Resumo

The Atlantic Forest is a major "hotspot" of biodiversity and may be an important source of new microbial species and bioproducts. Leaf surfaces (phyllospheres) have been shown to provide appropriate conditions for colonization by microorganisms including diazotrophic bacteria. These microorganisms may affect the growth and development of plants directly or indirectly. Previous studies on the microbial community in the Brazilian Atlantic Forest have shown a relatively high abundance of epiphytic bacteria on the phyllosphere and litter, with high N-fixation activity. In this study, we evaluated the possible mechanisms of plant growth promotion in diazotrophic bacteria isolated from leaf surfaces of the Atlantic Forest. Leaf samples were collected at the “Serra do Mar State Park” (São Paulo, Brazil) and free-living nitrogen fixing bacteria were isolated in nitrogen-free medium. Total nitrogenase activity was estimated using the acetylene reduction assay (ARA). The isolated strains that were positive for ARA, were further identified, based on their 16S rRNA gene sequences. A total of 120 isolates were obtained, being 50 from the phyllosphere and 70 from the litter. Forty-eight isolates showing nitrogenase activity, ranging from 5 to 41 nmol C₂H₄ h^-1 mg^-1 protein, were further analysed. 56% of these isolates showed indole acetic acid (IAA) production ranging from 1,74 to 51,77 μg mL^-1; 50% were able to solubilize inorganic phosphate and 17% were positive for siderophores. Phylogenetic affiliation of all the ARA positive isolates revealed the dominance of the class Gammaproteobacteria and the family Enterobacteriaceae, among them. Approximately 56% of the diazotrophic isolates were classified as Klebsiella sp., 32% as Ewingella, and 3% as Raoultella, Serratia, Rahnella e Enterobacter. In conclusion, our study confirmed that the phyllosphere is an environment prone for the prospection of diazotrophics, which may be able to promote plant growth.