**Background:** Phosphorus is an essential macronutrient for plants, but over 70% of soil phosphorus is bound in insoluble forms (inorganic minerals and organic phytates) unavailable to crops. Phosphate-solubilizing bacteria (PSB) offer an environmentally sustainable alternative to chemical fertilizers by converting insoluble phosphorus into bioavailable forms. The authors previously isolated Pantoea brenneri strains from Tatarstan soils that hydrolyze phytates via glucose-1-phosphatase and possess multiple plant-growth-promoting (PGP) traits. This study aimed to characterize their inorganic phosphate solubilization mechanisms and evaluate their potential to increase potato yield.
**Methods:** Four P. brenneri strains (3.1, 3.2, 3.5.2, 3.6.1) were tested on NBRIP agar and liquid media containing six insoluble phosphorus sources: tricalcium phosphate (Ca3(PO4)2), calcium hydroorthophosphate (CaHPO4), hydroxyapatite, phosphorite, aluminum phosphate (AlPO4), and iron phosphate (Fe3(PO4)2). Phosphate solubilization was quantified using the ammonium molybdate method. Culture conditions (pH, temperature, rotation speed, carbon/nitrogen sources) were optimized. Organic acids were analyzed by HPLC. Acid and alkaline phosphatase activities were measured using pNPP substrate. The genome of strain 3.5.1 was screened for phosphate metabolism genes. A seed germination assay was performed on two wheat cultivars (Zlata and Tulaikovskaya). Greenhouse trials on potato (Solanum tuberosum L., cv. Desiree) evaluated stem height, stem number, and tuber yield after pre-planting tuber treatment with or without additional soil irrigation.
**Key Results:** All four strains formed halo zones on solid media containing Ca3(PO4)2, CaHPO4, hydroxyapatite, and phosphorite, but not on AlPO4 or Fe3(PO4)2. The phosphate solubilization index (PSI) was highest on CaHPO4 and phosphorite (6.39 ± 0.4). In liquid media, strain 3.5.2 showed significantly (p < 0.01) higher efficiency: 68.74% on Ca3(PO4)2 and 61.22% on hydroxyapatite. Free phosphate concentration peaked at 1253.49 mg/L (87% efficiency) within 24 hours, with pH dropping from 7.0 to 2.85. Optimal conditions were: initial pH 7.0, 30°C, 200 rpm, with glucose and ammonium sulfate as carbon and nitrogen sources (74.56% efficiency). HPLC identified eight organic acids (oxalic, malic, formic, malonic, lactic, maleic, acetic, citric), with malic and formic acids exceeding 10 mM. An unidentified peak (retention time 6.6 min) was hypothesized to be gluconic acid based on genomic evidence (gcd and pqq operon). Maximum alkaline phosphatase activity was 4.85 U/100 mL (strain 3.2, day 7, hydroxyapatite); maximum acid phosphatase was 2.149 U/100 mL (strain 3.5.2, day 5, hydroxyapatite). Genomic analysis revealed genes for glucose dehydrogenase (gcd), pyrroloquinoline quinone biosynthesis (pqqB-F), phosphate transport (pstSCAB), phosphonate degradation (phnC-M), polyphosphate metabolism (ppx, ppk), and organic phosphate mineralization (phyK, agpP, phoA). In wheat seed assays, strain 3.5.2 increased germination to 97% (cv. Zlata) and 96% (cv. Tulaikovskaya) vs. controls (89% and 86%). Root length increased by 37.2% (cv. Tulaikovskaya) and 16.6% (cv. Zlata); shoot length increased by 10.7% and 15.4%, respectively. In greenhouse potato trials, strains 3.1 and 3.5.2 increased stem height by 26% and 14%. Tuber yield with strain 3.2 was 33.2 g/plant vs. 28 g/plant for controls. No significant differences were found between single pre-planting treatment and treatment plus soil irrigation.
**Clinical Implications:** P. brenneri strains, particularly 3.5.2, demonstrate strong potential as biofertilizers capable of solubilizing both inorganic and organic phosphorus while promoting plant growth through multiple PGP mechanisms. Their rapid phosphate solubilization (within 24 hours) and broad pH/temperature tolerance make them suitable for field application. The ability to reduce mineral phosphorus fertilizer use by up to 50% (as cited from literature) could significantly lower agricultural costs and environmental pollution. However, further field studies are needed to evaluate performance in diverse soil systems and assess impacts on native microbial communities.