Heme Oxygenase 1/Carbon Monoxide,Ferrous Iron System Is Involved In Hemin-induced Alleviation Of Heavy Metal Stress

Rice(Oryza saliva L.),as one of the most important cereal crops,is widely cultivated across the world and considered a staple food in many Asian countries,however,heavy metal pollution has become one of the major environmental problems,especially for the agriculture section due to highly anthropogenic activities.In this study,we investigated the possible role of hemin in alleviating zinc(Zn),lead(Pb)and chromium(Cr)toxicity in rice seedlings grown hydroponically by analyzing the morphological and physiological parameters.And several pharmacological experiments were subsequently performed to investigate the possible roles of HO1 and its catalytic products on hemin-induced attenuation of excess-zinc-toxicity.1.Our results showed that exposure of rice seedlings to excess Zn,Pb or Cr could cause severe leaf chlorosis,inhibit photosynthetic activity and consequently suppress plant growth.The concentration of O2·-and H2O2 significantly increased and the activities of antioxidative enzymes decreased in roots of rice seedlings under metal exposure.The combined treatments(hemin + ZnSO4,hemin + Pb(NO3)2 and hemin + K2Cr2O7),on the other hand,significantly enhanced the photosynthesis-and plant growth-related parameters compared with their corresponding heavy-metal-stress alone.Combined treatments dramatically stimulated the activities of superoxide dismutase(SOD),ascorbic peroxidase(APX)and glutathione reductase(GR)as well as the concentrations of ascorbic acid(AsA)and glutathione(GSH)as compared with the metal-stress alone.The concentrations of reactive oxygen species(ROS,e.g.O2·-and H2O2)were significantly reduced in the metal plus hemin treatments.Hemin addition also reduced metal accumulation in rice seedlings especially in root tissues.These findings suggest that hemin-elevated levels of antioxidants,activities of antioxidative enzymes and hemin-reduced accumulation of heavy-metal could confer resistance against Zn,Pb,and Cr stress in rice seedlings,resulting in improved pigments accumulation,photosynthetic attributes and plant growth.2.Results showed that hemin notably reduced the concentration of zinc in rice seedlings(including roots and shoots),this effect might be achieved by down-regulated the relative expression of OsZIP1,OsZIP3,OsZIP6,OsZIP7a and OsZIP8.However,Znpp application could reverse hemin-induced decrease of zinc uptake,and the relative expression of the above 5 genes were up-regulated by ZnPP administration.Those results showed that HO1 indeed get involved in mitigate zinc stress.The following byproducts experiments were conducted to illustrate the contributive effect of products in the hemin-induced mitigative effect.Results showed that BR dose not possess any alleviative effect,while CO has similar effect as hemin,and the mitigative effect of Fe2+ is even better than hemin.This might caused by Fe2+ trigged stronger expression of OsIRT1 and OsIRT2 than hemin,which resulted in more iron uptake in Zn + FeSO4 group.All these findings suggest that heimn through HO1/CO、Fe2+ system to reduce zinc accumulation and finally strengthen tolerance of rice seedlings against excess-zinc-toxicity.