| Título: | Phylogeny of nodulation and nitrogen-fixation genes in Bradyrhizobium: supporting evidence for the theory of monophyletic origin, and spread and maintenance by both horizontal and vertical transfer. |
| Autores: |
MENNA, P. HUNGRIA, M. |
| Fecha: |
2011 2011-11-30 |
| Publicador: | Repositório Acesso Livre à Informação Científica da Embrapa (ALICE) |
| Fuente: | |
| Tipo: | Artigo em periódico indexado (ALICE) |
| Tema: | Nodulação |
| Descripción: |
Bacteria belonging to the genus Bradyrhizobium are capable of establishing symbiotic relationships with a broad range of plants belonging to the three subfamilies of the Leguminosae (=Fabaceae) family, with the formation of specialized structures on the roots, the nodules, where fixation of atmospheric nitrogen takes place. The symbiosis is under the control of a finely tuned expression of common and host-specific nodulation genes and also of genes related to the assembly and activity of the nitrogenase, which, in the Bradyrhizobium strains investigated so far, are clustered in a symbiotic island. Information about the diversity of these genes is essential to improve our still-poor understanding of their origin, spread and maintenance, and, in this study, we provide information on forty Bradyrhizobium strains, most of tropical origin. For the nodulation trait we analyzed common (nodA), Bradyrhizobium-specific (nodY/K) and host-specific (nodZ) nodulation genes, while for fixation ability, the diversity of nifH was investigated. In general, clustering of strains in all nod and nifH trees was similar, and the Bradyrhizobium group was clearly separated from other rhizobial genera. However, the congruence of nod and nif genes with ribosomal and housekeeping genes was low. nodA and nodY/K were not detected in three strains by amplification or hybridization with probes using Bradyrhizobium japonicum and Bradyrhizobium elkanii type strains, indicating high diversity of these genes or that strains other than photosynthetic Bradyrhizobium must have alternative mechanisms to initiate the process of nodulation. For a large group of strains, the high diversity of nod genes - with an emphasis on nodZ, the low relationship between nod genes and the host legume, and some evidence of horizontal gene transfer might indicate strategies to increase host range. On the other hand, in a group of five symbionts of Acacia mearnsii, the high congruence between nod and ribosomal and housekeeping genes, in addition to shorter nodY/K sequences and the absence of nodZ, highlights a co-evolution process. Additionally, in a group of B. japonicum strains symbionts of soybean, vertical transference seemed to represent the main genetic event. In conclusion, clustering of nodA and nifH gives additional support to the theory of monophyletic origin of the symbiotic genes in Bradyrhizobium, and, in addition to the analysis of nodY/K and nodZ, indicate spread and maintenance of nod and nif genes through both vertical and horizontal transmission, apparently with the dominance of one or another of these events in some groups of strains. 2011 |
| Idioma: | en |