Comprehensive analysis of antioxidant mechanisms in Arabidopsis glutathione peroxidase-like mutants under salt-and osmotic stress reveals organ-specific significance of the AtGPXL's activities

Plant glutathione peroxidases contain cysteine in their active site instead of selenocysteine, and most of them use the thioredoxin (TRX) system more efficiently than the glutathione (GSH) system during the reduction of H2O2 and lipid peroxides. Recently, the more precise glutathione peroxidase-like (GPXL) name was adopted for the eight Arabidopsis thaliana isoenzymes. In this paper we have compared the effect of osmotic and salt stresses on the 6-week-old T-DNA insertion mutants (Atgpx11-8) grown hydroponically. The glutathione peroxidase (GPDX) activity measured with cumene hydroperoxide substrate in the wild type Arabidopsis shoots and roots was 2-5 times higher than the thioredoxin peroxidase (TPOX) activity. Mutation in one of the eight AtGPXLs resulted in decreased TPOX activity in untreated shoots and, in contrary to the wild type, this activity did not increase under stress, verifying the connection with TRX system. The level of reduced ascorbate significantly altered in shoots and the amount of GSH in roots under both control conditions and after 2 days of stress treatments. While positive correlations were found between GSH and TPOX activity in the wild type shoots and roots, the connection between the AtGPXLs and the GSH pool was stronger in roots than in shoots. Nevertheless, the TPOX activity increased in Aigpxls roots when the GSH content decreased, indicating the relationship between the GSH and TRX systems. The AtGPXL expression in mutant plants showed that some isoenzymes are regulated jointly. Furthermore, while AtGPXLs were generally down-regulated, several stress-inducible transcription factor genes were up-regulated, especially after applying osmotic stress.