The molecular and genetic manipulation of nitrogen fixation

Abstract

The fundamental importance of dinitrogen fixation for world agriculture, in relation to projected energy supplies, population pressure and food requirements over the next decades, obliges scientists to reconsider ways of exploiting this biological process. Genetic manipulation offers several options in principle. Existing symbiotic systems such as the legumes and seemingly inefficient systems such as the grass associations could be improved; new symbioses could be developed by nif gene transfer to rhizosphere commensals or by somatic hybridization of appropriate plants. A major advance would be to render plants independent of microbes by manipulation of expressable nif into the plant genome. This goal is discussed. It requires the complete genetic and physical characterization of nif in particular its regulation, and an understanding of the physiological background within which nif can be expressed, as well as the ability to fuse nif to alien genetic systems. Substantial progress in these directions has been made by using the n if genes of Klebsiella pneumoniae this progress is reviewed. Strategies for the further manipulation of nif towards regulated expression in the plant genome are considered.