Nitrate Assimilation by Plants

Abstract

Despite the fact that our knowledge of the biological reduction of nitrate has made considerable progress during the past decade, it appears that a number of important questions still remain open. These concern mainly the mechanism of nitrite reduction. The problem of the intermediates occurring between nitrite and ammonia and of the enzymes involved in their reduction has not yet been satisfactorily solved. In this connection it should also be stressed that the demonstration in vitro of enzymes capable of reducing certain inorganic nitrogen compounds does not necessarily mean that these enzymes also perform the same function in vivo. Thus some enzymes like aldehyde oxidase or xanthine oxidase which are able to reduce nitrate in vitro have nothing to do with nitrate reduction in vivo. Furthermore, a whole set of enzymes capable of reducing nitrate to ammonia has been obtained from a variety of animal tissues, although nitrate reduction does not normally play a significant role in animal metabolism. In addition, the possibility should still be kept in mind that the reduction of nitrite in vivo does not follow the inorganic pathway discussed so far, but that the N enters organic compounds before it has reached the level of reduction of ammonia. It has been known for a long time that nitrate, in addition to its role as a source of N, can serve in many bacteria as an electron acceptor for respiratory processes under anaerobic conditions. Some recent observations, however, seem to indicate that the so-called dis-similatory or respiratory reduction of nitrate might have a wider distribution among plants than was previously assumed. Thus Egami and co-workers were able to show that a dissimilatory reduction of nitrate to nitrite occurred in cotyledons of the legume Vigna sesquipedalis. After the demonstration by Kessler that the reduction of nitrite with molecular hydrogen in the green alga Ankistrodesmus braunii shows some resemblance to nitrate respiration, Czygan was able to separate the nitrite reductase system of this alga into an assimilatory and a dissimilatory component. Finally, the problem of the oxidation in inorganic N compounds in plants seems to deserve more interest in view of the demonstration by Cresswell and Hewitt that hydroxylamine is oxidized by peroxidase in extracts of higher plants. Similarly, a set of enzymes able to oxidize ammonia to nitrate has been obtained from yeast by Yamafuji and co-workers. There is a bibliography with 165 references.