Role of Endophytic Bacillus subtilis SSA4 and Melatonin in Enhancing Paddy Seedling Growth under Salt Stress

NaCl stress significantly limits the growth and yield of paddy crop plants. Plant growth-promoting endophytes significantly affected plant health. Melatonin (N-acetyl-5-methoxytryptamine; MT) is a plant hormone and multifunctional signalling molecule plays a significant role in NaCl stress tolerance. In the present study, paddy seedlings treated with B. subtilis strain SSA4 egg cell-based bioformulation and MT (50 μM and 100 μM) significantly (P < 0.001) enhanced paddy seedlings growth parameters, photosynthetic pigments, photosynthesis, respiration and leaves gas exchange parameters. A significant decline (P < 0.001) in oxidative stress biomarkers and antioxidant enzymes activities under NaCl (0-100 mM) lower and higher dose stress was noted due to B. subtilis strain SSA4 and MT treatments. Regression analyses exhibited positive relationship between O₂ evolution and respiration O₂ with antioxidant enzymes CAT (R² = 0.5487; R² = 0.3131), SOD (R² = 0.2943; R² = 0.262) and POX (R² = 0.6162; R² = 0.6266) indicates significant improvement in paddy seedlings. In conclusion, endophyte co-inoculation with signalling molecule found an effective bio-tools to mitigating NaCl stress in paddy seedlings.