ammonium assimilation occurs via glutamine synthetase EC 6.1.1.3 and enzyme, regardless of leaf age. L-glutamate is continuously synthesized and supplied to the phloem. Enzyme is localized to the phloem companion cells-sieve element complex in the leaf veins
enzyme is active both in light and dark. In transgenic leaf disks expressing antisense enzyme mRNA, low level of enzyme limits the synthesis of glutamate. In the dark, low level of enzyme in transgenic leaves is not the main limiting factor of ammonium assimilation
existence of the enzyme in the companion cells, which are adjacent to the sieve elements, of the central cylinder in rice roots is advantageous for the access and efficient utilization of glutamine and carbohydrate transported through the sieve elements under anaerobic conditions
in etiolated seedling, enzyme activity doubles within 1 h upon exposure to light, and a further 4fold increase in comparison with dark value is reached within 5 h