Diversidade genética e potencial simbiótico de bactérias de nódulos de Arachis spp. cultivados em solos do Semiárido

Abstract

Belonging to the Fabaceae family, the peanut (Arachis hypogaea) is cultivated worldwide, having great importance in the income of the cultivator. Because it is a culture that associates with bacteria that have the capacity to fix nitrogen, it has been studied in order to potentiate this symbiotic relationship and favor the increase of its production. Thus the objective of this work was to evaluate the diversity and symbiotic efficiency of the native bacteria associated with Arachis species. A collection of bacteria isolated from nodules of six species of the genus Arachis cultivated in samples of six soils from the semi - arid region was structured. The isolates were purified and evaluated for their phenotypic characteristics, and amplification of some symbiotic genes. The isolates considered as nifH, nodA and / or nodC positive we re selected for the symbiotic efficiency test, as well as for the partial sequencing reactions of the 16S rRNA gene. Bacteria selected in previous evaluations were tested in a greenhouse experiment to evaluate symbiotic efficiency. Bacteria classified as Bradyrhizobium, the 16S rRNA gene were also sequenced in reverse. In addition, the recA, nifD and nodC genes were also amplified and sequenced. For these same isolates, a BOX-PCR was performed to verify differences between these isolates. Phylogenetic trees were elaborated based on the genes gathered from the results. A collection of 365 isolates was obtained, with the majority of the isolates showing rapid growth, acidifying the culture medium, colony diameter of 1 to 2 mm, and yellowing. Of the total isolates, 253 had amplification in at least one of the evaluated symbiotic genes, 94 isolates amplified for all genes and primers evaluated. A total of 63 isolates were selected for sequencing of the 16S rRNA gene, as well as for the evaluation of symbiotic efficiency in greenhouse. From the obtained results, 45 sequences presented good quality and adequate size and were used for the comparison with those available in the EzBioCloud database. A total of 22 isolates were identified as belonging to the genus Bradyrhizobium, with 21 being classified in the clade I B. japonicum and only 1 in the clade II B. elkanii, and 23 others belonging to non-rhizobian genera. From the sequencing of the symbiotic genes, it was found that some isolates that had amplifications in the three evaluated genes, such as the isolates S2AB2, S2AD2, S2AH 9.1 and S2AH 4.1, remained in distinct groups in all three symbiotic gene trees, as well as in the 16S rRNA gene tree, confirming the low similarity of these isolates to the type strains, indicating that they are possibly isolated from new species. With the BOX-PCR it was confirmed that the isolates presented a high degree of polymorphism, and that these were actually different isolates. The isolates tested in greenhouse were able to nodulate commercial cultivar BR1. The co- inoculation with non-rhizobial bacteria may have favored some isolates in their symbiotic efficiency, mainly in the accumulation of N in plants. These results will support studies for future evaluations of the selected isolates in the recommendation of inoculants of native species for the peanut crop. Cultivated peanuts and wild species evaluated may establish symbiotic associations with native strains of rhizobia with high diversity and symbiotic efficiency.