A new Rhizobium species that nodulates PhaseoZus vulgaris L. and Leucaena spp. is proposed on the basis of the results of multilocus enzyme electrophoresis, DNA-DNA hybridization, an analysis of ribosomal DNA organization, a sequence analysis of 16s rDNA, and an analysis of phenotypic characteristics. This taxon, Rhizobium tropici sp. nov., was previously named Rhizobium Zeguminosarum biovar phaseoli (type I1 strains) and was recognized by its host range (which includes Leucaena spp.) and nifgene organization. In contrast to R. Zeguminosarum biovar phaseoli, R. tropici strains tolerate high temperatures and high levels of acidity in culture and are symbiotically more stable. We identified two subgroups within R . tropici and describe them in this paper.Members of the genus Rhizobium nodulate the roots of leguminous plants. The rhizobia that infect peas, clovers, and beans (Phaseolus vulgaris L.) are clustered in a single species, Rhizobium leguminosarum (29), which has three biovars (Rhizobium leguminosarum biovar viciae, Rhizobium leguminosarum biovar trifolii, and Rhizobium leguminosarum biovar phaseoli); these biovars contain different symbiotic plasmids that encode distinct nodulation specificities. Nevertheless, heterogeneity in Rhizobium leguminosarum biovar phaseoli has been identified by using such different criteria as protein pattern (50), antibiotic resistance (2), serological type (49), multilocus enzyme electrophoresis behavior (45), DNA-DNA hybridization data (10, 26, 54), plasmid profile (37) , and exopolysaccharide structure (70).We previously distinguished two different types among isolates obtained from bean nodules and found differences in their symbiotic plasmids (36, 38, 39). Type I strains have multiple copies of nitrogenase n i m genes (39, 46), a narrow nodulation host range, and hybridize with the psi (polysaccharide inhibition) gene (3). Type I1 strains have single copies of nifgenes, nodulate Leucaena spp., and do not hybridize with the psi gene (36, 39).Type I1 strains have received attention because their symbiotic plasmids promote an effective and completely differentiated symbiotic process in Agrobacterium tumefaciens recipients (5, 38). They are genetically stable, retaining their symbiotic plasmid after prolonged incubation at 37°C. Some are heat tolerant (31) or acid and aluminum resistant (12, 25, 30, 62). The nodulation genes from one of these strains have been cloned (64). The chemical composition and structure of the extracellular polysaccharides from one type I1 strain differ from the chemical composition and structure of the extracellular polysaccharides from type I isolates (23).Type I1 strains have been less successful in competition for bean nodule occupancy than the type I strains used (41). The former have been reported to occur less frequently in * Corresponding author bean nodules (39). Nodule occupancy by type I1 strains can be enhanced under acid conditions (47, 63).To define the taxonomic position and the genetic relatedness of type I1 strains, we analyzed 64 typ...

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Screening of Rhizobium leguminosarum bv. phaseoli strains showed some that were able to nodulate common beans (Phaseolus vulgaris L.) at high temperatures (35 and 38°C/8h/day). The nodulation ability was not related to the capability to grow or produce melanin-like pigment in culture media at high temperatures. However, nodules formed at high temperatures were ineffective and plants did not accumulate N in shoots. Two thermal shocks of 40°C/8 h/day at flowering time drastically decreased nitrogenase activity and nodule relative efficiency of plants otherwise grown at 28°C. Recovery of nitrogenase activity began only after seven days, when new nodules formed; total incorporation of N in tops did not recover for 2 weeks. Non-inoculated beans receiving mineral N were not affected by the thermal shock, and when growing continuously at 35 or 38°C had total N accumulated in shoots reduced by only 18%.