Evaluation of Stress-Tolerant Serratia and Enterobacter as PGPR for Nutrient Solubilization and Dose-Dependent Bioformulation to Enhance Tomato Seedlings

Plant growth-promoting rhizobacteria (PGPR) are eco-friendly and sustainable options for agrochemicals, particularly for enhancing crop productivity under stress conditions. The present research aims to isolate and characterize native PGPR from tomato rhizospheric soil and to evaluate their effectiveness as a dose-dependent response to enhance the growth of tomato seedlings. Out of 112 isolates, 10 bacterial strains were selected based on key PGPR traits, including indole-3-acetic acid (IAA), ammonia production, hydrogen cyanide (HCN), exopolysaccharide (EPS) synthesis, hydrolytic enzyme activity, potassium solubilization, antifungal activity against <i>Fusarium oxysporum</i>, and tolerance to pH and heat stress. Molecular identification via 16S rRNA gene sequencing confirmed that these isolates belong to the genera <i>Serratia</i> and <i>Enterobacter</i>. <i>S. marcescens</i> So-1 and <i>Enterobacter</i> sp. So-12 produced the highest levels of IAA (2.6-24.1 µg/mL). In vitro tomato seed germination tests using bacterial suspensions at three concentrations (10⁶, 10⁷, and 10⁸ CFU/mL) showed dose-dependent improvements, with T1 increasing germination up to 108.3% compared to the control. In polyhouse trials using cocopeat formulations, seedling growth improved noticeably. T2 increased the root length (28.3 ± 2.98 cm) by over 1560%, and the shoot length (35.7 ± 0.57 cm) increased by 55% against the control, whose root length is 1.7 ± 0.47. The chlorophyll amount of the treated leaves further showed significant results over the control. Collectively, these findings suggest that using native PGPR in a dose-dependent way can help tomato seedlings grow better and promote more sustainable crop production.