Strain-Specific Phosphate Mobilization in Enterobacter: Organic Acid Production and Genomic Architecture of Solubilization Mechanisms

Phosphate-solubilizing microorganisms (PSMs) show promise for sustainable agriculture, yet inconsistent field performance limits their application. We investigated phosphate solubilization mechanisms in <i>Enterobacter ludwigii</i> strains GMG278, GMG291, GMG378 and <i>Enterobacter soli</i> GMG1156 through greenhouse wheat experiments, high-performance liquid chromatography (HPLC) organic acid analysis, and comparative genomics. Greenhouse trials demonstrated that bacterial inoculation compensated for phosphorus deficiency, with GMG291, GMG1156, and GMG278 showing superior performance. HPLC identified malic acid as the predominant secreted organic acid, with <i>E. soli</i> producing threefold higher concentrations than <i>E. ludwigii</i> strains. Phosphate solubilization efficiency followed the order FePO₄ > AlPO₄ > Ca₃(PO₄)₂, with maximal release (95.9-97.7 μg/mL) from iron phosphate despite lower malic acid secretion, suggesting siderophore involvement. An inverse correlation between malic acid levels and soluble phosphate concentrations likely reflects competitive bacterial phosphate uptake and secondary precipitation processes. Comparative genomics revealed missense mutations in the LuxR transcriptional regulator of strain GMG378 (Asp86Asn and Arg97Leu) near predicted DNA-binding domains, correlating with reduced solubilization capacity. Phosphate solubilization in Enterobacter proceeds primarily through metal-malic acid complex formation, with strain-specific efficiency linked to LuxR-regulated biofilm formation genes. These findings suggest PSM screening should incorporate biofilm-related genetic markers alongside acid production measurements.