Sreening, stress toleranceexamination, and phosphate-solubilizing condition optimization of phosphate-solubilizing bacteria

Abstract: [Background] Phosphorus in the soil mainly exists in the form of insolublephosphate, which is inaccessible for crops. Phosphate-solubilizing microorganisms can convertinsoluble phosphorus into available phosphorus, significantly improving soil phosphorusbioavailability and regulating phosphorus transformation. [Objective] To screen the efficientphosphate-solubilizing microorganisms so as to provide the strain resources for developingmicrobial fertilizers. [Methods] The strain was identified based on the morphological, physiological,and biochemical properties and 16S rDNA gene sequence. The tolerance of the strains to salt, pH,temperature, and drought was examined. The phosphate-solubilizing and plant growth-promotingcharacteristics of the strain were investigated in terms of siderophore production, ammoniageneration, and plant hormone secretion. The organic acids were qualitatively analyzed by ultra-highperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-TOF-MS).[Results] Strain 4-4-4 capable of efficiently solubilizing phosphate was isolated from the rhizospheresoil of wheat in Xinjiang and identified as Enterobacter. Enterobacter sp. 4-4-4 could grow at pH4.5−9.5, 12−45 °C, and tolerate 10% NaCl and 80 g/L PEG6000, thereby demonstrating strong stresstolerance. This strain was capable of simultaneously solubilizing insoluble inorganic phosphorus(Ca3(PO4)2) and insoluble organic phosphorus (lecithin). It could secrete 28.07 mg/L ofindole-3-acetic acid (IAA) and fix 23.18 mg/L nitrogen, thus promoting the growth of tomato plants,which was evidenced by the plant height, root length, stem diameter, total dry weight, and total freshweight increases of 66.67%, 21.14%, 34.93%, 233.30%, and 211.70%, respectively. Under the cultureconditions optimized by response surface methodology, the strain achieved the soluble phosphoruscontent of 902.1 mg/L from Ca3(PO4)2. This result represents an increase of 30.1% compared withthat before optimization and is 1.25–16.20 times higher than that of most reportedphosphate-solubilizing Enterobacter strains. UPLC-TOF-MS results verified that gluconic acid wasthe key metabolite responsible for phosphate solubilization. [Conclusion] Enterobacter sp. 4-4-4shows an excellent phosphate-solubilizing capability, certain plant growth-promoting potential, andstrong stress tolerance, serving as a candidate strain for developing microbial fertilizers.