Isolation and Characterisation of Plant Growth-Promoting Rhizobacteria for Improving Growth and Water/Salt Stress Resilience in Lettuce

Plant Growth-Promoting Rhizobacteria (PGPR), represent a promising tool for the development of sustainable agriculture practices. Although numerous strains have been described in the literature, their characterisation often overlooks the ability to sustain functional activity under common abiotic stress conditions, such as water deficit and high salinity. The present study aimed to isolate putative PGPR strains from different environmental and biological matrices, characterise their key plant growth-promoting traits, and evaluate their effectiveness in improving plant growth under water and salt stress conditions. The isolated strains were initially tested in vitro for phytohormone production, phosphate solubilisation, and siderophore production. Selected Bacillus and Pseudomonas strains exhibiting the most promising traits were tested in a preliminary greenhouse pot test using lettuce (Lactuca sativa), followed by assays under drought stress (50% water reduction) and salt stress (100 mM NaCl). The results demonstrated that the two Bacillus velezensis strains (PB_8 and CSS_12) significantly enhanced plant growth by increasing foliar biomass and root development improving pigment content, and mitigating stress-induced damage. Overall, these findings support the potential of PGPR-based strategies for low-impact agricultural practices and enhancing plant resilience under stress conditions.