An investigation of N metabolism and pH regulation in Sphagnum using in vivo nuclear magnetic resonance and stable isotope mass spectrometry
Gerendás J., Heeschen V., Kahl S., Ratcliffe RG., Rudolph H.
Glutamine synthetase (GS) and glutamate dehydrogenase (GDH) are both present in Sphagnum and in principle both enzymes could be involved in the assimilation of ammonium. 15 NH 4 + application led to the formation γ- 15 N-glutamine, followed by α-amino glutamate/glutamine as observed by in vivo 15 N NMR. No assimilation was detected in the presence of the OS-specific inhibitor methionine sulfoximine. This labelling pattern suggests, that ammonium assimilation in Sphagnum is mediated solely by the GS/glutamate synthase pathway. The application of 15 N-labelled glutamate led to the liberation of 15 NH 4 + , indicating that GDH mainly catalyses the deamination of glutamate. It was possible to record good-quality in vivo 31 P NMR spectra of Sphagnum flexuosum tissue, and the chemical shift of the orthophosphate resonances indicated that cytoplasmic pH was not affected during the ammonium application, whereas the vacuolar pH was slightly reduced.