Small-subunit rRNA genotyping of rhizobia nodulating Australian Acacia spp.

The structure of rhizobial communities nodulating Acacia in southeastern Au stralia from south Queensland to Tasmania was investigated by a molecular a pproach. A total of 118 isolates from nodule samples from 13 different Acac ia species collected at 44 sites were characterized by small-subunit (SSU) ribosomal DNA (rDNA) PCR-restriction fragment length polymorphism analysis. Nine rhizobial genomospecies were identified, and these taxa corresponded to previously described genomospecies (B. Lafay and J. J. Burden, Appl. Env iron. Microbiol. 64:3989-3997, 1998). Eight of these genomospecies belonged to the Bradyrhizobium lineage and accounted for 96.6% of the isolates. The remaining genomospecies corresponded to Rhizobium tropici. For analysis of geographic patterns, results were grouped into five latitudinal regions re gardless of host origin. In each region, as observed previously for rhizobi al isolates taken from non-Acacia legumes (Lafay and Burden, Appl. Environ. Microbiol. 64:3989-3997, 1998), rhizobial communities were dominated by on e or two genomospecies, the identities of which varied from place to place. Despite this similarity in patterns, the most abundant genomospecies for A cacia isolates differed from the genomospecies found in the non-Acacia-deri ved rhizobial collection, suggesting that there is a difference in nodulati on patterns of the Mimosoideae and the Papilionoideae. Only two genomospeci es were both widespread and relatively abundant across the range of sites s ampled. Genomospecies A was found in all regions except the most northern s ites located in Queensland, whereas genomospecies B was not detected in Tas mania. This suggests that genomospecies A might be restricted to the more t emperate regions of Australia, whereas in contrast, genomospecies B occurs in different climatic and edaphic conditions across the whole continent. Th e latter hypothesis is supported by the presence of genomospecies B in sout hwestern Australia, based on partial SSU rDNA sequence data (N. D. S, Marsu di, A. R. Glenn, and M. J. Dilworth, Soil Biol. Biochem. 31:1229-1238, 1998 ).