Foliar Spraying With Silicon Controls Cadmium Accumulation In Rice Grains And Transcriptomic Analysis

Cadmium(Cd)is a highly toxic heavy metal and carcinogen.As an important food crop in China,rice is the main source of human cadmium intake.The excessive cadmium content of rice grain has become a serious food safety issue.Silicon(Si)has been proved to be beneficial to rice growth,and can alleviate the toxicity of cadmium on rice,and even reduce the accumulation of cadmium,but the relevant molecular mechanism,especially the use of foliar spraying with silicon to reduce the accumulation of cadmium in rice grain is relatively scarce.In this study,Huanghuazhan(HHZ)and Nipponbare(NIP)were used as materials to investigate the effect of foliar application of silica on the reduction of cadmium in crops through hydroponics,qRT-PCR and RNA-seq techniques were used to analyze cadmium-related genes in rice.The expression patterns of important genes and transcriptomic changes were explored to investigate the molecular mechanism underlying foliar silicon application controlling cadmium.The results obtained are as follows:1)The results of qRT-PCR analysis showed that the important genes related to cadmium uptake and translocation in rice,including OsNramp5,OsNramp1,OsHMA2,OsHMA3,OsLCD,and OsLCT1,were highly expressed in each specific tissue of HHZ and NIP during the tillering and filling stages.2)The results of foliar silicon spraying indicated that application of the silica sol on the leaves of Cd-treated rice(hydroponic culture)had the effect of reducing the cadmium content in rice grain.At day 1 and day 3 after spraying,the cadmium content in rice grain of HHZ decreased by 14.6% and 9.4%,respectively,while the content of cadmium in NIP decreased by 27.7% and 40.6%,respectively,and reach a significant level.This study also compared the effect of spraying silicon at differentgrowth stages of HHZ and found that foliar silicon spray had the most significant effect on the dry weight of rice grain.When silicon was applied on leaves during the seedling,tillering,filling and maturing stages,the dry weight of grain increased by15.0%,48.1%,25.5% and 19.3%,respectively,compared to the control without silicon spray.Foliar spraying of silicon can also affect the content of Cd in grains,especially brown rice,and the foliar spraying during grain filling stage had the most significant reduction of Cd content.Compared with the control,the Cd content in brown rice decreased by 67.2% after foliar silicon spray at the filling stage.3)It was found by qRT-PCR analysis that the sprayed silicon on the leaf surface at filling stage could change the expression level of cadmium uptake and transport related genes that has been reported so far in rice.Transcriptomic analysis showed that 176 genes and 506 genes in HHZ and NIP may be significantly differentially expressed by the control of silicon spray.Through GO analysis,it was found that these differential genes are mainly enriched in biological processes such as metabolism,stress,synthesis,and localization.KEGG analysis showed that these differential genes are mainly enriched in fatty acid elongation,signal transduction,RNA degradation,carbon metabolism,and glutathione metabolism,and the biological processes and metabolic pathways enriched in HHZ and NIP are basically the same.Through co-expression network analysis,it was found that 7 and 5 genes in HHZ and NIP,respectively,may play a key role in the control of cadmium by foliar spraying with silicon.These biological processes or metabolic pathways and the specific genes involved may be closely related to the regulation of silicon spray on the cadmium of rice,thus providing a molecular basis for the foliar spray control of cadmium in rice.4)Through qRT-PCR analysis,among the differential genes common to both HHZ and NIP,it was identified significant differences in the expression of the 4genes after foliar spraying of silicon at the grain filling stage.Of these four genes,Os02g0249600 is specifically expressed in node 1 of both HHZ and NIP and encodes a gluten B type 2 basic chain.However,the other three genes,Os01g0587300,Os09g0541000 and Os12g0194900 is specifically expressed in the flag leaf of HHZ and in the panicle of NIP,and encodes a protein containing C2 and GRAM domains,an amino acid permease and the aquaporin PIP2-7,respectively.In addition,from the key genes of the protein interaction network,it was identified that five genesspecifically expressed in HHZ,Os01g0618400,Os05g0589200,Os06g0270925,Os08g0545300 and Os09g0400300,and one gene,Os04g0684800,specifically expressed in NIP,which was still significantly differentially expressed after silicon spraying.qRT-PCR also identified one gene,Os03g0132900,in the node 1 of HHZ and also three genes,Os01g0296166,Os03g0427300 and Os07g0677200 in the node1 of NIP with significantly differential expression.These genes may play important roles in the foliar silicon spray controlling cadmium at the stage of rice filling,thus providing effective manipulation targets for the future application in the control of cadmium by silicon,and its genetic improvement and molecular breeding.