We investigated the role of glutathione (GSH) and phytochelatins (PCs) on the detoxification of selenite using Arabidopsis thaliana.The wild-type (WT) of Arabidopsis thaliana and its mutants (glutathione deficient Cad 2-1 and phytochelatins deficient Cad 1-3) were separately exposed to varying concentrations of selenite and arsenate and jointly to both toxicants to determine their sensitivities.The results of the study revealed that,the mutants were about 20-fold more sensitive to arsenate than the WT,an indication that the GSH and PCs affect arsenate detoxification.On the contrary,the WT and both mutants showed a similar level of sensitivity to selenite,an indication that the GSH and PCs do not significantly affect selenite detoxification.However,the WT is about 8 times more sensitive to selenite than to arsenate,and the mutants were more resistant to selenite than arsenate by a factor of 2.This could not be explained by the accumulation of both elements in roots and shoots in exposure experiments.The co-exposure of the WT indicates a synergistic effect with regards to toxicity since selenite did not induce PCs but arsenic and selenium compete in their PC binding as revealed by speciation analysis of the root extracts using HPLC-ICP-MS/E.SI-MS.In the absence of PCs an antagonistic effect has been detected which might suggest indirectly that the formation of Se glutathione complex prevent the formation of detrimental selenopeptides.This study,therefore,revealed that PC and GSH have only a subordinate role in the detoxification of selenite.