Molecular Characterization of Plant Growth-Promoting Bacteria Associated with Opuntia dillenii (Ker Gawl.) Haw (Cactaceae) in the Coastal Zone of Benin

Cacti thrive in arid and coastal environments partly through associations with beneficial endophytic and rhizosphere bacteria; however, current knowledge remains limited. This study aimed to assess the diversity of cultivable bacteria associated with Opuntia dillenii and evaluate their potential as Plant Growth-Promoting (PGP) agents. Endophytic bacteria were isolated from cladodes and roots, while rhizobacteria were recovered from rhizosphere soil. Bacterial isolates were identified using morphological characteristics and 16S rRNA/gyrB sequencing, followed by screening for PGP traits, pH and temperature tolerance. A total of 31 isolates were obtained, including 23 endophytes and 8 rhizobacteria, mainly affiliated with Firmicutes, Actinobacteria, and Proteobacteria. Bacillus (35.48%) and Priestia (32.25%) predominated, with Priestia flexa as the most prevalent species. The most frequent PGP traits were phosphate solubilization (80.65%), proteolytic activity (70.97%), siderophore production (67.74%), and nitrogenase activity (64.52%). The highest phosphate solubilization indices were observed for strain R3 (3.41), R6 (3.39) and S6 (3.21), whilst the highest indole-3-acetic acid yields were recorded for C9 (172.88 µg/mL), R11 (96.22 µg/mL) and C3 (90.94 µg/mL), and the strongest siderophore production for C3 (30.37 mm), C7 (27.96 mm) and S7 (27.88 mm). These findings highlight O. dillenii-associated coastal bacteria as promising resources for plant growth and plant stress resilience.