根瘤菌要与豆科植物建立共生关系，需要双方多种信号分子参与。而对信号分子的识别又存在着相对专一性。因此，通常情况下，根瘤菌与豆科植物之间存在着专一性共生关系，如苜蓿与Sinorhizobium meliloti共生，花生与慢生根瘤菌属菌共生，大豆与Sinorhizobium及Bradyrhizobium两个属的菌株共生，紫云英与Mesorhizobium huakuii共生，豌豆与Rhizobium leguminoarum等。这些都是相对专一性共生关系。

根瘤菌中有一个菌株Sinorhizobium sp. NGR234能与120属的豆科植物建立共生关系。这是目前发现的根瘤菌中宿主最为混杂的一个根瘤菌。

最近发表在MPMI杂志上的一篇论文，报道了一种对根瘤菌选择性很低的药用豆科植物苦参(Sophora flavescens)，它能与上面提到的宿主为苜蓿、花生、大豆、紫云英、豌豆等的根瘤菌建立共生关系，而这些根瘤菌在相互的宿主上却不能交叉结瘤。因此，苦参是一种极度混杂的植物。交叉结瘤试验还证实，苦参除能与alpha纲的根瘤菌结瘤外，还能与beta纲的根瘤菌结瘤。

苦参与不同根瘤菌之间的非共生专一性关系彻底打破了之前人们认为的专一性共生关系。苦参是植物界的“NGR234”。对苦参与不同根瘤菌共生关系的研究，将使人们更好地理解其非专一性共生的机理，以及苦参这一植物在发育上的地位。苦参与不同根瘤菌的共生关系将成为研究豆科植物-根瘤菌共生关系的一个极好的模式材料。

http://dx.doi.org/10.1094/MPMI-06-15-0141-R, http://apsjournals.apsnet.org/doi/abs/10.1094/MPMI-06-15-0141-R

论文摘要：

In present study, we report our extensive survey on the diversity and biogeography of rhizobia associated with Sophora flavescens, a sophocarpidine (matrine)-containing medicinal legume. We additionally investigated the cross nodulation, infection pattern, light and electron microscopies of root nodule sections of S. flavescens infected by various rhizobia. Seventeen genospecies of rhizobia belonging to five genera with seven types of symbiotic nodC genes were found to nodulate S. flavescens in natural soils. In the cross-nodulation tests, most representative rhizobia in class α-Proteobacteria, whose host plants belong to different cross-nodulation groups, form effective indeterminate nodules, while representative rhizobia in class β-Proteobacteria form ineffective nodules on S. flavescens. Highly host-specific biovars of Rhizobium leguminosarum (bv. trifolii and bv. viciae) and Rhizobium etli bv. phaseoli could establish symbioses with S. flavescens, providing further evidence that S. flavescens is an extremely promiscuous legume and it does not have strict selectivity on either the symbiotic genes or the species-determining housekeeping genes of rhizobia. Root-hair infection is found as the pattern that rhizobia have gained entry into the curled root hairs. Electron microscopies of ultra-thin sections of S. flavescens root nodules formed by different rhizobia show that the bacteroids are regular or irregular rod shape and nonswollen types. Some bacteroids contain poly-β-hydroxybutyrate (PHB), while others do not, indicating the synthesis of PHB in bacteroids is rhizobia-dependent. The extremely promiscuous symbiosis between S. flavescens and different rhizobia provide us a basis for future studies aimed at understanding the molecular interactions of rhizobia and legumes.

不同的根瘤菌的nodC系统发育树及与在苦参的结瘤情况如下图（Fig. 3）：

Fig. 3. Maximum likelihood (ML)phylogenetic tree based on nodC genes. The model T92+G+I was used to construct the tree. Only bootstrap values greater than 50% are shown at branch nodes. Bar, 5% nucleotide substitution per site. Strains used in nodulation werelabeled by: ▍, effective nodules, ○, no nodule, △, not stable but effective nodules, ◇,lower effective or inefficient nodules. The strains with boldfaced names wereisolated from Sophora flavescens.Host plants of origin of these rhizobia were added after the accession numberof each strain. Abbreviation: Ecoreg. = ecoregion; R. = Rhizobium; B. = Bradyrhizobium;S. = Sinorhizobium; M. = Mesorhizobium; T = type strain.

2017年9月22日，时隔近两年，我们又对苦参与不同根瘤菌的分子互作进行了研究，结果仍然发表在MPMI上，见：http://blog.sciencenet.cn/blog-3533-1077224.html ，或原网站：http://apsjournals.apsnet.org/doi/abs/10.1094/MPMI-05-17-0117-R?journalCode=mpmi。