Ammonium Assimilation and Metabolism in Rice

Rice sustains the world. It is a staple food in most Asian countries. Both a high yield and healthy growth of rice are highly dependent on nitrogen fertilizer. Ammonium is the most important nitrogen source for rice growth. Rice is grown in paddy fields in which nitrification is inhibited due to anaerobic conditions. Ammonium transporters (AMTs) transport ammonium from the soil into cells in plant roots. Glutamine synthetase (GS) ligates ammonium with glutamate in an ATP-dependent manner and is a key enzyme of ammonium assimilation. Glutamate synthase (GOGAT) transfers the amide residue of glutamine to 2-oxoglutarate to yield two molecules of glutamate. As GS and GOGAT provide substrates for each other, their reaction is coupled. The GS/GOGAT cycle is the major pathway in ammonium assimilation in plants, and the rice genome encodes several isozymes for GS/GOGAT. In this chapter, we describe the physiological functions and “job-sharing” of three cytosolic GS isoenzymes (GS1;1, GS1;2, and GS1;3) and two NADH-GOGAT enzymes (NADH-GOGAT1 and NADH-GOGAT2) using both a reverse genetic and a biochemical approach.