BIOCHEMICAL AND AGRONOMIC POTENTIALS OF PHOSPHATE-SOLUBILIZING BACTERIA FROM MEDICINAL PLANT RHIZOSPHERES FROM VARIOUS REGIONS OF ABIA STATE, NIGERIA

Phosphorus is a critical macronutrient essential for plant growth and development, yet its availability in agricultural soils is often limited due to fixation into insoluble forms. Excessive use of chemical phosphate fertilizers, aimed at addressing phosphorus deficiency, is associated with environmental risks such as eutrophication, soil degradation, and disruption of microbial ecosystems. In light of this, phosphate-solubilizing bacteria (PSB) offer a promising, eco-friendly alternative to enhance phosphorus bioavailability in soils. This study aimed to isolate, characterize, and evaluate the phosphate-solubilizing potential of bacteria from the rhizosphere of Ocimum gratissimum (scent leaf), a widely used medicinal and culinary plant grown in phosphorus-deficient soils in Benin City, Edo State, Nigeria. Rhizospheric soil samples were collected from three distinct locations (Ekewaun Road, Irhirhi, and Oko ADP), and PSB were isolated using serial dilution and cultured on Pikovskaya’s agar, modified with fluconazole to control fungal contamination. Six PSB isolates (TA1–TA6) showing halo zones around colonies were selected and subjected to morphological, biochemical, and functional assays. Their phosphate solubilization capacities were quantitatively assessed using solubilization index (PSI), solubilization efficiency (PSE), pH reduction, and production of indole-3-acetic acid (IAA), a key plant growth hormone. Isolate TA4 exhibited the highest PSI (1.57 ± 0.02) and PSE (57.13 ± 0.57%), along with the highest IAA production (15.7 µg/mL), marking it as the most potent solubilizer and plant growth-promoting rhizobacterium (PGPR). TA1 also demonstrated strong performance (PSI: 1.50 ± 0.02; PSE: 50.00 ± 0.75%; IAA: 15.0 µg/mL). All isolates caused a decline in medium pH during incubation, indicating organic acid production as a mechanism of phosphate solubilization. Biochemical tests identified isolates as belonging to beneficial genera such as Bacillus, Pseudomonas, and Actinomyces. Among them, Bacillus licheniformis (TA4) and Bacillus cereus (TA1) emerged as the most efficient candidates. These findings suggest that indigenous PSB from the O. gratissimum rhizosphere, particularly isolates TA4 and TA1, hold significant promise as biofertilizer agents. Their application could enhance sustainable agriculture by improving soil phosphorus availability, promoting plant growth, and reducing dependency on synthetic fertilizers. Further molecular characterization and field trials are recommended to validate these isolates for large-scale agricultural use. A essential macronutrient for plant growth, phosphorus is frequently found in forms that plants cannot use, especially in acidic tropical soils. In this study, phosphate-solubilizing bacteria (PSB) from the rhizosphere of Ocimum gratissimum (scent leaf), a commonly used medicinal and culinary plant, were isolated, screened, and biochemically analyzed. The phosphate solubilization efficiency of six bacterial isolates (TA1–TA6) was evaluated using in vitro assays, such as pH reduction, phosphate solubilization efficiency (PSE), phosphate solubilization index (PSI), and the generation of indole-3-acetic acid (IAA). With the greatest PSI (1.57±0.02e and 1.50±0.02d) and PSE (57.13±0.57f and 50.00±0.75e), respectively, and notable IAA production, TA4 and TA1 showed great promise as rhizobacteria that promote plant development. By means of morphological and biochemical tests, the isolates were identified to be members of the Actinomyces, Pseudomonas, and Bacillus genera. Based on these results, native PSB from scent leaf rhizospheres could be converted into biofertilizers, providing a sustainable way to enhance soil fertility and crop yield while lowering dependency on artificial fertilizers.