Studies Of Thallium Stress Effect On Oxalate Secretion And Tolerance Mechanism In Cabbage(Brassica Oleracea L.) And Rice(Oryza Sativa L.)

Thallium（TI） is a highly toxic heavy metal element.In China’s"Twelfth Five-Year Plan for Comprehensive Prevention and Control of Heavy Metal Pollution",heavy metal thallium is listed as a key pollution control.Thallium in the environment mainly comes from the release of mineral resources during mining.Plants absorb thallium in the environment,which poses a threat to food safety.Thallium has two valence states,Tl（Ⅰ）and Tl（Ⅲ）,to study the physiological response and tolerance mechanism of plants under two valence states of thallium stress.Provide theoretical basis.In this study,Brassica oleracea with important enrichment ability for thallium and important food crop rice were selected as the research materials.First,the organic acid secretion characteristics of the roots of cabbage under thallium stress and its effect on the absorption of thallium were determined;the previous studies have clarified thallium Based on the characteristics of oxalate secretion in rice roots under stress,the factors affecting the stability of Tl（Ⅲ）,the effect of root oxalate secretion on the absorption of thallium were clarified,and the rice tolerance mechanism under two valence states was studied from the cytological and molecular levels And differences.The research results are as follows:1.Cabbage will specifically secrete oxalic acid under thallium stress.Cabbage root will not secrete oxalic acid immediately,but will secrete oxalic acid after a period of thallium stress.After the NIF inhibitor is added,the anion channel inhibitor NIF will significantly inhibit the organic acid secretion in the cabbage root,and at the same time significantly reduce the absorption of thallium in the cabbage root,thereby reducing the thallium content in the above-ground stem and leaf.Under thallium stress,the oxalic acid secretion pattern of cabbage roots was initially determined to be delayed secretion type Ⅱ,and the organic acid content of the roots will affect the absorption of thallium by cabbage.2.The oxalic acid secretion of rice roots under Tl（Ⅲ）stress is about 3 times that of Tl（Ⅰ）stress.When the treatment time is extended to 6h,the difference in oxalate secretion of rice roots under two kinds of thallium stress is not Large;the rate of reduction of Tl（Ⅲ）to Tl（Ⅰ）is related to the concentration of oxalic acid secretion.High rice plant density and prolonged treatment time all promote the conversion of Tl（Ⅲ）to Tl（Ⅰ）.3.Treating rice with Tl（Ⅰ）and Tl（Ⅲ）for 2 h respectively,the rice roots showed Tl（Ⅰ）treatment(0.659 mg g^-1FW)>Tl（Ⅲ）treatment(0.291 mg g^-1FW),while the oxalate secretion of rice roots was treated with Tl（Ⅰ）(0.0008 mg g^-1)<Tl（Ⅲ）(0.0022mg g^-1);using scanning electron microscopy-energy dispersive X-ray to analyze the surface and interior of the rice root Morphology and element distribution,it is found that the thallium-containing particle complexes exist on the root surface in the bivalent state,but the number of Tl particle complexes in the root system under Tl（Ⅰ）treatment is more than that in Tl（Ⅲ）treatment;it is resistant to metals in the bivalent state Protein genes Os MTP5,Os MTP6,Os MTP7 and Os MTP8,metal chelate genes Os05g32110.1,Os05g34290.1,Os06g1260.1 were compared for expression differences:under Tl（Ⅰ）treatment,Os MTP6 expression was up-regulated with prolonged treatment time,speculating that this gene may be It is related to the tolerance of heavy metal thallium ion transport;under the treatment of Tl（Ⅲ）,Os MTP5,Os MTP6,Os MTP7 and Os MTP8 are generally inhibited by time prolonged expression;while the expression of the three metal chelate genes in the bivalent state is under 2h Generally suppressed,speculate The synth genes Os05g34290.1,Os05g32110.1,and Os06g1260.1 have a smaller contribution to the internal tolerance function of rice root cells.Tl（Ⅰ）is more likely to enter the rice root system than Tl（Ⅲ）,and the Tl ion inside the cell root It forms particles with a certain substance,fixes the position of Tl ions,reduces the toxicity of thallium ions,and the entry of Tl（Ⅰ）into the rice root system is related to the ion transport gene Os MTP6,and the contribution of metal chelate to plant Tl ion tolerance in the early stage Smaller.4.The contents of rice roots H₂O₂,O₂^-and MDA under Tl（Ⅰ）and Tl（Ⅲ）treatments are very different.The contents of rice roots MDA,H₂O₂and O₂^-under Tl（Ⅲ）treatments are significantly higher than Tl（Ⅰ）,It shows that Tl（Ⅲ）has higher oxidative stress toxicity treatment to plants.The metallothionein gene Os MT1 is up-regulated in the two valence states as the treatment time is extended,and the Os MT1b,Os MT3b,Os MT2c,and Os MT4 genes exhibit different expression patterns in the bivalent state.It is speculated that Os MT1 plays an important role in removing active oxygen from rice roots under thallium stress and improving the tolerance of rice roots to thallium.