EXPRESSION OF NADH-DEPENDENT GLUTAMATE SYNTHASE PROTEIN IN THE EPIDERMIS AND EXODERMIS OF RICE ROOTS IN RESPONSE TO THE SUPPLY OF AMMONIUM-IONS

The mRNA and protein for NADH-dependent glutamate synthase (NADH-GOGAT ; EC 1.4.1.14) in root tips of rice (Oryza sativa L. cv. Sasanishiki) plants increases dramatically within 12 h of supplying a low concentra tion (> 0.05 mM) of ammonium ions (T. Yamaya et al., 1995, Plant Cell Physiol 36: 1197-1204). To identify the specific cells which are respo nsible for this rapid increase, the cellular localization of NADH-GOGA T protein was investigated immunocytologically with an affinity-purifi ed anti-NADH-GOGAT immunoglobulin G. When root tips (> 1 mm) of rice s eedlings which had been grown for 26 d in water were immune-stained, s ignals for the NADH-GOGAT protein were detected in the central cylinde r, in the apical meristem, and in the primordia of the secondary roots . Signals for ferredoxin-dependent GOGAT CFd-GOGAT; EC 1.4.7.1) protei n were also seen in the same three areas. When the roots were supplied with 1 mM ammonium ions for 24 h, there were strong signals for the N ADH-GOGAT protein in two cell layers of the root surface, i.e. epiderm is and exodermis, in addition to the cells giving signals in the absen ce of ammonium ions. The supply of ammonium ions was less effective on the profile of signals for Fd-GOGAT. Although the supply of ammonium ions had less effect on the expression of cytosolic glutamine syntheta se (GS; EC 6.3.1.2), this enzyme was also found to be located in the e pidermis and exodermis, as well as in the central cylinder and cortex. The results indicate that NADH-GOGAT, coupled to the cytosolic GS rea ction, is probably important for the assimilation of ammonium ions in the two cell layers of the root surface.