Selenium inhibits the phytotoxicity of mercury in garlic (Allium sativum)

Abstract

To investigate the influence of selenium on mercury phytotoxicity, the levels of selenium and mercury were analyzed with inductively coupled plasma-mass spectrometry (ICP-MS) in garlic tissues upon exposure to different dosages of inorganic mercury (Hg²⁺) and selenite (SeO₃²⁻) or selenate (SeO₄²⁻). The distributions of selenium and mercury were examined with micro-synchrotron radiation X-ray fluorescence (μ-SRXRF), and the mercury speciation was investigated with micro-X-ray absorption near edge structure (μ-XANES). The results show that Se at higher exposure levels (>1 mg/L of SeO₃²⁻ or SeO₄²⁻) would significantly inhibit the absorption and transportation of Hg when Hg²⁺ levels are higher than 1 mg/L in culture media. SeO₃²⁻ and SeO₄²⁻ were found to be equally effective in reducing Hg accumulation in garlic. The inhibition of Hg uptake by Se correlates well with the influence of Se on Hg phytotoxicity as indicated by the growth inhibition factor. Elemental imaging using μ-SRXRF also shows that Se could inhibit the accumulation and translocation of Hg in garlic. μ-XANES analysis shows that Hg is mainly present in the forms of Hg–S bonding as Hg(GSH)₂ and Hg(Met)₂. Se exposure elicited decrease of Hg–S bonding in the form of Hg(GSH)₂, together with Se-mediated alteration of Hg absorption, transportation and accumulation, may account for attenuated Hg phytotoxicity by Se in garlic.

Introduction

Mercury (Hg) is one of the most hazardous pollutants in the environment. Its toxicity can be magnified in organisms at higher trophic levels due to its accumulation and transformation through food chain (Pickhardt and Fisher, 2007). Serious Hg contamination has been reported in Wanshan district (Guizhou Province, China), one of the world's largest Hg production centers. Total Hg can be up to 12 μg/L in surface water (Horvat et al., 2003, Zhang et al., 2010), and 790 mg/kg in paddy soil (Qiu et al., 2005). The average Hg concentrations in the crops from Wanshan district can even reach 78 μg/kg, and the food quality is seriously affected by the accumulation of Hg (Zhang et al., 2010). Besides high Hg levels, high selenium (Se) concentrations have also been found in soils (ranged from 0.16 to 36.6 mg/kg) and the crops (ranged from 0.02 to 0.67 mg/kg) from Wanshan area (Zhang et al., 2012). Se is an essential element for a wide variety of organisms including plants (Zhu et al., 2009). Although it plays an important role in coping with oxidative stress and heavy metal toxicity (Novoselov et al., 2002, Tran et al., 2007), Se can be toxic at high levels in organisms.

Se can protect animals from Hg toxicity (Parizek, 1967, Ng et al., 2001). The interaction between Hg and Se is one of the best known examples of biological antagonism (Khan and Wang, 2009). Various hypotheses for the antagonistic mechanism between Se and Hg have been proposed, including the redistribution of Hg in different living tissues (Fang, 1977), the competition for binding-sites (Leonzio et al., 1982), the formation of Hg–Se complexes along with or without proteins (Yoneda and Suzuki, 1997, Endo et al., 2005), and the facilitation of methylmercury (MeHg) demethylation (Cabañero et al., 2006). In recent years, the formation of highly stable organic MeHg–SeCys in organisms upon Hg/Se exposure has been extensively studied (Yang et al., 2008, Ralston et al., 2008). However, the effect of Se on Hg toxicity in plants remains largely unknown and thus needs further investigation.

As the base of the food chain, plants are implicated in heavy metals related food safety issues. Therefore, it is of great significance to investigate the Hg absorption, translocation, localization, speciation and the influence of Se on Hg biological processes in plant grown in Se and Hg co-exposure environment. However, most studies of the antagonism between Hg and Se have been carried out in animals, only a few researches focused on the antagonism in plants. Shanker et al. (1996) investigated the antagonism between Hg and Se in Radish irrigated with solutions containing Hg and Se. They have found a significant reduction in Hg uptake with increasing Se concentrations, which may result from the formation of an insoluble Hg/Se-containing complex in the rhizosphere. Thangavel et al. (1999) found that the protective effect of Se against Hg toxicity to Portulaca olerace was evident only at very low concentrations, and the protective effect decreased and simultaneously the toxicity increased while at high levels upon Hg and Se exposure. Yathavakilla and Caruso (2007) have reported that most of the water soluble Hg was found to be associated with Se as a high molecular weight entity in the roots. In a recent study, McNear Jr. et al. (2012) have found that Hg may bind to sulfhydryl groups or cell wall proteins of the green onion and in some places reacting with reduced Se to form a mercury selenide species. However, because of the distinct bioavailability and biological effects of different Se species, the variation of chemical forms of Se could complicate the process of Hg bioaccumulation and biotransformation in plants. In the present study, garlic, as a valuable economic crop, was cultured hydroponically in media with different dosages of inorganic mercury (Hg²⁺) and selenite (SeO₃²⁻) or selenate (SeO₄²⁻). The growth inhibition factor, accumulation, distribution and speciation of Hg affected by Se in garlic tissues were analyzed to elucidate the influence of Se on Hg phytotoxicity and the molecular mechanism thereof.