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 (snf1-5g) mutant, in which the formation of symbiotic nodules is deregulated, 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 chimeric Ca2+/calmodulin dependent protein kinase whose activity results in this `gain of function′ phenotype of spontaneous nodule formation. Furthermore the snf1-5g gene is shown to confer a spontaneous nodule formation phenotype to plants having a nodulation deficient genetic background. A gene of the invention, that confers a spontaneous nodulation phenotype, has utility for the transfer and establishment of nitrogen fixing capability in non-nodulating plants, and thereby reducing the nitrogen fertilizer dependence of non-nodulating crop plants.