Exogenously applied isothiocyanates enhance glutathione S-transferase expression in Arabidopsis but act as herbicides at higher concentrations.

Isothiocyanates (ITCs) are sulfur-containing compounds that are generated by the glucosinolate-myrosinase system in plants. Although previous greenhouse studies have demonstrated the phytotoxicity of ITCs, their action modes are still unknown. In this study, we report the physiological responses of Arabidopsis thaliana treated with three exogenous ITCs: methyl ITC, allyl ITC, and phenethyl ITC. Administration of a high dose of each ITC inhibited plant growth and induced severe bleaching in the rosette leaves. The bleaching was concomitant with the elevation of electrolyte leakage and the generation of hydrogen peroxide. Although the three ITCs showed bleaching symptoms, phenethyl ITC was the most potent. A low dose of phenethyl ITC, at which the ITC did not promote leaf bleaching, enhanced the accumulation of transcripts of glutathione S-transferases (GSTs) in Arabidopsis. When 16 GST genes were tested, the levels of transcripts corresponding to 5 of the GST genes were enhanced in response to the phenethyl ITC treatment. In particular, the expression of a Tau class gene (AtGSTU19, At1g78380) responded to the phenethyl ITC treatment. Enhancement of the AtGSTU19 gene expression also occurred in the treatment of both allyl ITC and methyl ITC. These results suggest that the administration of ITCs to Arabidopsis at high doses has an herbicidal effect by inducing oxidative burst-like responses, but that administration at lower doses enhances the expression of specific GST genes in Arabidopsis.