Carbon monoxide counteracts the inhibition of seed germination and alleviates oxidative damage caused by salt stress in Oryza sativa

Although carbon monoxide (CO) has always been regarded as a toxic gas, recent reports suggested that it is one of the products of heme oxygenase (HO; EC 1.14.99.3) catalysis in animals, and could confer beneficial cytoprotection against oxidative damage. Here, we investigated the effects of exogenous putative CO donor hematin and CO aqueous solution on rice seed germination under 100 mM NaCl salt stress. Both hematin and CO aqueous solution dose-dependently attenuated the inhibition of seed germination and seedling growth caused by salinity. Moreover, 1.0 μM hematin and 5% CO-saturated aqueous solution activated amylase activity, thus accelerating the formation of reducing sugar and total soluble sugar. Furthermore, hematin and CO aqueous solution induced catalase (CAT) and superoxide dismutase (SOD) activities, thus resulting in the alleviation of oxidative damage, as indicated by the decrease of thiobarbituric acid reactive substances (TBARS) content. Such above CO-induced effects were also confirmed by using semi-quantitative RT-PCR, including the up-regulation of expression patterns of α-amylase, CAT and Cu/Zn-SOD genes. Also, the similar function of hematin might contribute to endogenous HO-derived CO, since addition of the CO-specific synthetic inhibitor zinc protoporphyrin IX (ZnPPIX) and CO scavenger hemoglobin (Hb) reversed above effects. Meanwhile, further results also proved that treatment with hematin for 36 h could result in the potent induction of HO-1 transcript, HO activity and CO content in the germinating seeds. Collectively, these results indicated that CO performed an advantageous effect on attenuation of inhibition of seed germination and seedling growth induced by salt stress, and alleviated oxidative damage via activating anti-oxidant enzymes.