Enhanced tolerance to and accumulation of mercury, but not arsenic, in plants overexpressing two enzymes required for thiol peptide synthesis

Abstract

Arsenic and mercury are among the most toxic elemental pollutants in the environment, endangering human health and ecological integrity. Both elements are found in highly thiol‐reactive forms, arsenite and Hg(II), respectively, in plant tissues. Overexpression of Escherichia coliγ‐glutamylcysteine synthetase (ECS) or glutathione synthetase (GS) in Arabidopsis thaliana plants provided significant increases in the thiol peptides glutathione (GSH) and γ‐glutamylcysteine (γ‐EC), and/or phytochelatins (PCs), and some resistance to arsenic and mercury, but no substantial increases in the levels of these elements in above‐ground tissues. In contrast, the co‐expression of ECS and GS in ECS × GS lines produced significant increases in tolerance to toxic levels of mercury. The ECS × GS co‐expression line accumulated 35‐fold more biomass and three‐fold more mercury aboveground than the wild type (WT) when grown on Hg(II). No increases in arsenic accumulation were detected in the ECS × GS line. Increased resistance to and accumulation of mercury apparently resulted from enhanced root concentrations of PCs in ECS × GS co‐expression lines not seen in the wild type or lines expressing ECS or GS alone. Correlations between the levels of arsenic and mercury resistance and accumulation and increases in the accumulation of the various thiol peptides in the ECS, GS and ECS × GS transgenic plant lines are discussed.

Influence of glutathione chemical effectors in the response of maize to arsenic exposure

“…The increase in arsenic accumulation caused by DMS and DMPS is of special relevance. Thus, the increase in arsenic accumulated of between 2.5-and 4-fold more in plants co-treated with DMS or DMPS than in plants only treated with arsenic is a value comparable with the best results of enhanced plant metal accumulation provided by genetic engineering techniques (Dhankher et al, 2002;Li et al, 2006). As both DMS and DMPS are As(III)-chelating agents able to reduce As(V) to As(III) (Delnomdedieu et al, 1994) with a concomitant saving in GSH consumption, they might reduce arsenic toxicity and hence improve the plant potential for arsenic storage and translocation.…”

“…The absence of a correlation between NPT and arsenic levels in the treatments with flurazole, DMS and DMPS, suggests that the parameters are unrelated to each other or at least that they show a complex relationship. Increased production of non-protein thiols with only a weak or inclusive no incidence in arsenic accumulation has been described previously (Li et al, 2006). The increase in arsenic accumulation caused by DMS and DMPS is of special relevance.…”

“…Growing evidence indicates that glutathione (GSH) plays a central role in plant responses to the stress caused by arsenic through its participation in mechanisms related to tolerance, accumulation and distribution of the metalloid (Ahsan et al, 2008;Mrak et al, 2010;Norton et al, 2008). However, due to inconsistent results in certain studies showing a lack of correlation between the plant contents of GSH, or GSH-derived phytochelatins (PCs), and arsenic accumulation or tolerance (Li et al, 2006;Arnetolli et al, 2008), further investigations about GSH role in plant response to arsenic are required. In this study, we used a pharmacological approach to clarify the role of GSH in the responses of tolerance (toxicity), accumulation and distribution of arsenic in maize (Zea mays L.) plants after exposure to either arsenate [As(V)], the prevalent arsenic form taken up by plants in aerobic environments, or arsenite [As(III)], the active form of arsenic in processes of complexation with thiolreactive peptides [GSH and/or PCs] and the apparent predominant form in which free arsenic is transported in the xylem (Su et al, 2008).…”

Cesium Inhibits Plant Growth through Jasmonate Signaling in Arabidopsis thaliana

“…However, an urgent need for improved phytoremediation methods for radioactive Cs + has urged scientists to better understand the mechanism of Cs + uptake, transport and perception in plants. In Arabidopsis , a possible mechanism for tolerance to heavy metals, such as arsenic, mercury and cadmium, has been reported [43,44], but whether the same tolerance mechanism applies for Cs + is not yet known. A T-DNA insertion mutant which is impaired in Cs + uptake has been isolated; however, the identity of the gene disturbed is not revealed [45].…”

Phytochelatin synthase activity as a marker of metal pollution

“…Low molecular compounds rich in thiols including PCs can be determined by numerous methods [17][18][19][20][21], however, each method has its advantages and limitations and may serve a particular need in analysis. UV detection suffers from difficulties associated with insufficient selectivity, whereas using mass detection it may be difficult to quantify all thiol compounds in a single chromatogram.…”