Potentially Diazotrophic Endophytic Bacteria Associated to Sugarcane are Effective in Plant Growth-promotion

The bioprospection of microorganisms with the potential to promote sugarcane growth has been increasingly sought, mainly to minimize the insertion of inputs that increase the costs of the production chain. The objective of this work was to isolate, identify and select endophytic bacteria from sugarcane with potential ability for plant growth promotion. Also to evaluate the influence of different levels of salinity and pH values on plant growth-promotion and to analyze the genetic diversity of the isolates by the partial sequencing of the 16S rRNA gene. Samples of stems and roots of the varieties RB 92579, RB 867515 and RB 863129 were used for collecting the bacteria associated with sugarcane. The isolation was performed in LGI-P culture medium. The isolates were evaluated for biological nitrogen fixation (BNF), indole acetic acid (IAA) production, inorganic phosphate solubilization and exopolysaccharide (EPS) production. Bacterial growth and phosphate solubilization were evaluated at different salt concentrations and pH values of the culture medium. After isolation and identification were obtained 24 isolates, been that 54% grew in nitrogen-free medium. IAA was produced in 50% and 12.5% of the isolates in medium with and without the addition of L-tryptophan, respectively. In addition, 67% of the isolates were positive for phosphate solubilization and 75% produced exopolysaccharides. The presence of salt and different pH values of the culture medium influenced the growth of the isolates and the salt concentration influenced phosphate solubilization. Seven isolates showed similarity with Burkholderia gladioli and one with Burkholderia heleia, besides many genera only of Burkholderia sp., suggesting low genetic diversity. Burkholderia heleia had a lower growth rate than Burkholderia gladioli. It was a bacterium very sensitive to high amounts of salt when the pH was higher. The root bacterial isolate G27 obtained from RB 867515 presented the highest PSI in the lowest saline concentrations and the stem isolate G15 of the same variety was able to solubilize phosphate in high salt concentration. We conclude that sugarcane interacts with potentially diazotrophic endophytic bacteria involved with plant growth-promotion and can be used as inoculants in management programs of this crop, however, it is necessary to observe soil salinity levels and monitor pH to maximize plant growth-promotion characteristics of these bacteria.