Role of Thiol Compounds in Arsenic Tolerance in Pteris vittata

It has been suggested that thiol compounds such as glutathione (GSH) and phytochelatin (PC) contribute to arsenic (As) tolerance mechanisms in higher plant. In contrast, the role of thiol compounds in As tolerance and hyperaccumulation in Pteris vittata, an As hyperaccumulator fern, has not been well demonstrated so far. The purpose of this study was to elucidate that role. Gel-filtration chromatography showed that fronds of P. vittata contained high-molecular-weight As compounds containing thiol compounds. Localization of As showed similar trends to that of thiol compounds, increasing with getting higher in position of pinna. Besides, the molar ratio of thiol to As was nearly equal to the ideal molar ratio of As-thiol complexes. These results suggest that As makes complex with thiol compounds in fronds of P. vittata. When comparing P. vittata with Nephrolepis exaltata, an As nonaccumulator fern, As-induced enhancement of thiol synthesis was observed only in P. vittata. The enhancement of thiol synthesis was induced directly by As, not indirectly by As-mediated oxidative stress. γ-glutamylcysteine (γEC) synthetase is a key enzyme for the synthesis of GSH and PC. Two putative genes of γEC synthetase, PvECS1 and PvECS2, were found in P. vittata. Expression of PvECS2 in shoots increased at 48 h after the onset of As exposure, almost corresponding to the increase in concentrations of γEC and GSH. Moreover, As accumulation in shoots began just before these increases. Thus, this study strongly suggests the contribution of thiol compounds to As tolerance in P. vittata.