Bacillus velezensis HR6-1 enhances salt tolerance in tomato by increasing endogenous cytokinin content and improving ROS scavenging

The application of plant growth-promoting rhizobacteria (PGRP) is a promising and innovative strategy for alleviating salt stress in plants. However, the mechanism underlying PGRP-mediated alleviation of salt stress is currently unclear. In this study, we observed that the tomatoes inoculated with Bacillus velezensis HR6–1 exhibited better growth indicators and photosynthesis-related parameters than non-colonized tomatoes under salt stress. Physiological analysis showed that tomatoes inoculated with HR6–1 exhibited better Na+/K+ balance and lower ROS accumulation and oxidative damage, and higher activities of antioxidant enzymes compared with non-colonized tomatoes under salt stress. Transcriptome analysis revealed that under salt stress, HR6–1 treatment improved the expression of various transcription factors (especially WRKYs and ERFs) and many genes related to plant hormone signal transduction, the MAPK signaling pathway, the salt overly sensitive pathway, and detoxification in tomatoes. Moreover, HR6–1 inoculation increased the content of cZ- and tZ-type cytokinins in salt-treated tomato seedlings, which was consistent with the high expression of several cytokinin synthesis genes. Treatment with a cytokinin synthesis inhibitor prevented HR6–1-mediated improvement in salt tolerance in tomato seedlings, implying that HR6–1 stimulates cytokinin synthesis to enhance tomato resistance to salt stress. Our findings identify a potential biostimulant for improving tomato growth under salt stress and deepen our understanding of PGPR-mediated salinity alleviation in tomato seedlings.