Nitric oxide enhances copper tolerance by regulating cell wall composition and copper transporting-related transcripts in cotton roots

Little is known about nitric oxide (NO)-mediated cotton plants' response to copper (Cu) stress and the underlying tolerance mechanism. It was hypothesized that NO can alleviate Cu toxicity to cotton roots by regulating the root cell wall composition and the transcription of Cu ion transporting-related genes. Cu stress significantly increased NO synthase (EC 1.14.14.47) activity, leading to elevated endogenous NO content. Cu excess-induced growth inhibition was reversed by sodium nitroprusside (SNP, NO donor) application but exacerbated by cPTIO (NO scavenger) addition. The SNP + Cu treatment promoted more Cu ions accumulation in roots and less Cu ions transportation to leaves than Cu treatment, which also produced the largest Cu uptake amount per plant among all treatments. The concentration of cell wall pectin was significantly enhanced by 16.95% by the SNP application. Pectin methylesterase activity was up-regulated by 30.86% (p < 0.05), thus resulting in a reduction of 10.39% in pectin methylesterification degree in the Cu + SNP treatment than in Cu stress alone; additionally, Cu chaperons COX17, CCH, and ATX1, Cu chelator MT2, and Cu homeostasis regulator SPL7 exhibited higher transcriptional levels. Collectively, NO improved cotton roots' tolerance to Cu stress through the enhancement of Cu ions binding to cell wall due to increased polysaccharide biosynthesis and pectin demethylesterification degree, and via the promotion of Cu ions sequestration owing to up-regulated expressions of Cu chaperones and chelators. These findings should have significant implications for the phytoremediation of Cu-contaminated soils by using cotton plants, which needs further validation under field conditions.