Zinc-induced oxidative stress in Verbascum thapsus L. is caused by an accumulation of reactive oxygen species and quinhydrone in the cell wall

Abstract

Oxidative stress is one aspect of metal toxicity. Zinc, although unable to perform univalent oxido-reduction reactions, can induce the oxidative damage of cellular components and alter antioxidative systems. Verbascum thapsus L. plants that were grown hydroponically were exposed to 1 and 5 mM Zn²+. Reactive oxygen species (ROS) accumulation was demonstrated by the fluorescent probe H₂ DCFDA and EPR measurements. The extent of zinc-induced oxidative damage was assessed by measuring the level of protein carbonylation. Activities and isoform profiles of some antioxidant enzymes and the changes in ascorbate and total phenolic contents of leaves and roots were determined. Stunted growth because of zinc accumulation, preferentially in the roots, was accompanied by H₂O₂ production in the leaf and root apoplasts. Increased EPR signals of the endogenous oxidant quinhydrone, •CH₃ and •OH, were found in the cell walls of zinc-treated plants. The activities of the antioxidative enzymes ascorbate peroxidase (APX) (EC 1.11.1.11), soluble superoxide dismutase (SOD) (EC 1.15.1.1), peroxidase (POD), (EC 1.11.1.7) and monodehydroascorbate reductase (EC 1.6.5.4) were increased; those of glutathione reductase (EC 1.6.4.2), dehydroascorbate reductase (EC 1.8.5.1) and ascorbate oxidase (AAO) (EC 1.10.3.3) were decreased with zinc treatment. Zinc induced a cell-wall-bound SOD isoform in both organs. Leaves accumulated more ascorbate and phenolics in comparison to roots. We propose a mechanism for zinc-promoted oxidative stress in V. thapsus L. through the generation of charge transfer complexes and quinhydrone because of phenoxyl radical stabilisation by Zn²+ in the cell wall. Our results suggest that the SOD and APX responses are mediated by ROS accumulation in the apoplast. The importance of the POD/Phe/AA (ascorbic acid) scavenging system in the apoplast is also discussed.

The alleviation of zinc toxicity by silicon is related to zinc transport and antioxidative reactions in rice

“…4). Recently, the root cell wall of Verbascum thapsus is proposed to be the major pool of Zn under Zn toxicity that bound more than 60% of total root Zn (Morina et al 2010). It can be proposed that Si may increase the wall binding potential of Zn as has previously been demonstrated for Mn (Wiese et al 2007), which can explain the descripancy that the supply of Si did not decrease but even increased the total content of Zn (Fig.…”

Effects of Exogenous Zinc on the Cellular Zinc Distribution and Cell Cycle of A549 Cells

“…[16][17][18] The stunted growth of Verbascum thapsus due to preferential zinc accumulation in the roots has been shown to be accompanied by endogenous oxidative stress, e.g., a reduction of GSH, after the plants had been exposed to 1 and 5 mM Zn 2þ . 19) This exposure concentration may have been relatively high, resulting in the adverse effects of Zn 2þ seen in that study. However, zinc is considerably less toxic than such a redox-active metal as copper and is more soluble in oxygenated buffers than iron.…”

Formation of stable radicals in catechin/nitrous acid systems: Participation of dinitrosocatechin

“…This is deduced from the well-known charge-transfer complex formed from p-hydroquinone and p-benzoquinone (Regeimbal et al, 2003). Such stable radicals are also found in melanins, lignins, and so forth (Dreyer, Miller, Freeman, Paul, & Bielawski, 2012;Lü, Rosokha, Neretin, & Kochi, 2006;Morina, Jovanović, Mojović, Vidović, Panković, & Veljović-Jovanović, 2010;Sarna & Sealy, 1984;Seagel et al, 2005;Small, Zaitsev, Jung, Rosokha, Head-Gordon, & Kochi, 2004;Takahama, Oniki, & Hirota, 2001). The radicals in quinhydrones or charge-transfer complexes may also contribute to oxidation of cellular components and formation of reactive oxygen species, when absorbed into the body.…”