Overexpression of the Arabidopsis glutathione peroxidase-like 5 gene (AtGPXL5) resulted in altered redox status, plant development and salt tolerance

Plant’s glutathione peroxidase-like (GPXL) enzymes are thiol-based peroxidases catalysing the reduction of H2O2 or hydroperoxides to water or alcohols using reduced glutathione (GSH) or thioredoxin as an electron donor (Navrot et al., 2006). Arabidopsis thaliana possesses eight isoenzymes located in different plant’s organelles and having different roles in redox-dependent processes. Among them, AtGPXL5 is a poorly known plasma membrane-associated enzyme, although its role in salt stress tolerance was suggested (Gao et al. 2014). We have constitutively overexpressed the AtGPXL5 cDNA and investigated the role of AtGPXL5 in response to NaCl treatment and in development. Experiments were performed by using AtGPXL5-overexpressing lines (OX-AtGPXL5) and Atgpxl5 mutant plants. Based on our results, we have made the following observations: 1) 12-day-old Arabidopsis thaliana Atgpxl5 insertional mutants had higher level of superoxide radical anion and total ROS in untreated roots and shoots, respectively compared with Col-0. The higher level of ROS decreased the cells’ vitality in the shoot of Atgpxl5 seedlings even under control condition. After applying 7-day treatment with the concentration of 100 mM NaCl, the O2•− level in the root was elevated further and reached higher level than in the wild type. These indicate that AtGPXL5 might play an important role in the ROS homeostasis and maintaining the cell’s vitality. 2) The antioxidant mechanisms of the 6-week-old plants have altered, especially in the Atgpxl5 mutants compared to OX-AtGPXL5 plants. Several ROS processing enzymes worked in elevated level in Atgpxl5 mutant, but OX-AtGPXL5 plant exhibited similar activity to the Col-0 wild type. The GPOX activity was elevated in the lowest extent in Atgpxl5 plants while GPOX and TPOX enzymes in the AtGPXL5-overexpressing plants worked about on the level of wild type. 3) Under control conditions, significantly lower GSH was found in the Atgpxl5 mutant roots while its amount was elevated in the OX-AtGPXL5 shoot. The applied salt stress caused accumulation of the highest amount of reduced glutathione and the less oxidized form (GSSG) in the AtGPXL5-overexpressing plants among the investigated lines, while the GSSG increased most in the Atgpxl5 roots. The amount of reduced glutathione was higher and the calculated redox potential was more negative in the overexpressed line than in Col-0. The result confirms that AtGPXL5 has function in regulating the redox state, through which they can also influence the growth and development. 4) AtGPXL5 enzymes are required for healthy growth and development of the Arabidopsis thaliana seedlings. Deficiency of AtGPXL5 led to reduce the length of primary roots, biomass, rosette size, convex area, chlorophyll and anthocyanin contents compared to other investigated lines under normal conditions. In the presence of 100 mM NaCl, Atgpxl5 mutant and the Col-0 wild type seeds showed delayed germination, while germination of the OX-AtGPXL5 lines was not inhibited in the presence of 100 mM NaCl. Untreated OX-AtGPXL5 lines exhibited similar phenotype as Col-0, however the overexpressing plants grew better in the presence of 100mM NaCl: they had larger rosettes, larger convex area and lower convex percentage values with higher content of chlorophyll and anthocyanin than that of the wild type and Atgpxl5 plants. The reduced development of the shoots and decreased root length of the Atgpxl5 mutant indicate that this protein has a function even in the normal development.