School of Biotechnology, Institute of Agricultural Technology, Suranaree University of Technology.
Molecular and Applied Microbiology Laboratory, Center of Research and Service, Diponegoro University.
Microbes Environ. 2022;37(2). doi: 10.1264/jsme2.ME22008.
The symbiotic properties of rhizobial bacteria are driven by the horizontal gene transfer of symbiotic genes, which are located in symbiosis islands or on plasmids. The symbiotic megaplasmid pDOA9 of Bradyrhizobium sp. DOA9, carrying the nod, nif, fix, and type three secretion system (T3SS) genes, has been conjugatively transferred to different Bradyrhizobium strains. In the present study, non-nodulating B. cosmicum S23321, which shows a close phylogenetic relationship with Bradyrhizobium sp. DOA9, but lacks symbiotic properties, was used to carry pDOA9 (annotated as chimeric S2:pDOA9). The results obtained showed that pDOA9 conferred symbiotic properties on S23321; however, nodulation phenotypes varied among the DOA9, chimeric ORS278:pDOA9, and S2:pDOA9 strains even though they all carried symbiotic pDOA9 plasmid. S23321 appeared to gain symbiotic nodulation from pDOA9 by processing nodulation genes and broadening the host range. The present results also showed the successful formation of active nodules in Arachis hypogaea (Dalbergoid) and Vigna radiata (Millitoid) by chimeric S2:pDOA9, while Crotalaria juncea (Genistoid) and Macroptilium atropurpureum (Millitoid) formed nodule-like structures. The formation of nodules and nodule-like structures occurred in a nod factor-dependent manner because the nod factor-lacking strain (S2:pDOA9ΩnodB) completely abolished nodulation in all legumes tested. Moreover, T3SS carried by S2:pDOA9 exerted negative effects on symbiosis with Crotalaria juncea, which was consistent with the results obtained on DOA9. T3SS exhibited symbiotic compatibility with V. radiata when nodulated by S23321. These outcomes implied that pDOA9 underwent changes during legume evolution that broadened host specificity and the compatibility of nodulation in a manner that was dependent on the chromosomal background of the recipient as well as legume host restrictions.
根瘤菌的共生特性是由共生基因的水平基因转移驱动的，这些基因位于共生岛上或质粒上。 Bradyrhizobium sp. DOA9 的共生大质粒 pDOA9 携带 nod、nif、fix 和 III 型分泌系统 (T3SS) 基因，已通过共轭转移到不同的 Bradyrhizobium 菌株。在本研究中，非结瘤的 B. cosmicum S23321 与 Bradyrhizobium sp. DOA9 具有密切的系统发育关系，但缺乏共生特性，被用于携带 pDOA9（注释为嵌合 S2:pDOA9）。结果表明，pDOA9 赋予了 S23321 共生特性；然而，即使它们都携带共生 pDOA9 质粒，DOA9、嵌合 ORS278:pDOA9 和 S2:pDOA9 菌株的结瘤表型也有所不同。S23321 似乎通过处理结瘤基因和扩大宿主范围从 pDOA9 获得共生结瘤。本研究还表明，嵌合 S2:pDOA9 成功地在 Arachis hypogaea（Dalbergoid）和 Vigna radiata（Millitoid）中形成了活跃的根瘤，而 Crotalaria juncea（Genistoid）和 Macroptilium atropurpureum（Millitoid）形成了类似根瘤的结构。根瘤和类似根瘤结构的形成是依赖于结瘤因子的，因为缺乏结瘤因子的菌株（S2:pDOA9ΔnodB）完全废除了所有测试豆科植物的结瘤。此外，S2:pDOA9 携带的 T3SS 对与 Crotalaria juncea 的共生产生了负面影响，这与 DOA9 的结果一致。当 S23321 结瘤时，T3SS 与 V. radiata 表现出共生相容性。这些结果表明，pDOA9 在豆科植物进化过程中发生了变化，以依赖于受体染色体背景和豆科植物宿主限制的方式扩大了宿主特异性和结瘤的兼容性。
