Wheat growth promotion through inoculation with an ammonium-excreting mutant of Azospirillum brasilense

Azospirillum brasilense is a diazotrophic bacterium and one of the best studied plant-growth-promoting bacterium living in close association with several agronomically important crops. The production of plant-growth-regulating substances is a main mechanism of plant growth stimulation, although other mechanisms have also been proposed. Nitrogen transfer from the bacterium to the plant is one among the other possible mechanisms of plant growth stimulation. In this study, we investigated, by means of a greenhouse trial with winter wheat inoculation, the effect of a point mutation in the ammonium binding site of the A. brasilense glutamine synthetase. The glutamine synthetase is one of the main ammonium-assimilating enzymes and mutations in this enzyme generally result in the release of ammonium from the bacterium to its environment. The ammonium-excreting mutant used in this study was shown to perform better than the wild-type A. brasilense strain with respect to wheat growth parameters and yield. In the greenhouse conditions used, this effect was independent of the way fertilizer was applied.