Effects And Mechanisms Of Arbuscular Mycorrhizal Fungi On Uptake,Translocation Of Selenium In Winter Wheat And Selenium Fractions In Rhizosphere Soil

Selenium（Se）is one of indispensable trace elements for human beings,and its intake dose is very important for human health.Winter wheat（Triticum aestivum L.）is one of the four major food crops in China,and is considered to be an important source of Se supplement for humans.Se biofortification is a safe,economical and effective method for increasing dietary Se intake.Arbuscular mycorrhizal fungi（AMF）are ubiquitous in soil which can form symbiotic relationship with roots of winter wheat.The symbionts could promote the growth of wheat and the uptake of mineral nutrients such as phosphorus,zinc and Se.However,the effect and mechanism of AMF on the uptake of selenate and selenite in wheat and the changes of Se fractions in rhizosphere soil have not been systematically studied.The main winter wheat variety（xiaoyan 22）in Guanzhong area of Shaanxi Province in the present study was used,and Lou soil in Yangling of Shaanxi Province was taken as the test soil to carry out pot experiment.Firstly,the effect of AMF on the expression of sulfate transporter in roots of wheat at different level of selenate in soil was studied.The contribution of mycorrhizal absorption pathway to Se uptake in wheat was discussed,and the suitable selenate concentration for symbiosis between winter wheat and AMF was screened.Secondly,the effects of AMF on the uptake of selenate and selenite in wheat and the bioavailability of Se in the rhizosphere soil were elucidated.Thirdly,the effects of AMF on the distribution and accumulation characteristics of Se in wheat under different selenate fertilization application time were studied.Finally,the changes of P transporter gene expression in roots of mycorrhizal wheat and bacterial community structure in rhizosphere soil were studied to elucidate the effects and mechanisms of AMF on selenite uptake and accumulation in wheat under different P supply levels.The main results of the present research were summarized as follows:（1）Effects of AMF on Se uptake by winter wheat under different level of selenate in soil were studied via pot experiment.The result indicated that inoculation of F.m significantly（p<0.05）increased Se concentration in shoots and roots of wheat by11.41-42.46%and 15.78-34.12%under low level of selenate(≤5.0 mg·kg^-1)treatments,while inoculation did not increase the Se concentration in wheat under high level of selenate treatments(15 and 20 mg·kg^-1).Under selenate≤5 mg·kg^-1treatments,the expression of sulfate transporter gene（Tae Sultr1;1 and Tae Sultr1;3）in roots of mycorrhizal wheat was significantly up-regulated by 3.06-5.53 and 0.63-5.12 times,while mycorrhizal treatment did not significantly affect its expression under selenate>5 mg·kg^-1treatments,indicating that the effect of AMF on Se uptake of winter wheat depends on the level of selenate in soil.Partial least squares path modeling（PLS-PM）showed that both the changes of available Se concentration in soil and the relative expression of sulfate transporter gene are important factors affecting the selenate uptake of winter wheat.In addition,when selenate concentration in soil was up to 2.5 mg·kg^-1,the rate of mycorrhizal colonization,Se utilization efficiency and contribution of mycorrhizal absorption pathway to Se uptake in winter wheat reached the maximum,indicating that inoculation with AMF was the most beneficial to Se-enrichment in wheat under this level of selenate application.（2）Inoculation of F.m or G.v significantly（p<0.05）increased concentration of available Se in the rhizosphere soil by 30.21-189.19%and 12.03-76.35%in soil applied with 2.5 mg·kg^-1selenite or selenate.The effect of inoculating F.m was better than G.v.Se content of mycorrhizal wheat was significantly（p<0.05）enhanced by 24.11-56.06%and20.27-56.49%,respectively.Correlation analysis showed that soil p H,dissolved organic carbon（DOC）,easily extractable glomalin-related soil protein（EE-GRSP）and microbial quantities were the main factors affecting concentration of available Se in rhizosphere soil.There was a significant positive correlation（p<0.01）between the Se content in shoot and root of wheat and concentration of available Se in the rhizosphere soil,which showed that AMF could enhance Se uptake of winter wheat via increasing soil Se bioavailability.（3）Under different selenate application time,inoculation of F.m or G.e significantly（p<0.05）increased grain Se concentration of winter wheat by 5.93-42.62%and 4.14-18.10%,respectively,but decreased root Se concentration.Inoculation of AMF increased transportation factors of Se from root to stalk and leaf(TF_{stalk-leaf/root})and transportation factors of Se from root to grain(TF_grain/root)by 1.30-2.02 times and 1.10-1.81 times,respectively,indicating that inoculation of AMF can increase grain Se concentration in wheat by increasing the transport of Se from root to aboveground tissues.The Se concentration and Se utilization efficiency of grain in wheat growing in soil applied with selenate at heading stage were significantly higher than those applied at before sowing or jointing stage.TF_{stalk-leaf/root}and TF_grain/rootof winter wheat growing in soil applied with selenate at heading stage were significantly（p<0.05）higher than the corresponding TF value at before sowing or jointing stage.This result indicated that Se was more easily transported from root to aboveground tissues when Se applied at reproductive stage of wheat than that at early growth stage.（4）Under P levels≤100 mg·kg^-1,inoculation of AMF significantly up-regulated the relative expression of the AMF-inducible P transporter gene（TRIae;Pht1;10 and TRIae;Pht1;11）in roots of winter wheat.The relative expression level of TRIae;Pht1;10and TRIae;Pht 1;11 in inoculated wheat roots was the highest and was much higher than that of Ta PT4 and Ta PHT1.2.In addition,AMF inoculation significantly promoted the relative abundance（RA）of Proteobacteria,Bacteroidetes,Actinobacteria and Firmicutes as well as bacterial community diversity in rhizosphere soil.The correlation analysis illustrated that the four bacterial phyla were obviously positively correlated with SOL-Se and EXC-Se,and negatively correlated with OM-Se,indicating that Proteobacteria,Bacteroidetes,Actinobacteria and Firmicutes were beneficial to promote the transformation of conversion OM-Se into available Se fractions in soil.PLS-PM identified that both up-regulated of the expression of AMF-inducible P transporter gene（TRIae;Pht1;10 and TRIae;Pht1;11）in roots and improved Se bioavailability in rhizosphere soil contributed to enhancing plant Se concentration of wheat.（5）The rate of mycorrhizal colonization in roots at the flowering and mature stage increased first and then declined as the increase of soil P application level,and it reached the maximum when P concentration in soil was 50 mg·kg^-1,which indicated that this P level was the most conducive to enhance the AMF colonization.Under the treatment of phosphate concentration≤100 mg·kg^-1,inoculation of AMF improved the Se uptake and Se utilization efficiency in grains of winter wheat.When the P application level was reduced from 80.62 mg·kg^-1to 48.76 mg·kg^-1,grain Se concentration in wheat inoculated with F.m or G.v significantly increased by 16.90%or 12.53%,respectively.This study showed that inoculation of AMF could achieve the effect of reducing the application of P fertilizer and increasing the concentration of Se in grains of wheat.In conclusion,AMF improved the P nutrition and biomass of winter wheat,promoted the conversionof OM-Se in rhizosphere soil to available Se fractions,and up-regulate the expression of P and sulfate transporter in roots.Therefore,AMF promoted the uptake of Se in soil by winter wheat and improved the utilization efficiency of Se.In this study,mycorrhizal technology was associated with Se-enrichment in wheat,and the molecular mechanism of mycorrhizal technology regulating the uptake of Se in winter wheat was preliminarily clarified,which has guiding significance for improving Se concentration in grains of wheat and producing Se-enriched wheat.