Phosphate solubilizing bacteria from runner bean rhizosphere and their mechanism of action

摘要

Phosphorus is one of the most important nutrients for biological growth and development but also a limiting one because of the low availability of soluble forms. Microorganisms, especially rhizobacteria, have the ability to convert insoluble forms of phosphate to an accessible form for plants using different mechanisms. Therefore, the main goal of this study was to identify the mechanisms used by runner bean (Phaseolus coccineus L.) rhizospheric bacteria for inorganic phosphate solubilization. Ten out of twenty five isolated bacterial strains solubilized Ca3(PO4)2 in qualitative P-solubilization assays, with solubilization indices varying from 1.33 to 3.19. The strains that exhibited the highest potential to solubilize Ca3(PO4)2 in a quantitative assay were selected for further studies of the mechanisms involved in the process. The pH of the medium was monitored, the organic acids exported in the culture medium were identified by HPLC analysis and the acid and alkaline phosphatase activities were determined. An inverse relationship between pH and P solubilization was evidenced. The main mechanisms involved are the lowering of pH due to the production of organic acids such as tartaric and isocitric acids. No positive correlation between the soluble P and the phosphatase activity could be inferred.