Carbon monoxide enhances salt tolerance by nitric oxide-mediated maintenance of ion homeostasis and up-regulation of antioxidant defence in wheat seedling roots

Salt stress induced an increase in endogenous carbon monoxide (CO) production and the activity of the CO synthetic enzyme haem oxygenase (HO) in wheat seedling roots. In addition, a 50% CO aqueous solution, applied daily, not only resulted in the enhancement of CO release, but led to a significant reversal in dry weight (DW) and water loss caused by 150 mm NaCl treatment, which was mimicked by the application of two nitric oxide (NO) donors sodium nitroprusside (SNP) and diethylenetriamine NO adduct (DETA/NO). Further analyses showed that CO, as well as SNP, apparently up-regulated H+-pump and antioxidant enzyme activities or related transcripts, thus resulting in the increase of K/Na ratio and the alleviation of oxidative damage. Whereas, the CO/NO scavenger haemoglobin (Hb), NO scavenger or synthetic inhibitor methylene blue (MB) or N-G-nitro-l-arginine methyl ester hydrochloride (l-NAME) differentially blocked these effects. Furthermore, CO was able to mimic the effect of SNP by strongly increasing NO release in the root tips, whereas the CO-induced NO signal was quenched by the addition of l-NAME or cPTIO, the specific scavenger of NO. The results suggested that CO might confer an increased tolerance to salinity stress by maintaining ion homeostasis and enhancing antioxidant system parameters in wheat seedling roots, both of which were partially mediated by NO signal.