L. plantarum MR1 relieved the HC-induced oxidative stress in Nile tilapia. (a,b) The activities of superoxide dismutase (SOD) in the serum (a) and liver (b) (n = 6). (c,d) The contents of malondialdehyde (MDA) in the serum (c) and liver (d) (n = 6). (e,f) The contents of reduced glutathione (GSH) in the serum (e) and liver (f) (n = 6). (g,h) The protein expression and quantification of Nrf2 in the liver (n = 6). (i) The relative mRNA expression of genes for Nrf2 signaling pathway in the liver (n = 6). Statistics were analyzed by one-way ANOVA with Tukey’s adjustment and presented as mean ± SEM (*, p < 0.05; **, p < 0.01).