A prospective study of short-term apoplastic responses to ammonium treatment.

The integration of external stimuli into plant cells has been extensively studied. Ammonium is a metabolic trigger because it affects plant nutrition status; on the contrary, it is also a stress factor inducing oxidative changes. Plants, upon quick reaction to the presence of ammonium, can avoid the development of toxicity symptoms, but their primary ammonium sensing mechanisms remain unknown. This study aimed to investigate the different signaling routes available in the extracellular space in response to supplying ammonium to plants. During short-term (30 min-24 h) ammonium treatment of Arabidopsis seedlings, no indication of oxidative stress development or cell wall modifications was observed. However, specific changes in reactive oxygen species (ROS) and redox status were observed in the apoplast, consequently leading to the activation of several ROS (RBOH, NQR), redox (MPK, OXI), and cell-wall (WAK, FER, THE, HERK) related genes. Therefore, it is expected that immediately after ammonium supply, a defense signaling route is initiated in the extracellular space. To conclude, the presence of ammonium is primarily perceived as a typical immune reaction.