Uptake And Tanslocation Of As(Ⅲ) In Rice Plants And Regulation Mechanisms By The Silicon Nutrition

Arsenic(As)is a known class-I carcinogen.Contamination of As in soil has became a global environmental problem.Rice is one of staple crops in South and Southeast of Asia.Furthermore,rice consumption is a major route of As exposure for people in these regions and countries and thus threated human health.Higher content of As in rice grains compared with those of other crops is mainly attributable to the flooded cultivation of rice plants,and the efficient uptake and transport system of arsenous acid through transporters of silicic acid in rice plants.Under flooded anaerobic condition,As is easily to be desorbed from the soil particles and enters the soil solution.As a result,the mobility and toxicity of As was increased.Currently,there are many studies on the uptake and transport of silicic acid in rice,but how to reduce the uptake and transport of arsenous acid without affecting the silicon nutrition is rare.In this study,rice cultivar Youyou 128 was used as the material to study the effect of application of foliar silica sol on the uptake and transport of arsenite in rice by hydroponic culture.The molecular and physiological mechanisms of reducing As in seedling stage and entire growth period of rice plants by silica sol was discussed with special reference to the quantitative analysis of arsenite-related genes.The main conclusions are as follows:(1)In order to explore the key period(s)and key transporter(s)of arsenite uptake and transport,As accumulation and the expression levels of arsenite-related genes in rice plant were studied during the entire growth period.The results showed that the order of As concentration in different tissues of rice plants was: root > stem and leaf > husk > brown rice.Before heading,As preferentially accumulated in the root;after heading,a large amount of As was transferred to the shoot and then accumulated in the straw.Arsenite stress also changed the expression levels of arsenite-related genes.In the root,adding As treatment significantly increased the expression of Os Lsi1 at the heading stage,the expression of Os Lsi2 at the jointing and heading stage,and the expression of Os ABCC1 in roots at the jointing stage,but significantly inhibited the expression of Os ABCC1 in the root at the flowering stage.In the rice plants tissues above the ground,adding As treatment significantly inhibited the expression of Os ABCC1,such as the nodes from the flowering to milk stage.However,adding As treatment promoted the expression of Os Lsi6 in most aboveground tissues of rice plants,especially in node I from the heading to milk stage,indicating that the downregulation of Os Lsi6 in the node I is a critical process to reduce As translocation from node I to rice grains.Therefore,the inhibition of expressions of Os Lsi1,Os Lsi2 and Os Lsi6 at the jointing stage and reproductive growth period,and the disinhibition of Os ABCC1 expression in the nodes during the flowering stage may play a critical role in reducing As accumulation in the rice grains.(2)This paper also studied the the mechanism(s)of As reduction in rice plants at seedling stage by foliar silicon nutrition.In root cells,foliar sprayed silicon promoted the segregation of As in vacuoles in the symplast and reduced its transfer to other organelles.Meanwhile,foliar sprayed silicon also significantly increased the expression of Os ABCC1 in the roots,indicating that foliar sprayed silicon could increase vacuolar As segregation through increasing Os ABCC1 expression in the roots.Under low level(2 μM)of arsenite stress,foliar silicon significantly inhibited the expression of Os Lsi1 and Os Lsi2 in the root.Besides,foliar silicon significantly reduced As concentration in the shoot,and slightly reduced As concentration in the root,indicating that foliar silicon may reduce the As(Ⅲ)uptake by the root and transport to the shoot via reducing the expression of Os Lsi1 and Os Lsi2 in the root.Under the stress of high level(8 μM)of arsenite,foliar silicon promoted the expression levels of Os Lsi1 and Os Lsi2,and greatly promoted the expression of Os ABCC1 in the root.Correlation analysis showed that the expression levels of Os ABCC1 and Os Lsi2 in the root was significantly and positively correlated(r=0.461,P < 0.05),indicating that high expression level of Os Lsi2 is accompanied by high expression level of of Os ABCC1 in the root.Besides,under the stress of high level(8 μM)of arsenite,foliar sprayed silicon decreased translocation factor of As from root to shoot of rice plants,indicating that significant upregulation of the expression level of Os ABCC1 in the root may play a crucial role for decreased As translocation to shoot.In addition,the cell wall of the apoplast space isolated a small amount of As(Ⅲ),and foliar silicon significantly increased the amount of As isolated in the cell wall,thereby reducing its transport to the symplast.(3)In order to further explore the effect of foliar silicon on As accumulation in rice grains,the uptake of As(Ⅲ)and the expression levels of As(Ⅲ)-related transporter genes at the jointing,booting,flowering and filling stages by foliar silica sol application were analyzed with reference to the the key period(s)of arsenite uptake and transport obtained.In general,foliar silicon under 0.5 μM arsenite stress reduced the As content in brown rice.At the jointing,booting,flowering and filling stages,the As concentration in brown rice was decreased by 28.2%,23.9%,21.4% and 27.4%,respectively.In addition,foliar silicon application reduced the As accumulation in rice roots,stems,leaves and husks at the jointing stage,but increased the expression of Os Lsi1 in roots.The increase of Os Lsi1 expression level may result in an increase of arsenite efflux from the roots,thereby reducing the accumulation of As in rice root and translocation to shoot and rice grains.Moreover,at the milk stage,foliar silicon application increased the As accumulation in roots,but reduced the As accumulation in stems,leaves and husks.At the same time,the expression level of Os Lsi2 in the root was significantly reduced and this may lead to the increase of As accumulation in roots and thus decreased As translocation to the shoots.In the rice tissues above the ground,foliar sprayed silicon significantly reduced the values of As transfer factor of husk/node I and brown rice/node I,indicating that foliar spray silicon can reduce the As accumulation in rice grains by reducing the transport of As from node I to the grains.At the filling stage,the decrease of Os Lsi6 expression in node I may be the key process to the decrease of As translocation from node I to grains.In conclusion,foliar sprayed silicon regulated the expression of As(Ⅲ)-related transporter genes,thereby(i)reducing the uptake of As(Ⅲ)by rice roots,(ii)reducing the transport of As from roots to shoots,and(iii)reducing the redirection of As from the nodes to rice grains.Foliar sprayed silicon can significantly reduce the As accumulation in rice grains,especially at the jointing stage and filling stage.Therefore,in lightly As-contaminated and silicon-deficient rice fields,foliar sprayed silicon at jointing and filling stages can be used as an effective measure to rice safety production.