Legume Root Nodule Associated Bacteria

Abstract

Root nodules have intrigued mankind ever since their role in the maintenance of soil fertility has been known. The earlier school of thought amongst microbiologists and agronomists was that root nodules are highly specialised structures rich in leghaemoglobin, which house the diazotrophic bacterium Rhizobium, whose primary role was to fix atmospheric nitrogen in association with the host plant. But several path-breaking discoveries over the past few decades have thrown light on the plethora of bacterial occupants of the root nodules and their possible role in nodulation and N fixation besides several other beneficial roles. Recent technological advances in bacterial taxonomy and microbial ecology have unearthed a wide range of microbial nodule occupants, some of which have been encompassed under the classical umbrella of rhizobia, purely based on their ability to nodulate the host and fix atmospheric nitrogen, while other closely or even distantly related bacterial genera devoid of the ability to nodulate and fix nitrogen in nodules are often referred to as endophytes or simply nodule inhabitants. This chapter attempts to capture the existing knowledge on the root nodule associated bacteria both rhizobial and non-rhizobial and their possible roles in sustaining plant growth.