In rice roots, the mRNA and protein for NADH-GOGAT increase dramatically within 12 h of supplying a low concentration (<50 mM) of ammonium ions.25,26 Immunolocal-ization studies clearly showed that two cell layers of the root surface, i.e., epidermis and exodermis, are responsible for the rapid increase in NADH-GOGAT protein.27 Recent anatomical studies indicate that there is a Casparian strip between the epidermis and the sclerenchyma cells, which locate one cell layer outside the first layer of cortex in rice roots,28 indicating that solute transport between these cell types should be a symplastic process. Because cytosolic root GS is also localized in these two cell layers, NADH-GOGAT could have a responsibility to supply glutamate to the GS reaction for the rapid assimilation of ammonium ions in these two cells. Our recent
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