Abstract. Two ferredoxin-dependent proteins, nitrite reductase and glutamate synthase, play a role in nitrate assimilation during the anaerobic germination of rice (Oryza sativa L.). This paper reports the expression of the root form of ferredoxin-NADP + oxidoreductase (FNR), the protein responsible for providing reduced ferredoxin in rice coleoptiles. Using an antibody against FNR, a protein with the expected molecular mass for root FNR (35 kDa) was recognized by Western blot analysis in extracts from aerobic and anaerobic coleop- tiles. The enzyme is synthesized de novo, as shown by immunoprecipitation of the radiolabeled 35-kDa protein from anaerobic seedlings grown in the presence of [ 35 S]methionine. Northern blot analysis with speci®c probes for root and leaf FNR showed the presence of mRNA for the root form but not for the leaf form, in both aerobic and anaerobic rice coleoptiles. The induc- tive eect of exogenous nitrate on the expression of FNR is further evidence for the presence of the root type of FNR in rice coleoptiles. The importance of the expression of root FNR during the anaerobic develop- ment of rice seedlings is discussed. Key words: Anoxia ± Coleoptile ± Ferredoxin-NADP + oxidoreductase ± Light ± Nitrate ± Oryza Introduction Ferredoxin (Fd)-NADP + oxidoreductase (FNR) was originally puri®ed and demonstrated to be a component involved in NADP + photoreduction during oxygenic photosynthesis (Shin et al. 1963). The leaf enzyme was puri®ed, characterized and the amino acid sequences described for spinach (Karplus et al. 1984; Jansen et al. 1988; Kna and Hirasawa 1991), pea (Newmann and Gray 1988) and ice plant (Michalowski et al. 1989) as well as for other photosynthetic organisms. Recently, FNR was puri®ed from nonphotosynthetic tissues of higher plants (Hirasawa et al. 1990; Wada et al. 1989; Morigasaki et al. 1990a,b; Green et al. 1991; Bowsher et al. 1993; Morigasaki et al. 1993). In nongreen tissues, the activity of FNR represents an essential link between the oxidative pentose-phosphate pathway and the for- mation of reduced Fd (Bowsher et al. 1989). The function of this enzyme in heterotrophic tissues was related to nitrate assimilation, since the reduced Fd could serve as an electron donor for the reduction of nitrite to ammonia (catalyzed by nitrite reductase, NiR), and to glutamate synthesis (catalyzed by Fd-dependent glutamate synthase, Fd-GOGAT) (Bowsher et al. 1992). The primary sequences for higher-plant FNR, both from leaves and roots, were obtained mainly from dicotyledonous species. Then, the two sequences of the full-length cDNA encoding the leaf and the root FNRs from rice were published (Aoki and Ida 1994; Aoki et al. 1994). The molecular weights of the two isoenzymes were very similar, being 34 and 35 kDa for the leaf and root forms, respectively. Nevertheless, there was shown to be only 49% homology between the rice leaf and root FNRs, while the rice leaf enzyme shared more than 80% sequence identity with the photosynthetic FNRs from higher plants (Aoki and Ida 1994). The root FNR transcript was reported to accumulate in response to nitrate ions in roots of maize and rice (Aoki and Ida 1994; Ritchie et al. 1994). Moreover, nitrate was sucient for the expression of the root FNR gene while the leaf FNR gene was induced by light without nitrate (unpublished results). In previous papers, we reported the ability of rice to assimilate nitrate during anaerobic germination (Matt- ana et al. 1993; Reggiani et al. 1993a,b). Nitrate was reduced and its nitrogen incorporated into amino acids by the coleoptile, which is a nonphotosynthetic tissue (Reggiani et al. 1995). All the enzymes required for Planta (1997) 202: 397±401 The root form of ferredoxin-NADP 1 oxidoreductase is expressed in rice coleoptiles for the assimilation of nitrate Monica Mattana 1 , Shoji Ida 2 , Remo Reggiani 1 1 Istituto Biosintesi Vegetali, C.N.R., via Bassini 15, I-20133 Milano, Italy 2 Research Institute for Food Science, Kyoto University, Uji, Kyoto 611, Japan Received: 7 October 1996 / Accepted: 22 January 1997 Abbreviations: Fd = ferredoxin; FNR = ferredoxin-NADP + ox- idoreductase; GOGAT = glutamate synthase; NiR = nitrite re- ductase Correspondence to: R. Reggiani; Fax: 39 (2) 23699411; E-mail: reggiani@ibvh10.ibv.mi.cnr.it