Distribution And Transport Of Cadmium In Different Genotype Rice Roots

It is well known that China is facing an increasingly serious issue that rice(Oryza sativa L.)paddy is being contaminated by cadmium(Cd).Cd has strong mobility in the soil-crop system,so the main pathway of Cd taken by human is through the food chain,which finally causes serious health problems to human.Rice is a staple food in our country.Understanding the mechanisms of Cd uptake and accumulation in rice is essential for developing agronomic measures to reduce Cd transfer from soil to grain,or screening low-Cd-accumulation rice cultivars,which are both effective in guaranteeing the food security of rice grown in lightly or moderately Cd contaminated soil.However,the whole mechanisms of uptake,translocation and accumulation of Cd in rice are not entirely clear until now.For revealing the physiologicaland biochemical mechanism in rice gain Cd accumulation,synchrotron radiation-X-ray fluorescence(SR-XRF),inductively coupled plasma mass spectrometry(ICP-MS),and quantitative Real-time PCR(qRT-PCR),were conducted to characterize the difference in uptake,translocation and accumulation of Cd in root of two rice genotypes,high-Cd-accumulation genotype(HA)and low-Cd-accumulation genotype(LA),in this study.The main results are shown as following:1.With several hydroponic experiments,the characteristics of Cd uptake and translocation in HA and LA rice by root and its relation with grain Cd accumulation were investigated by ICP-MS and qRT-PCR.The results of Cd time-course accumulation in rice roots indicated that HA and LA rice did not show significant difference in Cd uptake.However,HA rice had lower Cd content in root but a higher root-to-shoot translocation rate compared with LA rice.Furthermore,the analysis of Cd content in xylem sap confirmed that HA rice had higher Cd loading rate in root xylem.Therefore,HA rice exhibited higher Cd accumulation in grain owning to its effective root xylem loading and root-to-shoot translocation,not the root absorption.Moreover,qRT-PCR analysis suggested that high Cd exposure could remarkably upregulate the expression of OsHMA3 associated with Cd tonoplast transporters in LA rice,while this trend could not be observed in HA rice.In contrast,the expression of OsHMA2,a gene related to Zn and Cd transporter in root xylem loading,was elevated in HA rice,which indicates that Zn and Cd transporter OsHMA2 mediated Cd root xylem loading in HA rice.Further research found that addition of exogenous Zn2+ could enhance Cd root-to-shoot translocation rate,with a greater increase in LA rice.All these results illustrated that the grain Cd level was closely related root-to-shoot translocation rate,and Zn and Cd transporter OsHMA2 probably contributes to Cd root xylem loading.2.To compare the characteristics of Cd micro-area distribution at the tissue and cell level between HA and LA rice root,ICP-MS and SR-XRF were performed.The results verified that Cd was variously accumulated in different parts of root.No matter how long the sample treatment time is,short-term(24 h)or long-term(7 d),the content of Cd in lateral root is much higherthan that in other parts of root,indicating that lateral root is the main part where Cd is absorbed in root.Compared with the two genotypes of rice,Cd level in lateral root of HA rice was a little higher than that in lateral root of LA rice after 24 h Cd treatment,but the content of cadmium in the other parts was significantly lower than that in the latter..After 7 days of Cd treatment,Cd content in LA rice lateral root sharply raised and significantly exceeded that in HA rice.It was possibly owing to that xylem loading played a pivotal role in Cd translocation form lateral root to shoot in HA rice in the high cadmium stress.Then,The XRF analysis of the whole cadmium of the roots further confirms the results above,verifing that the difference of cadmium accumulation between HA and LA increased with the prolongation of cadmium treatment time.To further study Cd distribution characteristics at cell level,the?-XRF was utilized,and it was observed that Cd was more distributed in epidermis and stele of root was observed.Compared with LA rice,the concentration distribution of cadmium in the stele of lateral root of HA rice was more obvious.It can be seen that the cadmium of HA rice is more prone to being absorbed into the xylem region of the stele,then it was loaded to the xylem vessels and transported to the ground.All these results illustrated that the Cd absorption capacity varies in different parts of rice root,while lateral root is the most important area of absorbing Cd.The Cd accumulation ability in rice is closely related to the rapid xylem-loading process in root.