Rhizobium spp.,Sesbania Rostrata,Leguminous Plants,Azorhizobium Caulinodans,Rhizobium Cells,Bacterial Cells,Nitrogenase

	Rhizobium spp. - These are Gram negative soil bacteria capable of forming root nodules in most leguminous plants and some nonleguminous plants. But Azorhizobium caulinodans nodulates the stems of Sesbania rostrata. The root nodulating rhizobia are divided into several species depending upon the legume species they are able to nodulate, e.g., R. leguminosarum nodulates pea (Pisum sativum), khesari (Lathyrus sativus) and lentil (Lens culinaris); R. phaseoli nodulates kidney/French bean (Phaseolus vulgaris); R. meliloti, nodulates lucerne (Medicago sativa) and fenugreek (Trifolium foenumgraecum), etc.
Rhizobium cells contain genes for nitrogen fixation (nif genes) on a megaplasmid. The bacteria enter the roots through root hairs, the interaction being highly specific and progressing through several steps; it ultimately results in nodule, formation. Many genes of Rhizobium as well the host legume are involved in the infection process. Inside the nodule, many bacterial cells change into nondividing bacterioids, which produce nitrogenase, the enzyme which reduces atmospheric nitrogen into ammonia. Nitrogenase is highly sensitive to 02; it is protected from 02 by the pink pigment leghaemoglobin, which binds to 02, produced by the legume and present in the nodules.
Different strains of a Rhizobium species differ in their ability to fix nitrogen; this trait is also affected by the genes of host legumes as well. Therefore, extensive screening for efficient N2-fixers is undertaken (strain development). The energy and carbon requirements of the bacteria are provided for by the host legume. In return, the ammonium produced by the bacterial cells is made available to the host. The estimated N2 fixed by Rhizobium species ranges from 50 to 150 kg/ha or even more, especially in the case of clovers. Field trials in India suggest a 10-15% yield increase in inoculated pigeonpea and chickpea over the uninoculated controls.