| Thallium(Tl)is a highly toxic heavy metal.It exists in two valence of Tl(Β)and Tl(Ⅲ)in the natural environment.Tl(Ⅲ)is thousands of time more toxic than Tl(Β).Tl can not only affect the germination and growth of plants,but also enrich into human body through the pathway of groundwater-soil-plant.Rice is an important food crop in China.In recent years,with the massive exploitation of thallium containing minerals in China,a large area of farmland has been polluted by Tl,which lead to the poisoning events occurred from time to time because of daily food intake in the surrounding areas.In order to reduce the Tl pollution of rice and ensure food safety,it has great theoretical value to reveal the pathway of thallium into rice and the external tolernal response of rice under thallium stress.This paper studied the characteristics of thallium absorption of indica and japonica rice,and the molecular pathway of thallium absorption,enzyme activity related to oxalate secretion and gene expression were further analyzed.Two rice varieties with significant differences in thallium absorption were used for comparison and verification.The aim is to reveal the absorption characteristics of two valence thallium in rice.The main results are as follows:1.From eleven indica and japonica rice varieties with different thallium content after 12 hours of Tl(Β)stress,indica rice Huangguangjinzhan and japonica rice Nipponbare with higher thallium content,indica rice Zhongjiazao17 and japonica rice Xiushui123 with lower thallium content were selected to measure thallium absorption and oxalic acid secretion respectively.The thallium absorption of the four varieties of rice treated with Tl(Ⅰ)were 0.205 mg·g-1 DW、0.191 mg·g-1 DW、0.176 mg·g-1 DW、0.136 mg·g-1 DW,which were significantly higher than the thallium absorption that 0.162 mg·g-1 DW、0.171 mg·g-1 DW、0.133 mg·g-1 DW、0.111 mg·g-1 DW treated with Tl(Ⅲ),the same rice treated with Tl(Ⅰ)generally had higher thallium absorption than Tl(Ⅲ);The quadratic regression analysis of thallium absorption and oxalic acid secretion of four rice varieties showed that the correlation coefficient was r=0.474.The correlation between thallium absorption and oxalic acid secretion among four varieties of indica and japonica rice was low.2.In the roots of indica rice Zhongjiazao17,the expression levels of potassium channel protein genes OsKAT1.1 and OsAKT1 in Tl(Ⅰ)treatment were significantly higher than those in the control check and were similar to those in K treatment.The expression levels of heavy metal transporter genes OsHMA9 and OsYSL14 in Tl(Ⅰ)treatment were significantly higher than those in the control check;The expression levels of OsKAT1.1,OsHKT2;4,OsHMA9 and OsYSL14 were different between Tl(Ⅰ)and Tl(Ⅲ)treatments.3.When Zhongjiazao17 was treated with thallium of two valence states,The addition of ATPase inhibitor Na3VO4 can effectively inhibit thallium absorption,Na+/K+-ATPese activity,the expression level of potassium transporter gene OsHKT2;4,heavy metal transporter gene OsHMA9 and OsYSL14 under the treatment of Tl(Ⅰ),indicating that the thallium absorption in rice are affected by the ATPase activity of rice roots;However,Na3VO4 had no significant effect on Tl(Ⅲ)treatment,indicating that there may be differences in the absorption pathways of two valence thallium in rice;NFA,an anion channel inhibitor,can effectively inhibit the oxalic acid secretion of roots under two valence thallium treatment,and the thallium absorption under Tl(Ⅰ)treatment,but has no effect on the thallium attached to the root surface under two valence thallium treatment;The results showed that oxalic acid secretion was not a direct factor affecting thallium absorption;The oxalic acid secretion of rice roots of this variety was significantly higher in Tl(Ⅲ)treatment than that in Tl(Ⅰ)treatment,and the activity of H+-ATPase,the expression level of proton pump genes OsA2、OsA3 and OsA7 encoding this protein were significantly higher in Tl(Ⅲ)treatment than that in Tl(Ⅰ),indicating that proton pump may be involved in mediating oxalic acid secretion.4.Further verify the relationship between thallium absorption and enzyme activities and transport genes in indica and japonica rice roots.The results showed that under the same valence treatment,thallium absorption of indica rice Huahang75 was higher than that of japonica rice xiushui123;Moreover,the thallium absorption of the two varieties under Tl(Ⅰ)treatment was significantly higher than that under Tl(Ⅲ)treatment.This may be related to the higher Na+/K+-ATPase activity in roots of Huahang75 and the higher expression of OsHKT2;4 and OsHMA9 transporter genes than Xiushui123;In addition,the oxalic acid secretion of Xiushui123 was higher than that of Huahang75;Moreover,Tl(Ⅲ)treatment secreted more oxalic acid than Tl(Ⅰ)treatment.This may be related to the high expression level of H+-ATPase activity and OsA2 and OsA3 in roots.This result is basically consistent with the thallium absorption characteristics of indica rice zhongjiazao17. |
