Characterization of phosphate solubilizing bacteria from rice (Oryza sativa L.) rhizosphere soils and their potential for plant growth promotion

Agricultural soils contain substantial amount of phosphorous (P) in complexed-forms that have gradually accumulated through repeated P-fertilization events. Reliance on chemical P-fertilizers in crop production can be reduced by mobilizing P from soil reserves. Biofertilizers containing phosphate solubilizing bacteria (PSB) are considered an eco friendly approach to mobilize soil-P for plant uptake. Before their application in biofertilizer formulations, PSB should be investigated for their ability to solubilize different P-forms as well as for other associated plant growth-promoting benefits and potential risks. The study aimed to identify and characterize eleven isolates of PSB obtained from rice fields in Dry and Intermediate zones of Sri Lanka. The isolates were identified using 16S rRNA gene sequencing. Their ability to solubilize tri-calcium phosphate (Ca-P), aluminum phosphate (Al-P), and iron phosphate (Fe P), and to produce catalase, protease and Indole-3-Acetic Acid (IAA) were determined using culture-based bioassays. Bio priming effect of isolates was tested for six rice varieties. The isolates belonged to seven genera: Staphylococcus, Bacillus, Xanthomonas, Pseudomonas, Lysinibacillus, Acinetobacter, and Stenotrophomonas. Five isolates produced IAA, six showed catalase activity, and three exhibited extracellular protease production. Phosphate solubilizing index of the isolates ranged between 3.48±0.51 to 5.87±0.81, and all isolates demonstrated the ability to solubilize Ca-P, Fe-P and Al-P. The isolates were more effective in solubilizing Ca-P than the other two forms. Bio priming seeds with PSB significantly (p<0.05) promoted early development of rice seedlings and improved seedling vigour of all six varieties. The studied PSB isolates are promising candidates for biofertilizer development targeting rice farming in Sri Lanka.