Mitigation of salt stress in plants by the salt-tolerant bacterium Pantoea ananatis JBR3-16 isolated from sand sedge (Carex pumila Thunb.)

Abstract

Salt stress is a major abiotic stress that limits plant growth, development, and productivity of crops. However, some salt-tolerant plant growth-promoting bacteria (ST-PGPB) can confer abiotic stress tolerance to plants. This study isolated 22 endophytic bacteria from the halophyte sand sedge (Carex pumila Thunb.) cultivated on sandy beach soil. One of the bacterial isolates, strain JBR3-16, survived in R2A broth supplemented with 10% NaCl, showing various plant growth-promoting properties under salt conditions. The salt tolerance mechanism of strain JBR3-16 was investigated through fluorescent microscopy, analysis of growth characteristics, and gene expression. Inoculation of plants with strain JBR3-16 significantly improved the salt-induced reduction of rosette diameter, leaf and root fresh weights, and chlorophyll contents in 100 and 150 mM NaCl. Moreover, strain JBR3-16 promoted salt tolerance by stimulating the production of indole-3-acetic acid, exopolysaccharides, proline, and catalase activity when cultivated on 50？200 mM NaCl medium. Furthermore, reducing reactive oxygen species (ROS) and expressing key abiotic stress-related genes under salinity conditions were the primary resistance mechanisms to high salt concentrations in JBR3-16. Finally, CoroNa Green as a fluorescent Na+ indicator was observed in the root tips under salt conditions but restricted by JBR3-16 co-cultivations. Our results revealed that ST-PGPB Pantoea ananatis JBR3-16 alleviates NaCl stress in plants by (1) inducing osmolytes and plant hormones, (2) lowering ROS generation and expression levels of salt-responsive genes, and (3) reducing Na+ uptake into plants. This study provides an approach to comprehensively understand and evaluate ST-PGPR as a biological inoculant for plants under salt stress.