Mitigating salinity and cadmium stress in rice (Oryza sativa L.) using PGPR and salicylic acid: rhizosphere, health risk, and physiological insights.

Abstract

Soil contamination with salinity and heavy metals such as cadmium (Cd) is becoming a serious global problem due to the rapid development of the social economy. Although plant growth-promoting rhizobacteria PGPR and organic agents such as salicylic acid (SA) are considered major protectants to alleviate abiotic stresses, the study of these bacteria and organic acids to ameliorate the toxic effects of salinity and Cd remains limited. Therefore, the present study was conducted to investigate the individual and combined effects of PGPR and SA on enhancing the phytoremediation of salinity (100 mM NaCl) and Cd (50 µM CdCl₂) using rice (Oryza sativa L.) plants. The research results indicated that elevated levels of salinity and Cd stress in soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, and gas exchange attributes. However, salinity and Cd stress also induced oxidative stress in the plants by increasing malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) by 44% and 38%, respectively, which also induced increased compounds of various enzymatic and nonenzymatic antioxidants, and also the gene expression and sugar content. Furthermore, a significant (P < 0.05) increase in cadmium accumulation, potential health risk indices, proline metabolism, the AsA-GSH cycle, and the pigmentation of cellular components was observed. Although the application of PGPR and SA showed a significant (P < 0.05) increase in plant growth and biomass, gas exchange characteristics, microbial diversity, functional gene abundance in the rhizosphere, enzymatic and nonenzymatic compounds, and their gene expression, and also decreased oxidative stress. In addition, the application of PGPR and SA enhanced cellular fractionation and decreased metal accumulation by 37% in shoots, proline metabolism, and the AsA-GSH cycle in O. sativa plants. These results provide new insights for sustainable agricultural practices and hold immense promise in addressing the pressing challenges of salinity and heavy metal contamination in agricultural soils.