The Genus Rhizobium
Bacteria responsible for the formation of morphologically defined nodules on the roots of members of the family Leguminosae constitute the genus Rhizobium. The common capacity of such symbiosis to reduce dinitrogen to ammonia and to incorporate this produ
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The Genus Rhizobium JAMES M. VINCENT
Bacteria responsible for the formation of morphologically defined nodules on the roots of members of the family Leguminosae constitute the genus Rhizobium. The common capacity of such symbiosis to reduce dinitrogen to ammonia and to incorporate this product into the nitrogen metabolic stream of the host plant gives the genus a place of outstanding importance in natural ecosystems and agricultural production. Rhizobium and Agrobacterium are the two genera of the family Rhizobiaceae in the eighth edition of Bergey's Manual of Determinative Bacteriology (Jordan and Allen, 1974). Their distinction rests on the capacity of Rhizobium to nodulate a legume host, but there can be little doubt that, symbiotic behavior apart, there is closer relatedness between the fastgrowing rhizobia and Agrobacterium than there is between fast- and slow-growing forms of the present genus Rhizobium. Moreover, the presently defined species of Rhizobium obviously need fresh definition and leave many strains unspecified. This account deals chiefly with the Rhizobium bacterium, its ecology, and economic utilization. Features, uniquely or particularly, of the symbiosis will be found in two recent comprehensive accounts: The Biology of Nitrogen Fixation (Quispel, 1974) and A Treatise on Dinitrogen Fixation, in four sections under the general editorship of R. W. F. Hardy (1977-1978; Hardy and Silver, 1977). Wherever possible, citations in these two publications will be used to keep the present bibliography within reasonable limits.
HABITATS
Factors Determining Occurrence, Distribution, and Persistence Evidence of the natural occurrence of rhizobia chiefly depends on data on nodulation of the host plant. Most species of legume so far examined can nodulate, but about 70% of the subfamily Caesalpinioideae seem to be inherently resistant to M. P. Starr et al. (eds.), The Prokaryotes © Springer-Verlag Berlin Heidelberg 1981
a rhizobium-induced symbiosis (Allen and Allen, 1947, 1961, Lange, 1959, Grobbelaar et aI, 1964, Corby, 1971, cited by Vincent, 1974; Allen and Allen, 1976). Although rhizobia of a specific kind are likely to be found in soils where their appropriate host occurs naturally or has been long established, those with a different host requirement are likely to be absent. The soybean provides a good example of common initial absence of specific rhizobia; once introduced with the sown crop, it seems well able to establish itself as a member of the soil population. In Australia, although none of the rhizobia of its many indigenous legume species nodulates the introduced Trifolium species, those which came in with early plant material have spread widely with the hosts they nodulate. Rhizobial strains of a given locality are likely to be diverse, whether this is judged on their surface antigens or patterns of symbiotic effectiveness with different hosts (Purchase and Vincent, 1949, Purchase et al. 1951, cited by Vincent, 1974). There can be some broad geographical relationships in connection with surfac
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