Bacterial-Mediated Induction of Systemic Tolerance to Salinity with Expression of Stress Alleviating Enzymes in Soybean (Glycine max L. Merrill)

Abstract

This study explored several features related to salt tolerance in soybean plants through plant growth-promoting rhizobacteria (PGPR; Pseudomonas sp. strain AK-1, and Bacillus sp. strain SJ-5). We report the significant effect of 1-aminocyclopropane-1-carboxylate deaminase, indole-3-acetic acid production and exopolysaccharide production from both bacterial strains on physical parameters and biochemical activities in Glycine max plants under salt stress. In this report, we investigated the leaf water content, osmolyte accumulation, and activities of stress-responsive enzymes in the absence and presence of salt stress. Control (plants devoid of bacterial strains) and PGPR-inoculated soybean plants were grown in half Murashige and Skoog medium subjected to saline and non-saline conditions. Results showed that PGPR-inoculated plants had superior tolerance against salt stress, as shown by their enhanced plant biomass (fresh weight), higher water content, higher photosynthesis activity, and lower osmotic stress injury. The increased proline accumulation and lipoxygenase activity in PGPR-inoculated plant roots contributed to increased plant tolerance to salt stress. SJ-5-inoculated plants (0.414 U/mg protein) and AK-1-inoculated plants (0.403 U/mg protein) showed higher LOX activity than control plants (0.366 U/mg protein). Proline content was higher in SJ-5-(120 µg/g f.w.) and AK-1-(135 µg/g f.w.)inoculated plants than control plants (90 µg/g f.w.). Peroxidase activity was also higher in PGPR-inoculated plant roots during salinity. These results suggest that, in PGPR-inoculated roots, lipoxygenase plays a role in mitigating the adverse effect of salt stress. Furthermore, enhanced proline maintains osmotic balance and a positive water potential for water entrance into the roots, and peroxidase enzyme reduces oxidative damage by lowering reactive oxygen species level under salt stress. Our results indicated that both Pseudomonas and Bacillus are multifunctional PGPR strains that can promote plant growth, development and reduce salinity stress. However, our Bacillus bacterium strain had more ACC deaminase, phosphate solubilization, and siderophore activity under salt stress as compared to the Pseudomonas strain.