Plant Growth-Promoting Rhizobacteria (PGPR) Reduce Adverse Effects of Salinity and Drought Stresses by Regulating Nutritional Profile of Barley

Abstract

With the growing emphasis on sustainable agriculture, food security, and environmental protection, the use of beneficial soil microbes is imperative, as the use of chemicals such as fertilizers, pesticides, and herbicides has resulted in food contamination, disease, weed resistance, and negative environmental consequences, which ultimately impacted human health. Climate change is a major factor and is of great concern for crop production. Abiotic stresses, including salt and drought stress, restrain the crop yield. The aim of this particular study is to understand what role do plant growth-promoting rhizobacteria (PGPR) play in combating the salinity and drought stresses through modification of nutritional profile. In the current study, inoculated barley (Hordeum vulgare L.) plants were subjected to various stresses such as 200 mM and 1000 mM salinity stress as well as drought stress, and then their various parameters such as seed germination as well as shoot and root biomasses and photosynthetic activity were compared with non-treated stressed barley plants. Our data depicted an improvement or significant enhancement of these parameters in PGPR (Pseudomonas fluorescens SBW25 and Pseudomonas putida KT2440) applied barley plants. Furthermore, the particle-induced X-ray emission (PIXE) technique was used for the elemental analysis of PGPR-inoculated and non-inoculated plants under stress vs. no stress conditions. Our PIXE analysis of various macro- and micronutrients revealed an enhancement of Ca, Mg, K, P, S, Al, and Si uptake in PGPR-treated plants. PGPR applications depicted reduced Cl− contents in 200 mM salt-stressed barley roots (KT2440 = 7.7 mg/kg and SBW25 = 6.3 mg/kg) and stems (KT2440 = 406.4 mg/kg and SBW25 = 365.5 mg/kg) as compared to controls (roots = 8.9 and stems = 469.5), while they displayed a significant increase in the barley leaves (KT2440 = 405 mg/kg and SBW25 = 416.4 mg/kg) when compared to control (110.6 mg/kg) under the same stress condition. In 1000 mM salt stress, a significant reduction in the Cl− content was observed in PGPR-applied barley roots (KT2440 = 7.6 mg/kg), stems (KT2440 = 1205.8 mg/kg and SBW25 = 1008.3 mg/kg), and leaves (KT2440 = 967.8 mg/kg and SBW25 = 530.8 mg/kg) when compared to controls (roots = 15.2 mg/kg, stems = 1605.2 mg/kg, and leaves = 1165.2 mg/kg). On the other hand, a significant increase in the Cl− content was noticed in PGPR-applied barley roots (KT2440 = 29.5 mg/kg and SBW25 = 25.8 mg/kg), stems (KT2440 = 1023.8 mg/kg and SBW25 = 894.9 mg/kg), and leaves (KT2440 = 369.2 mg/kg and SBW25 = 409.8 mg/kg) when compared to controls (roots = 13.5 mg/kg, stems = 505.3 mg/kg, and leaves = 219.9 mg/kg) under drought stress condition. PGPR application was also found to be effective for enhancing the uptake of micronutrients (Mn, Fe, Co, Ni, Cu, and Zn) in barley plant parts under control and also under stressed conditions. Overall, our findings revealed an improvement in the uptake of macro- and micronutrients for the enhancement of salinity and drought stress tolerance. Conclusively, these PGPR species are an effective source of plant stress tolerance and elevated growth of barley and related plants under stress conditions.