Biochemical background and compartmentalized functions of cytosolic glutamine synthetase for active ammonium assimilation in rice roots

Rice plants in paddy fields prefer to utilize ammonium as a major nitrogen source. Glutamine synthetase (GS) serves for assimilation of ammonium in rice root, and ameliorates the toxic effect of ammonium excess. Among the three isoenzymes of the cytosolic GS1 gene family in rice, OsGLN1;1 and OsGLN1;2 were abundantly expressed in roots. Analysis of the purified enzymes showed that OsGLN1;1 and OsGLN1;2 can be classified into high-affinity subtypes with relatively high V-max values, as compared with the major high-affinity isoenzyme, GLN1;1, in Arabidopsis. Low-affinity forms of GS1 comparable to those in Arabidopsis (GLN1;2 and GLN1;3) were absent in rice roots. The OsGLN1;1 and OsGLN1;2 transcripts showed reciprocal responses to ammonium supply in the surface cell layers of roots. OsGLN1;1 accumulated in dermatogen, epidermis and exodermis under nitrogen-limited condition. By contrast, OsGLN1;2 was abundantly expressed in the same cell layers under nitrogen-sufficient conditions, replenishing the loss of OsGLN1;1 following ammonium treatment. Within the central cylinder of elongating zone, OsGLN1;1 and OsGLN1;2 were both induced by ammonium, which was distinguishable from the response observed in the surface cell layers. The high-capacity Gin synthetic activities of OsGLN1;1 and OsGLN1;2 facilitate active ammonium assimilation in specific cell types in rice roots.