Diversity and adaptability of soybean and cowpea rhizobia in tropical soils

The current interest on biodiversity, allied with the use of powerful molecular biology techniques, are revealing the nodulation abilities of tropical legumes, their preferences in terms of relationships with rhizobia, the dynamics of the rhizobia population, and data are accumulating on the cowpea group of tropical rhizobia. Studies on the ecology of cowpea-nodulating rhizobia in soils from the semi-arid region of Brazil, are providing an understanding of the ecology of soybean nodulating rhizobia in Brazilian soils and are revealing a large community of native strains capable of nodulating soybeans in soils where soybeans never have been planted. Many isolates showed a limited ability to nodulate soybean roots and formed nodule-like structures, or pseudo-nodules. Recent host range analysis of known rhizobia species has generated substantial data that is rapidly changing recognized patterns of symbiotic specificity. Cowpeas, soybeans and common beans nodulate with many different species of rhizobia, and the occurrence of both slow- and fast-growing strains in the same plant species has been reported for many genera of tropical legumes. This review will focus on recent data on the diversity of tropical rhizobia and their adaptability to tropical soils.