Resumo

Diazotrophs are microorganisms that fix atmospheric nitrogen gas into a more usable form such as ammonia, and grow without external sources of fixed nitrogen. Multiple experiments during recent years failed to show a substantial contribution of Biological Nitrogen Fixation to plant growth in most cases. Now it is clear due to previous studies that diazotrophs need a combination of different mechanisms to promoting plant growth. The aim of this study was to isolate, identify and characterize diazotrophic bacteria from the rhizosphere of sisal, antagonistic activity against sisal pathogen Aspergillus níger and plant growth promoting activities. Bacteria were recovered from the soils of sisal fields located in Conceição do Coité – Bahía-Brazil .To recover these isolates, was used Burk's nitrogen free medium. Followed nifH gene fragments were amplified using "nested PCR." Two pairs of primers were used: the first pair Pol F -Pol R and the second pair: nifHF - nifHRev .The Diversity among isolates was carried out using the primer BOX A1-R and to calculate the similarity matrix was used the D-UPGMA tool.The bacterial strains were identified sequencing by Sanger method the 16S rRNA gene. To do the physiological characterization were performed tests to measure the IAA, cellulose, pectinase production and inorganic phosphate solubilization.All isolated bacteria were tested for antifungal activity against Aspergillus níger. Forty seven strains were isolated and were grouped in 11 clusters. One strain showed a considerable IAA production, 4 an inorganic phosphate solubilization, 2 a cellulase activity. Others studies have shown that Diazotrophs can affect indirectly plant growth decreasing or preventing the effects of pathogenic microorganisms, mostly through the synthesis of antibiotics, competition for nutrients, the synthesis of phytohormones and vitamins, improving nutrient uptake, enhancing stress resistance, solubilization of inorganic phosphate , mineralization of organic phosphate or by the induction of systemic resistance to pathogens.Two strains showed antagonistic activity against Aspergillus níger and sequencing associated them with Burkholderia sp. This genus has species known like a highly effective nitrogen-fixing symbionts. The knowledge of the mechanisms involved in plant growth promotion will help in the employment of diazotrophic bacteria in plants of economic interest.