Populações de alfa e beta-rizóbios de feijão-caupi naturalmente estabelecidas em solos do semiárido tropical

Abstract

Recently, significant contributions to the advancement of the knowledge of the ecology and diversity of microorganisms in different Brazilian ecosystems have led to the enrichment of the taxonomy of rhizobia, including the description of new species of microsymbionts. The great diversity of soil and climatic conditions (soil types, mean annual precipitation, etc.) and characteristics of the vegetation of the caatinga make this biome a vast field for studies of rhizobia ecology and for the search of new elite strains for a recommendation as inoculants. The objective of this work was to determine the characteristics of naturally occurring alpha and beta-rhizobia populations in Pernambuco semi-arid soils covered with native vegetation. To perform a representative sampling of the different edaphoclimatic conditions, points were selected in areas with dense caatinga cover (with little anthropic interference) and in the main soil classes occurring in the biome: Ultisols, Leptosols, Arenosols, Regosols, Oxisols, Alfisols and Planosols. The native rhizobial communities of the collected soil samples were accessed in a greenhouse experiment using the cowpea as bait plant. This species was chosen for its known ability to establish symbiosis with alpha and beta-rhizobia communities. Of the nodules collected in the bait-plant experiment, 434 were obtained, of which 338 were positive for nodC (nodC +) gene amplification. Most are fast-growing isolates, with a metabolism that acidifies the culture medium and producers of much mucus. The 338 isolates were submitted to Amplified Ribosomal DNA Restriction Analysis (ARDRA) to evaluate the genetic variability of the nodC + collection. The analysis of the restriction profiles of the 16S rRNA obtained from the endonucleases HaeIII, MspI and Hin6I, allowed the formation of 209 groups at 100 % of similarity, evidencing a high genetic variability among the isolates. From the dendrogram of the ARDRA, a threshold of 75 % of similarity was determined and, from this grouping, representatives of the groups (a total of 60) were selected to be submitted to the amplification and sequencing of the 16S rRNA gene. Of the 60 isolates, 30 were randomly selected to be sequenced the nodC, recA and gyrB genes. Taking into consideration the sequencing of 16S, Paraburkholderia (18) was the genus with the highest number of isolates, followed by Rhizobium (13) and Bradyrhizobium (10). Analyzing the tree based on 24 isolates with a good sequence of the recA gene, it was verified that eight isolates belong to the class of betaproteobacteria belonging to the genus Paraburkholderia. For the alpha-rhizobia, only 2 isolates grouped with the genus Rhizobium and 12 were grouped with Bradyrhizobium. Most of the isolates grouped with the genus Paraburkholderia in the nodC gene tree showed high similarity with the reference strains. Because of divergent recA and gyrB sequences in the concatenated tree, isolates C88-4 and C59-4 formed a group with low similarity to the reference strains of beta-rhizobia. In alpha-rhizobia the behavior was similar. This pattern of grouping demonstrates the high genetic variability of the isolates and points to the existence of new taxonomic groups among the isolates obtained.