# Bacillus velezensis SQR9-Emitted Volatiles Enhance Arabidopsis Salt Tolerance via ROS Scavenging and Ion Transport Regulation

## TL;DR

A soil bacterium's volatile compounds help plants tolerate salt stress by reducing damage and improving ion balance.

## Contribution

Identifies 2,3-butanediol as a key volatile compound from Bacillus velezensis SQR9 that enhances plant salt tolerance.

## Key findings

- SQR9 VOCs increase Arabidopsis salt tolerance by reducing oxidative stress and improving ion homeostasis.
- 2,3-butanediol mimics the salt-tolerance-enhancing effects of SQR9 VOCs when applied exogenously.
- VOCs upregulate ion transporter genes and activate antioxidant defense mechanisms in plants.

## Abstract

Salinity stress severely limits crop productivity worldwide. While plant growth-promoting rhizobacteria (PGPR) are known to alleviate abiotic stress, the specific mechanisms mediated by their volatile organic compounds (VOCs) remain largely elusive. In this study, an in vitro split-plate system was used to investigate the effects of VOCs emitted by Bacillus velezensis SQR9 on Arabidopsis thaliana seedlings under salt stress. Exposure to SQR9 VOCs significantly enhanced Arabidopsis salt tolerance, evidenced by increased biomass and root growth. Mechanistically, SQR9 VOCs mitigated salt-induced damage by increasing chlorophyll content, modulating osmolytes, and reducing malondialdehyde (MDA) levels. SQR9 VOCs alleviated oxidative stress by decreasing ROS (H2O2, O2−) accumulation and enhancing antioxidant enzyme (SOD, CAT, POD) activities. Furthermore, SQR9 VOCs maintained ion homeostasis by significantly reducing leaf Na+ accumulation, maintaining a high K+/Na+ ratio, and upregulating key ion transporter genes. Analysis of the headspace from SQR9 cultured on MSgg medium identified 2,3-butanediol (2,3-BD) as a major active VOC. Exogenous application of 2,3-BD successfully mimicked the growth-promoting and salt-tolerance-enhancing effects of SQR9. Our findings demonstrate that SQR9 VOCs, particularly 2,3-BD, systemically prime Arabidopsis for salt tolerance by co-activating the antioxidant defense system and the SOS ion homeostasis pathway.