NITRATE ASSIMILATION UNDER ANOXIA IN RICE

Anaerobic nitrate assimilation was studied in rice seedlings. Incorpor ation of (NO3)-N-15 into amino acids was detected both in 3-day-old ae robic rice seedlings subjected for 24 h to anaerobic conditions and in anaerobically germinated seedlings. The combined incorporation of N-1 5 into glutamate, glutamine, and alanine accounted for 89% and 85% of its 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 a ctivities of the enzymes involved in the nitrate assimilation (nitrate reductase, NR; nitrite reductase, NiR;,glutamine synthetase, GS; ferr edoxin-dependent glutamate synthase, Fd-GOGAT; Fd-NADP oxidoreductase, FNR) were found in the anaerobic coleoptile. All these activities, wi th 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 exert any effect on the transcript level of cytosolic GS. These data indicat e that, in rice, the nitrate assimilation pathway is operative under a noxia.