primers and bacterial re-isolation. Colonization was established in all tested plant growth conditions. The highest level of colonization was achieved in the liquid media without mineral nitrogen. Microscopic observations showed that G. diazotrophicus colonizes B. distachyon through the lateral root emergence sites, root hairs, and root tips. The colonization was found to be limited to root tissues and not found in other organs. These results indicate that B. distachyon roots are the primary organ of interaction and colonization by the bacterium. Plant fresh weight and lateral root numbers were significantly increased in inoculated plants indicating that G. diazotrophicus can stimulate growth of B. distachyon. This study indicates that B. distachyon is a useful model to study the mechanisms of G. diazotrophicus colonization and growth enhancement in monocot plants.