Hemin Through the Heme Oxygenase 1/Ferrous Iron, Carbon Monoxide System Involved in Zinc Tolerance in Oryza Sativa L

Several pharmacological experiments were performed to investigate the possible roles of heme oxygenase 1 (HO1) and its catalytic products on hemin-induced attenuation of excess zinc toxicity. The results showed that hemin markedly reduced concentrations of zinc in rice seedlings (including roots and shoots); this effect may be achieved by down-regulating the relative expression of OsZIP1, OsZIP3, OsZIP7, and OsZIP8. However, zinc protoporphyrin IX (Znpp, an HO1 inhibitor) application could reverse hemin-induced decreases in zinc accumulation and upregulate the relative expression of the above four genes. These results showed that HO1 is indeed involved in the mitigation of zinc stress. The following experiments were conducted to illustrate the contributive effect of different hemin by-products in the hemin-mediated attenuating effect. The results showed that bilirubin (BR) does not have any alleviative effect, whereas carbon monoxide (CO) has a similar effect as hemin; the alleviative effect of ferrous ion (Fe2+) is even better than that of hemin. Altogether, these findings suggested that hemin, through the HO1/Fe2+, CO system, may reduce zinc accumulation in rice seedlings and ultimately strengthen Zn tolerance of rice seedlings.