Preliminary Studies On The Mechanisms Of Methylarsenate Toxicity To Arabidopsis Thaliana

The US Environmental Protection Agency has classified inorganic arsenic and its compounds as group one carcinogens.Arsenic pollution has become a worldwide health problem affecting tens of million people worldwide.Arsenic has been used in medicine,industry and agriculture.For example,in agriculture,arsenic-containing herbicides,such as sodium dimethylarsenic(DSMA),are used to for weed and pest control.Arsenic accumulation in soil will not only pollute the soil,but also bring serious negative impact on the ecological environment.In this project,we studied the resistance mechanisms of Arabidopsis thaliana to DMA(?)by analysing the toxic effect of three forms of arsenic(As(?),MMA(?),DMA(?))on A.thaliana and comparing the tolerance of A.thaliana to the three arsenic species.In the first experiment,when A.thaliana mutants cadl-3(PC-deficient mutant),cad2-1(GSH-deficient mutant)and abccl-2(ABCC transporter-deficient mutant)and Col-0(wild type)were treated in different arsenic species,we found that the three mutants showed a much higher sensitivity to As(?)or MMA(?),but not to DMA(?).In addition,L-buthionine-[S,R]-sulfoximine(BSO),a specific inhibitor of y-glutamylcysteine synthetase,greatly increased the sensitivity of Col-0 to As(?)or MMA(?),but not to DMA(?).The results suggest that the toxicity and tolerance mechanisms are similar between As(?)and MMA(?),but different for DMA(?).In the second experiment,the content of non-protein thiols(NPT)was determined in A.thaliana wild type Col-0 in response to exposure to different arsenic species.The results showed that NPT concentration significantly increased in tissues of A.thaliana exposed to MMA(?)or As(?).In contrast,DMA(?)treatment did not increase NPT content.We analyzed the hydrogen peroxide and malondialdehyde contents as indicators of oxidative stress.The results showed that both contents increased in shoots and roots treated with As(?),MMA(?)and DMA(?),with the response being greatest in the DMA(?)treatment.These data indicated that reactive oxygen metabolism occurred and the cell membranes were damaged when A.thaliana was exposed to three forms of arsenic,and that DMA(?)caused more oxidative stress than the other two arsenic species.Arsenic contents in root and shoot tissues of A.thaliana(Col-0)exposed to different forms of arsenic were determined.Plants exposed to DMA(?)in hydroponic conditions accumulated ten times higher As concentration in the shoots than in the roots,indicating that DMA(?)was preferentially accumulated in the shoots.In a pot experiment with vermiculite as the growth medium,As(?)treatments did not significantly affect growth.In contrast,both MMA(?)and DMA(?)significantly inhibited plant growth.The fresh weights of shoots and seeds were both significantly reduced with the increasing concentrations of MMA(?)and DMA(?).Arsenic accumulated in the shoots of A.thaliana exposed to DMA(?)more than the As(?)or MMA(?)treatments.Arsenic concentration in seeds was lowest in the As(?)treatment.The above results suggests that the translocation rate of arsenic from shoots to seeds was highest in MMA(?)when compared to As(?)and DMA(?),and the lowest rate was found in the As(?)treatment.The results indicated that DMA(?)is highly mobile in the translocation from roots to shoots.Finally,we also completed the establishment of an EMS mutagenesis library of the A.thaliana cad1;3 mutant,laying the foundation for the subsequent screening.