QTL Mapping Of Grain Arsenic Concentration And Root Transcriptome Analysis At Grain Filling Stage In Rice

Arsenic is a poisonous metalloid found in nature.In recent years,due to human activities such as mineral mining,metal smelting process,agricultural irrigation containing arsenic wastewater,and the use of pesticides,the concentration of arsenic in the environment exceeds the standard,and the arsenic pollution in farmland soil is also very serious.The contaminated soil will increase the risk of human absorption of arsenic through the food chain.Rice is a crop with high arsenic accumulation due to its special submerged environment.Arsenic in soil mainly exists in the inorganic form of As（V）and As（III）.Under aerobic conditions,the main existing form is As（V）,which is mainly absorbed by plant roots through phosphate transporters,while under anaerobic conditions,the main existing form is As（III）,which enters plant roots through aquaporins.Plants have the ability to reduce As（V）to As（III）,and can also chelate As（III）with phytochelatins（PC_S）and transport them into vacuoles for storage.It is an important scientific problem to study how arsenic is absorbed from roots and accumulated in grains.Different rice varieties have different arsenic accumulation capacity,but the specific genetic mechanism is not clear.This paper focuses on two rice varieties P491 and P979 with high arsenic accumulation in grain and TQ with low arsenic accumulation in grain,and map the quantitative trait locus（QTL）controlling the grain As concentration in P491 and P979.Moreover,RNA-seq was used to identify the differentially expressed genes between the roots of P491 and TQ at late tillering and grain filling stages.The main results are as follows:1)It was found that arsenic concentration in grains and husks of P491 and P979 was significantly different from TQ.As concentration in grains of P491 and P979 was40%-50%higher than that of TQ,and arsenic concentration in husks of P491 and P979was 70%-80%higher than that of TQ.At the same time,different tissues arsenic concentration of TQ P491and P979 was measured.It was found that arsenic concentration of P491 and P979 was not only higher than that of TQ in grain,but also higher in flag leaves shade,nodes I and nodes II.Arsenic concentration in flag leaves shade of P491 and P979 was 4 times higher than that of TQ.Arsenic concentration in the nodes I and nodes II is 35%to 70%higher than TQ.The arsenic accumulation in the roots of P491 and P979was significantly different from that in TQ roots under 5μM As（V）treatments for 3 days and 10μM As（V）treatments for 5 days without phosphorus.The arsenic tolerance test of young roots showed that P491 and P979 could still grow when treated with 2μM As（V）for48 hours,while TQ roots could not grow normally,indicating that P491 and P979 when treated with 2μM As（V）had stronger arsenic tolerance than TQ.2)In order to map the QTL controlling arsenic concentration in grains of P491 and P979,we crossed P491 and P979 with TQ to obtain F₁,and constructed F₂ population after self-crossing.From the F₂ population,we selected the individual plants with extreme high or low arsenic accumulation,and the mixed pools with high arsenic and low arsenic were constructed respectively.Through BSA-seq we found that the two populations had overlapping regions associated with the target trait in the region of 37 Mb-41 Mb on chromosome 1.3)The differentially expressed genes of TQ and P491 were analyzed through RNA-seq,and a total of 5,544 and 7,891 differentially expressed genes were obtained in rice roots at late tillering and grain filling stages.According to the gene expression analysis related to arsenic absorption and transport that has been reported,we found that in the grain filling stage,the expression level of Lsi1,Lsi2 and Os NIP3;1 in P491 was 6,13 and 5times that of TQ respectively.We found that the expression level of OSHAC1;1 and OSHAC1;2 in P491 was 21 times and 3 times that of TQ.However,there was no significant difference in the expression levels of these genes between P491 and TQ in late tillering period.We thought that the growth period more closely associated with arsenic accumulation in grain was grain filling stage,and the differentially expressed genes between TQ and P491 were mainly related to the absorption and the reduction of As（III）and As（V）.In conclusion,arsenic accumulation in rice grains is a complex physiological process.Two rice accessions with high arsenic accumulation in grains were found in this study.The arsenic concentration of straw in grains at mature stage and the related physiological experiments at seedling stage were conducted.QTL mapping of the two F₂ populations showed that they had overlapping regions on chromosome 1,indicating that the preliminary mapping results were reliable.The RNA-seq of TQ and P491 at late tillering stage and grain filling stage showed there are some genes specific expression at grain filling stage related to arsenic uptake and transport.