Nitric oxide mitigates copper toxicity in upland cotton via increasing antioxidant defense response and copper sequestration

Whether and how NO is implicated in cotton Cu detoxification remains obscure. A hydroponic experiment was conducted to assess the effects of sodium nitroprusside (SNP, NO donor) on cotton growth, antioxidative responses, Cu uptake and distribution as well as related gene expression under Cu stress. Cotton seedling’s growth and photosynthetic pigment concentration were decreased by Cu excess. The inhibitory effects could be reversed by the application of SNP with 50 μM SNP performing the best. Cu stress stimulated the massive production of reactive oxygen species (ROS), while SNP application can help to abolish them. The addition of SNP promoted more Cu ions deposition into roots and binding to the cell wall, thus limiting their transportation up to shoots and distribution in the leaf cell protoplast; the proportions of the two active Cu chemical forms were decreased by 10%–30.45% in leaves, and by 49.83%–56.31% in roots, respectively. SNP-induced transcription increase in <i>GhHIPP39</i> and <i>GhMT2</i> decreased Cu ion bioavailability within cells. Collectively, the SNP application alleviated the Cu toxicity in cotton plants by increasing ROS scavenging ability and promoting the sequestration of Cu into the cell wall and the immobilization of Cu inside cells. The present study demonstrated SNP application contributes to increasing cotton tolerance to Cu excess, and provides an effective method for the phytostabilization of Cu-contaminated soil using cotton plants. The SNP application can limit the transportation of Cu ions from roots to shoots under Cu stress, thus decreasing the damage to aerial parts. Although more Cu ions were accumulated in roots due to the SNP application, a large portion of them were attached to root cell walls and existed in inactive chemical forms inside cells, thus Cu phytotoxicity was significantly alleviated. In addition, the mechanism of NO-mediated ROS scavenging was elucidated.