The Study On Alternation Of Wetting And Drying Treatment For The Properties Of Iron Plaque On Rice(oryza Sativa)root Surface And Formation Mechanism

Iron plaque（IP）is a layer of crystalline or amorphous iron（hydr）oxides precipitated on root surface of helophytes,e.g.rice（Oryza sativa）.IP has large surface area and it can adsorb various nutrients and toxic ions.However,current understanding on IP formation mechanism is limited,so it’s not clear how the factors influence crystallinity and surface properties of IP.It’s known that sufficient root oxidizing capacity（ROC）and ferrous iron(Fe²⁺)are essential for IP formation,and alternation of wetting and drying（AWD）influence availability of rhizosphere Fe²⁺and crystallinities of iron（hydr）oxides in soil and IP on root surface by changing soil redox condition.ROC consists of molecular oxygen mediated ROC that radical oxygen loss participates and antioxidant enzyme mediated ROC that oxidative substances participate.The understanding on the latter one is more deficient.But it can be confirmed that hydrogen peroxide（H₂O₂）and peroxidase participate the establishment of antioxidant enzyme mediate ROC.In this study,quartz sand-soil-cultivation with rice was conducted to explore the effect of AWD on crystalline iron（CI）,amorphous iron（AI）,crystalline ratio（CR,which was defined as the proportion of to total IP to describe IP crystallinity）,ROC and surface properties of IP with different treatment durations and at different stages.Results indicated that AWD remarkably promoted IP formation and CR raising for several cultivars.Fourteen-day（4-cycle）AWD treatment increased CI to 2.20 times of that after continuous wetting（CW）but decreased AI to 72.3%of that after CW.Correspondingly CR was raised to 6.89%from 4.08%.However,long term（28-day）AWD decline IP formation and its CR.Remarkably,CR of IP after AWD ending in submergence and ending in drying were was6.89%and 4.23%,respectively.In addition,ROC after 14-d AWD was enhanced by 1.13times of that after CW,and activities of many antioxidant enzymes including Class III peroxidase（Prx）were also enhanced.Inactivation experiment suggested that both sufficient ROC and AWD treatment promoted increasing of IP content,and sufficient ROC significantly promoted the formation of CI.Results from scanning electronic microscope suggested that 14-d DSA induced thinner sheets with finer particles in IP compared to that after CW.Results from X-ray diffraction revealed that IP contained higher proportions of goethite,lepidocrocite,magnetite and hematite after DSA than those after CW.Variable charge and surface area of IP after DSA were only 26.5%and 32.0%of those after CW,respectively.RNA-sequencing,histochemical staining,confocal laser microscopy,and transgenic techniques were used to evaluate the contributions of ionic-bonded cell-wall-located class III peroxidases（i Prxs）and hydrogen peroxide（H₂O₂）to IP formation in rice under AWD condition with sand cultivation.AWD significantly enhanced i Prx activities of several cultivars,of which was most significant in Zhonghua 11 cultivars.Assay with enzyme inhibitors and reactive oxygen species scavengers indicated that i Prx and H₂O₂played key roles in IP formation.Results from transcriptomic sequencing and q RT-PCR suggested that Os Prx24,a gene encoding i Prx,significantly response to AWD.During AWD process,Os Prx24 expression,i Prx activity and H₂O₂concentration presented periodic variation trend that increased during wetting phase and decreased during drying phase.Compared with the results of AWD treatment without additional Fe²⁺and continuous wetting with additional Fe²⁺,it can be inferred that sufficient Fe²⁺was benefit to H₂O₂concentration elevation while AWD was main factor leading to periodic variation of i Prx activities and H₂O₂concentration.Microscopic image analysis of of histochemical staining section and laser confocal fluorescent section indicated that Os Prx24 was mainly expressed in root epidermis,where H₂O₂accumulation and IP precipitation also occurred.In an Os Prx24-overexpression line（Os Prx24-OX）,the H₂O₂concentration was 28.2%lower,and IP formation was 42.6%higher than in the wild type（WT）.Treatment with both an i Prx inhibitor and an H₂O₂scavenger reduced the IP content,but it was higher in Os Prx24-OX than in the WT under these treatment conditions.Intermediate compounds analysis of Prx cycles implied that AWD promoted Prx mediated peroxidative cycle and hydroxyl cycle that H₂O₂participated in.Phosphorus concentrations were higher and more IP was found in the roots of Os Prx24-OX than the WT.Together,our results indicated that proper strength AWD promotes ROC elevation,and transformation from AI to CI,consequently increases CR of IP,meanwhile changes its surface properties,including decline of adsorption ability.The mechanism that AWD enhances IP formation on rice root surface is AWD induces periodic variations of rhizopshere water content and Fe²⁺concentration,and subsequently induces periodic variations of i Prx activity in root.Prxs can both produce and consume H₂O₂.In case of Prx activity generally increasing,H₂O₂concentration raises while the consumption of H₂O₂also enhance.Therefore,Fe²⁺in rhizosphere is oxidized and precipitated on root surface,i.e.IP forms.The Prx encoded by Os Prx24 mainly consumes H₂O₂during this process.