Rhizobacterial coordination of reactive oxygen species homeostasis underpins mulberry resilience to waterlogging

Rhizobacteria are crucial for plant adaptation to abiotic stresses; however, their contributions to waterlogging resilience of woody plants in fragile riparian zones remain poorly understood. Here, we investigated whether and how the rhizobacterium Klebsiella variicola HWS1, isolated from a riparian zone, improved waterlogging tolerance in mulberry. Our results demonstrated that HWS1 exhibited multiple plant-beneficial traits in vitro and harbored genomic signatures associated with plant growth promotion and stress adaptation. Co-cultivation with HWS1 significantly promoted mulberry growth and elevated antioxidant enzyme activities under waterlogging conditions. Transcriptomic profiling further revealed that stress primarily reprogrammed genes related to oxidoreductase activity, while HWS1 specifically enriched genes involved in hydrogen peroxide metabolism, implicating that alleviation of oxidative stress is a key mechanism for enhanced resilience. Interestingly, waterlogging triggered excessive accumulation of reactive oxygen species (ROS) in mulberry, whereas HWS1 displayed a robust capability to tolerate and detoxify hydrogen peroxide, thereby helping maintain host ROS homeostasis. Genetic analyses identified the bacterial ahpC and katG genes as essential for oxidative stress tolerance. In particular, deletion of katG significantly impaired its ability to scavenge plant-derived ROS and consequently compromised mulberry stress resistance. These findings reveal a mechanism in rhizobacteria-plant interaction whereby rhizobacterium coordinates the reprogramming of host ROS metabolic genes with its own antioxidant defenses to enhance plant resilience, highlighting ROS-detoxifying rhizobacteria as promising agents for mitigating waterlogging in riparian and agricultural ecosystems.