NADPH oxidase-dependent H₂ O₂ production mediates salicylic acid-induced salt tolerance in mangrove plant Kandelia obovata by regulating Na⁺ /K⁺ and redox homeostasis

Salicylic acid (SA) is known to enhance salt tolerance in plants. However, the mechanism of SA-mediated response to high salinity in halophyte remains unclear. Using electrophysiological and molecular biological methods, we investigated the role of SA in response to high salinity in mangrove species, Kandelia obovata, a typical halophyte. Exposure of K. obovata roots to high salinity resulted in a rapid increase in endogenous SA produced by phenylalanine ammonia lyase pathway. The application of exogenous SA improved the salt tolerance of K. obovata, which depended on the NADPH oxidase-mediated H₂ O₂ . Exogenous SA and H₂ O₂ increased Na⁺ efflux and reduced K⁺ loss by regulating the transcription levels of Na⁺ and K⁺ transport-related genes, thus reducing the Na⁺ /K⁺ ratio in the salt-treated K. obovata roots. In addition, exogenous SA-enhanced antioxidant enzyme activity and its transcripts, and the expressions of four genes related to AsA-GSH cycle as well, then alleviated oxidative damages in the salt-treated K. obovata roots. However, the above effects of SA could be reversed by diphenyleneiodonium chloride (the NADPH oxidase inhibitor) and paclobutrazol (a SA biosynthesis inhibitor). Collectively, our results demonstrated that SA-induced salt tolerance of K. obovata depends on NADPH oxidase-generated H₂ O₂ that affects Na⁺ /K⁺ and redox homeostasis in response to high salinity.