Plant growth promoting microorganisms can improve germination, seedling growth and potassium uptake of soybean under drought and salt stress

Soybean (Glycine max (L.) Merr; Leguminosae) is an important source of protein and oil for human and animal consumption. Understanding the interactions between plant growth promoting microorganisms (PGPM) and their host plants is necessary especially under stressful conditions. In this study, therefore, a set of twenty-one PGPM as well as a control (non-inoculation) treatment, were evaluated on seed germination, seedling growth and potassium uptake of soybean. Seed germination and soybean growth significantly enhanced when PGPM were applied. However, eight PGPM were selected in experiment I (optimum condition) based on the results of cluster analysis. These PGPM were then tested at three conditions including optimum (non-stress), drought (− 0.17 MPa, prepared using PEG8000) and salt (100 mM, prepared using NaCl) stress. Our findings indicated that four PGPM [Bacillus cereus (B1), Bacillus megaterium (B2), Trichoderma longibrachiatum (F1) and Trichoderma simmonsii (F2)] out of eight PGPM were better than the others in terms of promoting soybean seed germination and seedling growth at all conditions. Furthermore, the laboratory and pot experiments were carried out under the mentioned conditions to find the best combination of these PGPM (e.g., single, double, triple and quadruple forms). Therefore, single inoculation of B2, F1, F2 and triple inoculation of B1B2F2 were considered to be the best treatments due to improving seed germination, seedling growth and potassium uptake of soybean plants in both experiments. Consequently, in order to achieve the sustainable agriculture, application of these PGPM can be recommended due to either their positive effects on germination characteristics and/or improvement of soybean seedling growth under optimum and stressful conditions.