Sulphur and biothiol metabolism determine toxicity responses and fate of mercury in Arabidopsis

Abstract

Mercury (Hg) is one of the most hazardous pollutants released by humans and is of global environmental

concern. Mercury causes oxidative stress and strong cellular damages in plants, which can be attenuated by the

biosynthesis of thiol-rich peptides (biothiols), including glutathione (GSH) and phytochelatins (PCs). We ana lysed Hg tolerance and speciation in five Arabidopsis thaliana genotypes, the wild-type Col-0, three knockdown

γ-glutamylcysteine synthetase (γECS) mutants and a knockout PC synthase (PCS) mutant. Mercury-PC complexes

were detected in roots by HPLC-ESI-TOFMS, with its abundance being limited in γECS mutants. Analysis of Hg biothiol complexes in the xylem sap revealed that HgPC2 occurs in wild-type Col-0 Arabidopsis, suggesting that

Hg could be translocated associated with thiol-rich metabolites. Twenty genes involved in sulphur assimilation,

GSH and PCs synthesis were differentially expressed in roots and shoots, implying a complex regulation, possibly

involving post-translational mechanisms independent of GSH cellular levels. In summary, the present study

describes the importance of biothiol metabolism and adequate GSH levels in Hg tolerance and identifies for the

first time Hg-PC complexes in the xylem sap. This finding supports the notion that Hg-biothiol complexes could

contribute to Hg mobilisation within plants.