Symbiosis Contribution of Non-nodulating <i>Bradyrhizobium cosmicum</i> S23321 after Transferal of the Symbiotic Plasmid pDOA9

Abstract

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:pDOA9Ω nodB ) 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.

Comparative genome analysis of Sesbania cannabina-nodulating Rhizobium spp. revealing the symbiotic and transferrable characteristics of symbiosis plasmids

“…Among them, the strains within different Rhizobium species presented extraordinarily highly conserved symbiosis genes [17], indicating that they might have acquired the symbiosis genes through HGT in a short evolutionary history. The symbiosis genes are usually located on pSyms [4,[37][38][39] or chromosomal symbiotic islands [40][41][42][43][44]. As for other fast-growing rhizobia, the symbiosis genes of S. cannabina-nodulating rhizobia A. pusense IRBG74 and Sinorhizobium alkalisoli YIC4027 were also located in a pSym [24,45].…”

Complete genome sequence of Bradyrhizobium sp. 62B, a native nitrogen-fixing rhizobium isolated from peanut nodules

“…62B genome contains them in the plasmid ( 20 , 21 ). It also carries genes predicted to be related to conjugative transfer proteins and the type three secretion system ( 22 ), features related to broader host specificity and putative plasmid transfer toward other strains ( 23 ).…”