Respiratory burst oxidases and apoplastic peroxidases facilitate ammonium syndrome development in Arabidopsis

Ammonium-nitrogen (NH4+) nutrition is linked to metabolic over-reduction for plants. The characteristic symptom of sole NH4+ nutrition is growth suppression, signifying this condition as the ammonium syndrome. In the present study, we investigated the mechanism of perception of high NH4+ conditions in Arabidopsis thaliana plants by examining apoplastic reactive oxygen species (ROS) metabolism. Major enzyme activity and a special pattern of expression of NADPH-dependent respiratory burst oxidases (RBOH) was found in Arabidopsis individuals cultured under NH4+ as the sole nitrogen source. This oxidative burst is independent of RBOHD/F expression and does not activate typical intracellular signalling pathways. In addition, elevated superoxide dismutase and apoplastic secretory peroxidase activities contributed to hydrogen peroxide (H2O2) accumulation in plants exposed to NH4+ nutrition. Consequently, higher H2O2 contents were determined in the extracellular space and were localised cytochemically. H2O2 is a substrate for cell wall cross-linking peroxidases, which showed enhanced activity in the presence of NH4+. Increase of cell wall polymerisation, could in turn inhibit cell elongation and slow down growth, as observed under NH4+ toxicity