Isolation and Characterization of Phosphate-Solubilizing Rhizobacteria from Solanum tuberosum with Plant Growth-Promoting Activity

Phosphate-solubilizing rhizobacteria associated with the Solanum tuberosum L. cultivar ‘Superchola’ were isolated and characterized to improve our understanding of plant growth promotion in agricultural systems. Bacteria were isolated by serial dilutions, and the morphology of the colonies was characterized on nutrient agar culture medium. In addition, morphological identification was achieved by Gram staining. The ability to solubilize phosphate was assessed in Pikovskaya agar culture medium, while molecular identification involved the amplification of the partial 16S rRNA gene using the polymerase chain reaction. In the Píllaro canton, the highest number of colony-forming units per gram of soil was recorded at 9.72 × 109. Among the isolated strains, 62% exhibited circular morphology, 92% had a smooth texture, and 85% displayed entire margins. Notably, 83% of the isolates were Gram-negative, with 50% exhibiting a bacillary form. The most effective phosphate solubilizers were from the Mocha canton, particularly the isolate CC-FCAGP-BSF6, which showed superior solubilization capacity. Molecular identification revealed bacterial isolates from four genera, i.e., Bacillus, Pseudomonas, Lysinibacillus, and Paenibacillus. These strains exhibited significant phosphate solubilization in vitro and resulted in increased leaf area (0.21–0.49, p = 0.038), fresh mass (0.46–0.87, p = 0.014), dry mass (0.092–0.096, p = 0.047), and leaf area index (0.14–0.33, p = 0.026) in the S. tuberosum cultivar ‘Superchola’ in vitro plants. This study identifies bacterial species associated with the rhizosphere of S. tuberosum in Ecuador and highlights their potential for promoting plant growth and solubilizing phosphates.