The Regulatory Mechanism Of Zinc On Iron-induced Phytotoxicity In Wheat Seedlings

In recent years,with the acceleration of industrialization,the improper disposal of living wastes and the unreasonable use of fertilizers and pesticides,heavy metal contamination in agricultural soils has become a serious problem,reducing crop yields and threatening human health on account of lowering food safety.Iron?Fe?,as a transition element for plants,is involved in such fundamental biological processes as photosynthesis and respiration,but if excessive,it can lead to the generation of highly toxic hydroxyl radicals?·OH?through the Fenton reaction,which can cause oxidative damage of macromolecules and even inhibit the growth and development of plants.As an essential trace element,zinc?Zn?plays an important role in maintaining the integrity and stability of plasma membrane,protecting cells from oxidative stress and alleviating the multiple heavy metals-induced toxicity.However,Zn at high levels is also toxic to plants and disrupts the normal metabolism in plants.In the present study,wheat?Triticum aestivum L.?cv Xihan 4 was chosen as the experimental material to investigate the effects of adding different concentrations of Zn(50?mol·L^-1,250?mol·L^-11 and2 mmol·L^-1)on 300?mol·L^-1Fe-induced phytotoxicity,aiming to explore the regulatory mechanism of Zn-alleviating effect on iron toxicity or their synergetic effect when plants were exposed to Fe treatment in combination with Zn.The main results are as follows:1.Fe treatment alone significantly inhibited the growth of wheat seedlings and the addition of50 or 250?mol·L^-11 Zn notably lowered the inhibition effects on root and shoot growth,but high Zn concentation(2 mmol·L^-1),on the contrary,exacerbated the inhibition of root growth in Fe-stressed wheat seedlings.2.300?mol·L^-11 Fe stress significantly elevated the accumulation of Fe content in roots and leaves and Mg level in roots,but reduced Zn and Na levels in roots as well as Ca and Mg contents in leaves.In comparison with Fe stress alone,the application of low Zn levels remarkably raised the amount of Zn,Na,potassium?K?and Ca in roots and leaves as well as Mg accumulation in leaves.However,high Zn concentration resulted in the maximum amount of Fe,Zn and Na in roots and leaves,but decreased Ca content in roots and leaves as well as root Mg content.3.Compared with the control,the treatment of 300?mol·L^-11 Fe weakened cell viability,but increased the molondialdehyde?MDA?and protein carbonyl contents together with the generation of total and apoplastic reactive oxygen species?ROS?in wheat roots.The application of 50 or 250?mol·L^-11 Zn lowered the loss of cell viability as well as the levels of lipid peroxidation and protein carbonylation in Fe-sterssed roots,and the effects of Fe+50?mol·L^-1Zn were more obvious.250?mol·L^-1Zn did not affect apoplastic hydrogenperoxide?H₂O₂?content,while lower Zn concentration blocked Fe-induced ROS accumulation in roots.Moreover,except for total and apoplastic superoxideanion(O₂^·?)as well as apoplastic·OH,these parameters were notably elevated as the result of Fe treatment combined with 2 mmol·L^-11 Zn.4.The significant enhancements of total and apoplastic superoxide dismutase?SOD?,peroxidase?POD?and ascorbate peroxidate?APX?activities were obsevered in Fe-treated roots,while total catalase?CAT?and glutathione reducase?GR?activities as well as apoplastic GR activity decreased.The addition of different Zn concentrations decreased total SOD,POD and APX activities as well as apoplastic CAT activity,but further stimulated total CAT and GR as well as apoplastic POD and GR in Fe-stressed roots.5.Fe stress not only resulted in the significant increases of such cell wall materials as hemicellulose I?HCI?,hemicellulose II?HCII?and pectin,but elevated the activities of sucrose synthase?SuSy?,cellulose synthase?CesA?and pectin methylesterase?PME?in wheat roots.Compared with Fe treatment alone,the application of 50 or 250?mol·L^-11 Zn reduced the accumulation of these substances and decreased the activities of Fe-induced SuSy,CesA and PME.In concusion,low Zn concentrations alleviated Fe-induced phytotoxicity in wheat roots and reduced the loss of cell viability as well as the level of membrane lipid peroxidation and protein carbonylation in response to Fe stress,which might be associated with the reduction of Fe-induced ROS generation in the presence of low Zn levels.Low Zn concentrations blocked Fe-induced accumulation of ROS,thus decreasing total SOD and POD activities as well as apoplastic SOD,CAT and APX ones and increasing the activities of total and aopolastic GR as well as apoplastic POD.In addition,Zn at low levels decreased the generations of cell wall polysaccharides,the activities of cellulose synthesis-related enzymes and the level of pectin methyl esterification in Fe-treated roots.However,synergetic effects of Zn at high levels and Fe further enhanced the phytotoxicity in wheat roots.