Diazotrophic bacteria in soils under rubber tree

Several reports have shown the benefits of atmospheric nitrogen-fixing prokaryotes in growth and nutrition of many plant species; however, these microorganisms in the rhizosphere of rubber trees (Hevea brasiliensis) have not been investigated so far. Thus, the objective of this study was to verify the occurrence of diazotrophic bacteria in soils and roots of a plantation of rubber trees, and to isolate and characterize these bacteria. For this purpose, soil and fine roots of rubber trees were sampled on the Experimental Campus of the Federal University of Lavras, Minas Gerais, for inoculation in semi-solid culture medium without N, to favor the growth of some diazotrophic bacteria species. Nineteen isolates were obtained from soil samples, and no growth of N-fixing bacteria was verified in the cultures with root samples. The characterization of the cells and colonies of these isolates indicated that 17 of them produce large amounts of elastic and sometimes cartilagenous exopolysaccharide. They are all Gram-negative, have rod-shaped, motionless cells and a globule of poly-β-hydorxybutyrate (PBH) at either cell end. The sequencing of the 16S rDNA and its phylogenetic analysis confirmed that representative isolates of this group belong to the genus Beijerinckia (B.indica and B. derxii), and that the other two Gram-positive isolates belong to the genus Bacillus. The presence of the enzyme nitrogenase, responsible for biological nitrogen fixation (BNF), was confirmed by the acetylene reduction technique. It was concluded that the soil under rubber tree contained predominantly freeliving diazotrophs of the genus Beijerinckia (B.indica and B.derxii), with no evidence of endophytic or rhizospheric bacteria.