Bacillus subtilis B21-Mediated Salt Tolerance in Cucumber via Auxin Signalling Activation

Soil salinization is a major constraint on global crop productivity. While certain plant growth-promoting rhizobacteria (PGPR) improve plant survival under salt stress, their underlying molecular mechanisms remain largely uncharacterised. In this study, a highly halotolerant PGPR strain, Bacillus subtilis B21, was isolated from saline - alkali soils in Ningxia, China, and investigated its mechanism of action in cucumber (Cucumis sativus). Inoculation with B21 significantly promoted root growth, re-established ion and reactive oxygen species (ROS) homoeostasis, and alleviated osmotic stress under high salinity. Transcriptomic analysis revealed that B21 activated host plant hormone signalling pathways and induced the expression of genes encoding antioxidant enzymes. Full-genome annotation and pharmacological assays confirmed that B21 synthesises indole-3-acetic acid (IAA) via the tryptamine pathway and that the bacterial IAA is perceived by the host plant to modulate cucumber salt tolerance. Specifically, B21-derived IAA upregulated key genes involved in the cucumber auxin signalling pathway, which in turn enhanced the expression of ROS-scavenging genes and reduced salt-induced oxidative damage. Collectively, these findings demonstrated that cucumber roots absorbed IAA production by Bacillus subtilis B21, thereby activating the expression of IAA signalling and ROS detoxification that had been inhibited by salt stress. These findings provide new mechanistic insights into PGPR-mediated plant salt tolerance.