High-affinity glutamine synthetase which catalyzes the synthesis of glutamine from ammonium and glutamate (PubMed:14757761). May contribute to the homeostatic control of glutamine synthesis in roots. ATP + L-glutamate + NH4(+) = ADP + H(+) + L-glutamine + phosphate The KM value for ammonium is smaller than 10 uM. Measured at pH 7.8 and 30 degrees Celsius for all experiments. Homooctamer (By similarity). Interacts with CRK3 and GRF3. Expressed in root tips, root hairs and epidermis. Ubiquitously expressed with higher levels in siliques and roots. By nitrogen deprivation, sucrose, glucose and fructose. Down-regulated by ammonium supply. Induced during leaf senescence. Phosphorylated by CRK3. Belongs to the glutamine synthetase family.