Response Of Wheat To Fe Stress Based On Lignin Synthesis

Owing to the acceleration of urbanization and urban industry,various heavy metal ions enter the ecosystem,and continue to migrate,accumulate and transform,resulting in enhancing environmental pollution of heavy metals and causing serious disadvantage to animals and plants.Iron（Fe）,as a transition metal element,participates in plant life activities such as photosynthesis,respiration and nitrogen metabolism.Although iron in the soil under aerobic conditions will form insoluble complexes at neutral or alkaline p H,which is not easily used by organisms,excessive iron will generate toxic free radicals through the Fenton reaction and destroy the biological macromolecules in the plant,thus seriously inhibiting normal metabolism.In this study,spring wheat cv Xihan 3 was used as experimental material to investigate the effects of different concentrations of FeCl₃treatments(0,100,300 and 500μmol·L^-1)on wheat seedlings growth,cell wall polysaccharide components,lignin content,enzyme activity and gene expression related to lignin synthesis,to investigate the response to lignin synthesis metabolism of wheat seedlings under excessive iron.The main results are as follows:1.Low Fe concentration could stimulate the root growth of wheat seedlings,while high Fe concentration could restrain the root growth and reduce the root activity.Among them,500μmol·L^-1FeCl₃had the most obvious inhibitory effect.The plant height of wheat seedlings was decreased when treated by 300 and 500μmol·L^-1FeCl₃.Under high Fe concentration,fresh weight,dry weight and the root-shoot ratio of wheat seedlings root all decreased,while Fe stress had little effect on the dry weight of wheat seedlings leaves.The fresh weight of leaves only decreased under 500μmol·L^-1FeCl₃treatment.2.Fe treatments significantly induced the accumulation of apoplast reactive oxygen species（ROS）in the roots of wheat seedlings.DPI（Diphenyleneiodonium chloride,NADPH oxidase inhibitor）treatments alone significantly increased the contents of apoplast H₂O₂and·OH,while the apoplast O₂^·-content decreased significantly;compared with iron stress alone,the adding of DPI improved the apoplast H₂O₂ and·OH contents,while significantly decreased the apoplast O₂^·-content.In addition,the NADPH oxidase activities of wheat seedlings roots were remarkably raised with the increase of iron concentration at 2 and 6 days.3.The amount of uronic acid and total sugar in the cell wall cellulose of wheat seedling roots increased significantly under different consistences of iron;there were no evident divergences on the uronic acid and total sugar contents in pectin and hemicellulose I（HCⅠ）of wheat seedling roots treated by low Fe concentration,but the indicators were significantly increased under high Fe concentration.The uronic acid content in hemicelluloseⅡ（HCⅡ）only increased under high Fe concentration,and its total sugar content raised with Fe concentration increasing.4.Iron treatments could enhance the lignin content of wheat seedling roots,and the degree of lignification raised with the increase of iron concentration.Iron stress could induce lignin biosynthesis-related enzymes:phenylalanine ammonialyase（PAL）,cinnamyl alcohol dehydrogenase（CAD）,4-coumarate:Co A ligase（4CL）,laccase（LAC） and two cell wall peroxidases（guaiacol peroxidase（GPX）and coniferyl alcohol peroxidas（CAPX））to increase activities and changes in the expression of Ta PAL,Ta CAD,Ta4CL and Ta LAC,resulted in the aggravation of lignification of wheat seedling roots and the severe inhibition of root elongation.The above results indicated that iron stress inhibited the normal growth of wheat seedlings,incurred the increase of NADPH oxidase activity in plasma membrane,led to a large accumulation of ROS and lignin,and the activities of lignin synthesis-related enzymes PAL,4CL,CAD,LAC,GPX and CAPX enhanced and variations in the expression of TaPAL,TaCAD,Ta4CL and TaLAC,thus exacerbated lignification in wheat seedling roots and severely inhibited root elongation.