Bacillus subtilis PE7-Mediated Alleviation of Phosphate Starvation and Growth Promotion of Netted Melon (Cucumis melo L. var. reticulatus Naud.)

Members of <i>Bacillus</i> species are able to enhance the level of available phosphorus (P) for plant absorption through mechanisms of P solubilization and mineralization. In our study, <i>B. subtilis</i> PE7 showed P-solubilizing activity in simple phosphate broth (SPB) medium, and acetic acid, iso-butyric acid, and iso-valeric acid were major organic acids responsible for the increase in soluble P and decrease in pH of SPB medium. In addition, strain PE7 released phytase on phytase-screening agar (PSA) medium, and analysis of semi-quantitative reverse transcription and polymerase chain reaction (sqRT-PCR) revealed that the <i>phyC</i> gene expression was the highest at 1 day after incubation. A low concentration of KH₂PO₄ in SPB medium induced more biofilm formation than a high concentration of KH₂PO₄. Strain PE7 showed swimming and swarming motilities in TY and TrA agar media. Under P starvation, inoculation with higher cell numbers of strain PE7 enhanced biomass and nutrient acquisition by melon plants, resulting in higher values of growth parameters and nutrient contents. Moreover, the persistence of bacterial cells on the root surface and in the rhizosphere of melon plants indicated colonization of the plants by strain PE7. Due to its capacity for P solubilization and mineralization, <i>B. subtilis</i> PE7 could be utilized as an alternative to synthetic fertilizer for P deficient-stress management in crop plantation.