Exogenous Cysteine Improves Mercury Uptake and Tolerance in Arabidopsis by Regulating the Expression of Heavy Metal Chelators and Antioxidative Enzymes

Abstract

Cysteine (Cys) is an essential amino acid component of the major heavy metal chelators, such as glutathione (GSH), metallothioneins (MTs), and phytochelatins (PCs), which are involved in the pathways of mercury (Hg) tolerance in plants. However, the mechanism through which Cys facilitates Hg tolerance in plants remains largely unclear. In this study, we investigated the effects of exogenous Cys on Hg uptake in the seedlings, roots, and shoots of Arabidopsis throughout 6 and 36 h of Hg exposure and on the regulation of Hg detoxification by heavy metal chelators and antioxidative enzymes. The results showed that exogenous Cys significantly improved Hg tolerance during the germination and seedling growth stages in Arabidopsis. Exogenous Cys significantly promoted Hg uptake in Arabidopsis roots by upregulating the expression of the Cys transporter gene AtLHT1, resulting in increased Hg accumulation in the roots and seedlings. In Arabidopsis seedlings, exogenous Cys further increased the Hg-induced glutathione synthase (GS1 and GS2) transcript levels, and the Hg and Hg + Cys treatments greatly upregulated MT3 expression after 36 h exposure. In the roots, MT3 was also significantly upregulated by treatment of 36 h of Hg or Hg + Cys. Notably, in the shoots, MT2a expression was rapidly induced (10-fold) in Hg presence and further markedly increased (20-fold) by exogenous Cys. Moreover, in the seedlings, exogenous Cys upregulated the transcripts of all superoxide dismutase (CuSOD1, CuSOD2, MnSOD1, FeSOD1, FeSOD2, and FeSOD3) within 6 h and subsequently increased the Hg-induced GR1 and GR2 transcript levels at 36 h, all of which could eliminate the promotion of reactive oxygen species production and cell damage caused by Hg. Additionally, exogenous Cys upregulated all the antioxidative genes rapidly in the roots and subsequently increased the expression of CuSOD1, CuSOD2, and MnSOD1 in the shoots. These results indicate that exogenous Cys regulates the transcript levels of heavy metal chelators and antioxidative enzymes differently in a time- and organ-specific manner under Hg stress. Taken together, our study elucidates the positive functional roles of exogenous Cys in the Hg uptake and tolerance mechanisms of Arabidopsis.

Gamma-Aminobutyric Acid Enhances Cadmium Phytoextraction by Coreopsis grandiflora by Remodeling the Rhizospheric Environment

“…The results were different from previous findings that foliar spraying of exogenous GABA [ 18 ] and its addition in hydroponics [ 14 , 15 ], semihydroponics [ 16 ], or nutrient media [ 13 ] reduced HM uptake by plants. A similar phenomenon has also been observed for cysteine, for example, which increased Cd accumulation in Solanum nigrum in soils [ 19 ] and Hg accumulation in Arabidopsis in Hoagland nutrient solution [ 20 ] but reduced Cr uptake in B. napus in MS media [ 21 ] and in maize shoots in Hoagland nutrient solution [ 22 ]. These results indicate that cysteine did not produce a consistent effect on different HM-plant systems even in the similar environmental media [ 20 , 22 ].…”

“…A similar phenomenon has also been observed for cysteine, for example, which increased Cd accumulation in Solanum nigrum in soils [ 19 ] and Hg accumulation in Arabidopsis in Hoagland nutrient solution [ 20 ] but reduced Cr uptake in B. napus in MS media [ 21 ] and in maize shoots in Hoagland nutrient solution [ 22 ]. These results indicate that cysteine did not produce a consistent effect on different HM-plant systems even in the similar environmental media [ 20 , 22 ]. Accordingly, the effects of GABA on HM uptake by plants in different soil–HM–plant systems are also mutable and should attract further research.…”

New Insights into MeHg Accumulation in Rice (Oryza sativa L.): Evidence from Cysteine

“…The promotion of Hg uptake by Cys is likely due to the presence of Hg–Cysteine complexes, which can effectively cross the root barrier of the plant. Further, Cys also plays a role in scavenging ROS, thus reducing intracellular Hg toxicity …”

“…Further, Cys also plays a role in scavenging ROS, thus reducing intracellular Hg toxicity. 60 Pro is an amino acid that plays a crucial role in maintaining cellular homeostasis via osmotic regulation. 27 Some plant species (e.g., rice, Lemna minor) caused lipid peroxidation and hydrogen peroxide (H 2 O 2 ) accumulation after Hg stress, which is able to produce high Pro levels to alleviate oxidative stress.…”

“…82 The upregulation of Cys transporter gene AtLHT1 by amendment of Cys enhanced the accumulation of Hg by Arabidopsis. 60 The upregulation of the Cys transporter is thought to be associated with the internalization of Cys by plants as AAs or amino acidderived molecules, leading to increased uptake of MeHg− Cysteine complexes. 83 Here, we proposed that the differences in the MeHg accumulation in rice plants might be related to that both differ in molar ratio of MeHg to Cys and a certain group of amino acid transporters for the transportation of the MeHg−Cysteine complex.…”