Lafay Bénédicte, Bullier Erika, Burdon Jeremy J
CSIRO Plant Industry, PO Box 1600, Canberra, ACT 2601, Australia.
UMR CNRS-IRD 2724, Centre IRD, 911, Avenue Agropolis - BP 64501, 34394 Montpellier Cedex 5, France.
Int J Syst Evol Microbiol. 2006 May;56(Pt 5):1013-1018. doi: 10.1099/ijs.0.63897-0.
Rhizobial bacteria almost exclusively nodulate members of the families Fabaceae, Mimosaceae and Caesalpiniaceae, but are found on a single non-legume taxon, Parasponia (Ulmaceae). Based on their host-range, their nitrogen-fixing ability and strain competition experiments, bacterial strains isolated from Parasponia were thought to constitute a separate lineage that would account for their exceptional host affinity. This hypothesis was investigated by focusing on four isolates that are representative of the morphological and cultural types of Parasponia-nodulating bradyrhizobia. Their evolutionary relationships with other rhizobia were analysed using 16S rRNA gene sequences and their nodulation properties were explored using the nodA gene as a proxy for host-range specificity. Phylogenetic analyses of the 16S rRNA and nodA gene sequences revealed that bacterial isolates from Parasponia species are embedded among other bradyrhizobia. They did not cluster together in topologies based on the 16S rRNA or nodA gene sequences, but were scattered among other bradyrhizobia belonging to either the Bradyrhizobium japonicum or the Bradyrhizobium elkanii lineages. These data suggest that the ability of some bradyrhizobia to nodulate species of the genus Parasponia does not represent a historical relationship that predates the relationship between rhizobia and legumes, but is probably a more recent host switch for some rhizobia.
根瘤菌几乎只与豆科、含羞草科和苏木科的植物形成根瘤，但也在一个非豆科分类单元——帕拉豆属（榆科）中被发现。基于它们的宿主范围、固氮能力和菌株竞争实验，从帕拉豆属植物中分离出的细菌菌株被认为构成了一个单独的谱系，这可以解释它们特殊的宿主亲和力。通过聚焦于四种代表帕拉豆属慢生根瘤菌形态和培养类型的分离株来研究这一假设。使用16S rRNA基因序列分析它们与其他根瘤菌的进化关系，并使用nodA基因作为宿主范围特异性的替代指标来探究它们的结瘤特性。对16S rRNA和nodA基因序列的系统发育分析表明，从帕拉豆属植物中分离出的细菌菌株嵌于其他慢生根瘤菌之中。在基于16S rRNA或nodA基因序列的拓扑结构中，它们并未聚在一起，而是分散在属于日本慢生根瘤菌或埃氏慢生根瘤菌谱系的其他慢生根瘤菌之中。这些数据表明，一些慢生根瘤菌与帕拉豆属植物形成根瘤的能力并不代表早于根瘤菌与豆科植物关系的历史关系，而可能是一些根瘤菌最近发生的宿主转换。
