Rhizobacteria Enterobacter sp. LHB11 and Bacillus sp. PIXIE Induced Systemic Tolerance Against Drought Stress in Tomato (Solanum lycopersicum)

Plant growth-promoting rhizobacteria (PGPR) induce changes in the plant metabolism, improving plant growth under drought stress conditions by employing different mechanisms of interaction. In this study, two bacterial strains (Enterobacter sp. LHB11 and Bacillus sp. PIXIE) were evaluated in vitro regarding their PGPR traits, including the ACC-Deaminase enzyme activity. Both PGPR strains produced indole acetic acid (40.65–75.81 µg−1mL−1), exopolysaccharides (39.23–40.20 µg eq CR mL−1), proline (61.5–106.1 mM), and volatile organic compounds. Furthermore, both solubilized phosphate (1.15–1.53 ratio, halo/colony) and fixed the atmospheric nitrogen. Only LHB11 showed ACC-Deaminase activity. Furthermore, both strains tolerated osmotic stress induced in liquid media with up to 20% of Polyethylene glycol-6000. In a drought stress pot experiment, both strains were applied to tomato roots, exposed to normal irrigation (100%) and drought stress (decreasing irrigation by 50%). The inoculation of both strains improved the plant growth parameters under stress conditions significantly, e.g., the root dry weight (+41.0–43.4%), while the proline content decreased to a level similar to the unstressed control. In addition, strain inoculation increased the total phenolic content and the antioxidant capacity measured as the inhibitions of the ABTS radical and as the reduction in ferric ions and increased the catalase and ascorbate peroxidase enzyme activity. The bacterial contribution to the changes in biochemical parameters is higher than in morphological parameters. At the same time, the strains modulate specific parameters depending on the stress condition, e.g., ABTS, catalase activity, and proline content. In conclusion, both strains Induced Systemic Tolerance (IST), regardless of their capacity to use the ACC-Deaminase mechanism, by modulating several mechanisms of plant response to drought stress. Our results showcase the relevance of considering the orchestration of several plant response mechanisms in order to fully assess the potential and efficiency of the plant–PGPR interaction under drought stress.