Biochemistry Division, Indian Agricultural Research Institute, New Delhi
Online published on 25 January, 2012.
Nitrogenase and nitrate reductase enzyme complexes contain molybdenum as a functional element and it has been suggested that in Rhizobium, common genetic determinants may be present for the two enzymes (Kondorosi, A., Barabas, I., Svab, Z., Oroiz, L., Sik, T., and Hotchkiss, R. D. (1973). Nature, (New Biology), 246,153–154). The reducing power required for the assimilation of nitrate as well as for nitrogen fixation in plants is supplied in the final ‘ analysis by photosynthetic reactions. The end product of both these processes is ammonia, which is a represser of nitrogenase synthesis.
In the leaves of higher plants, activity of nitrate reductase is very high as compared to that in the roots. The root nodules of legumes can reduce nitrate as well molecular nitrogen. However, it is known that if sufficient combined nitrogen is available in the soil, nitrogenase synthesis is repressed.
The interaction of these two vital processes which supply essential nitrogen for crops is discussed. The two pathways compete for reductants generated by photosynthetic reactions. The enzymes probably have common molybdenum-containing sub units as well as common genetic determinants. Genes for nitrate reductase are very active in the leaves of higher plants but genes for nitrogenase are not present or have not been discovered.
