Cytosolic and chloroplastic glutamine synthetase of sugarbeet (Beta vulgaris) respond differently to organ ontogeny and nitrogen source

Changes in the activity and subunit composition of cytosolic glutamine synthetase (GS 1; EC 6.3.1.2) and chloroplastic GS (GS 2) were studied in response to an internal (organ ontogeny) and external signal (N source: NO 3 − or NH 4 + ). Maximum GS 1 activity of all organs examined was measured in the fibre roots, irrespective of the N source. The response of GS 1 to the N source was, however, organ specific. In the fibre roots, NH 4 + nutrition resulted in a 2‐ to 7‐fold (based on protein or freshweight, respectively) increase of GS 1 activity compared to NO 3 − ‐grown plants. In contrast to the roots, GS 1 activity in the leaf blades was 2‐fold lower with NH 4 + nutrition, whereas only minor changes occurred in the petioles. GS 2 activity was highest in the mature and senescing leaf blade; activity was 2‐fold higher with NH 4 + than with NO 3 − nutrition. Not only activity, but also subunit composition of GS 1 changed during organ ontogeny as well as in response to the N source. In contrast to GS 1, only minor changes were evident in GS 2 subunit composition, despite significant changes in GS 2 activity. Up to 5 different GS 1 subunits of ≈41–43 kDa were separated; they were identical in all organs examined. GS 2 was composed of 4 different subunits of ≈48 kDa.