Rhizobium -Legume Nodulation: Life Together in the Underground Review

How good and how pleasant it is for organisms to dwell together in unity! And, probably, how common it is in the biological world. This review concerns a dramatic association, one of the few that has been studied in detail: the nitrogen fixing symbiosis between certain plants and microbes Rhizobium bacteria stimulate leguminous plants to develop root nodules, which the bacteria infect and inhabit. Ultimately, the two organisms establish metabolic cooperation: the bacteria reduce (fix) molecular nitrogen into ammonia, which they export to the plant for assimilation; the plant reduces carbon dioxide into sugars during photosynthesis and translocates these to the root where the bacteria use them as fuel. The plant family Leguminosae (Fabaceae) is the third largest family in the Angiosperms, spreads from the tropics to arctic regions, and includes forms varying from annual herbs to large trees. It doubtlessly owes at least some of this diversity and success to its ability to grow independently of often scarce soil nitrogen. Only one non-legume plant, Parasponia, has been found to form symbiotic root nodules with Rhizobium. The question of what makes the legumes unique is an important and provoking one. There is also considerable specificity of individual strains or species of Rhizobium for particular groups of plants, as shown in Table 1. The ecological and economic importance of nitrogen fixation has justly earned research attention for the Rhizobium-legume symbiosis. The system has an additional, fundamental attraction. During a complex series of developmental steps, the bacteria and the plant each influence in the other such fundamental activities as ceil division, gene expression, metabolic function, and cell morphogenesis. Analysis of the bacterial influence on these processes may lead to identification of otherwise elusive components that are parts of the indigenous plant systems for signal transduction, gene regulation, cell division, and cell wall formation. The driving forces for recent study of Rhizobium-plant symbioses include bacterial genetics, plant molecular biology, and detailed microscopy of the bacteria-plant interaction. This review highlights several questions of recent interest, with the focus on genetics and molecular biology; the references are representative, not exhaustive. A more complete view of the field can be found in two recent symposium volumes (Bothe et al., 1988; Verma and Palacios, 1988).