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Carbon monoxide (CO) has recently proven to be an important bioactive or signaling molecule in mammalian cells. Its effects are mainly mediated by nitric oxide (NO) and cyclic GMP (cGMP). In Vicia faba leaves, CO production and heme oxygenase (HO) activity, an important CO synthetic enzyme, are first reported to increase in response to ABA treatment, which could result in stomatal closure. Interestingly, ABA-induced stomatal closure in V. faba guard cells is partially blocked when the synthetic CO inhibitor ZnPP, or the CO/NO scavenger Hb is added. Furthermore, we show that, exogenously applied CO donor, hematin, and CO aqueous solution not only result in the enhancement of CO release, but also time-dependently induce stomatal closure, and the latter is mimicked by the application of an NO donor SNP. The above-mentioned stomatal closure effects are differentially reversed by the addition of tungstate, a potent inhibitor of NO synthetic enzyme nitrate reductase (NR), the specific NO scavenger cPTIO, ZnPP, or Hb. During treatment for 4 h, SNP, 0.01 % CO aqueous solution or hematin significantly triggers NO synthesis, whereas cPTIO, or tungstate approximately fully inhibits NO fluorescence. Additionally, application of the GC inhibitor ODQ blocks CO-induced stomatal closure. This inhibition could be reversed when 8-Br-cGMP is added. Thus, the above results suggest that CO produced by HO is involved in ABA-induced stomatal closure, and NO and cGMP may function as downstream intermediates in the CO signaling responsible for stomatal closure. [ABSTRACT FROM AUTHOR]

Abstract: 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 100mM NaCl salt stress. Both hematin and CO aqueous solution dose-dependently attenuated the inhibition of seed germination and seedling growth caused by salinity. Moreover, 1.0μM 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 36h 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. [Copyright &y& Elsevier]