Nitrate assimilation under anoxia in rice

Anaerobic nitrate assimilation was studied in rice seedlings. Incorporation of (NO3)-N-15 into amino acids was detected both in 3-day-old aerobic rice seedlings subjected for 24 h to anaerobic conditions and in anaerobically germinated seedlings. The combined incorporation of N-15 into glutamate, glutamine, and alanine accounted for 89% and 85% ofits total incorporation into the coleoptile and root, respectively, in aerobic seedlings transferred to anoxia, while other amino acids were heavily labeled in anaerobically germinated seedlings. The specific activities of the enzymes involved in the nitrate assimilation (nitrate reductase, NR; nitrite reductase, NiR;,glutamine synthetase, GS; ferredoxin-dependent glutamate synthase, Fd-GOGAT; Fd-NADP oxidoreductase, FNR) were found in the anaerobic coleoptile. All these activities, with the exception of GS, increased in the presence of exogenous nitrate (5 mM). The Northern blot analysis revealed the presence of mRNAs for NR, NiR, cytosolic GS, Fd-GOGAT, and root FNR. Nitrate did not exertany effect on the transcript level of cytosolic GS. These data indicate that, in rice, the nitrate assimilation pathway is operative under anoxia.