Formation of nitrogen fixing root nodules in legumes is induced by perception of lipochitin-oligosaccharide signal molecules secreted by compatible Rhizobium bacteria, which triggers a common symbiotic pathway. The present invention provides a spontaneous nodule formation (snf2) mutant, in which the formation of symbiotic nodules is spontaneous, leading to nodule development in the absence as well as in the presence of Rhizobium bacteria and/or exogenous rhizobial signals. The invention further provides an isolated DNA sequence encoding a mutant cytokinin-independent histidine kinase whose activity results in this `gain of function` dominant phenotype of spontaneous nodule formation. Furthermore the snf2 gene is shown to confer a phenotype, characterised by regulated organogenesis of spontaneous nodules, to plants having a nodulation deficient genetic background. A gene of the invention, that confers this spontaneous nodulation phenotype, has 15 utility for the transfer and establishment of nitrogen fixing capability in non-nodulating plants, and thereby reducing the nitrogen fertiliser dependence of non-nodulating crop plants.