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: NO3- or NH4+). 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, NH4+ nutrition resulted in a 2- to 7-fold (based on protein or freshweight, respectively) increase of CS 1 activity compared to NO3--grown plants. In contrast to the roots, GS I activity in the leaf blades was 2-fold lower with NH4+ nutrition, whereas only minor changes occurred in the petioles. CS 2 activity was highest in the mature and senescing leaf blade; activity was 2-fold higher with NH4+ than with NO3- 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 approximate to 41-43 kDa were separated; they were identical in all organs examined. GS 2 was composed of 4 different subunits of similar to 48 kDa.