The Manifestation of the Dual ROS-Processing and Redox Signaling Roles of Glutathione Peroxidase-like Enzymes in Development of Arabidopsis Seedlings

Plant glutathione peroxidase-like (GPXL) enzymes are thiol-based peroxidases that reduce H₂O₂ or hydroperoxides to water or alcohols using electrons principally from thioredoxin. <i>Arabidopsis thaliana</i> possesses eight isoenzymes (AtGPXL1-8) located in different plant organelles and have various roles in redox-dependent processes. The determination of the redox potential of 6-day-old T-DNA insertional mutants (<i>Atgpxl1</i>-<i>Atgpxl8</i>) using a cytosolic redox-sensitive fluorescent probe (roGFP2) uncovered more oxidized redox status in the shoot and/or root of the untreated mutants, except for <i>Atgpxl5</i>. To investigate the involvement of AtGPXLs in the growth and abiotic stress responses of seedlings, the 4-day-old <i>Atgpxls</i> were exposed to salt and osmotic stresses for two weeks. The evaluation of the reactive oxygen species (ROS) levels of untreated 18-day-old plants using fluorescent microscopy revealed the elevated accumulation of total ROS in the shoots and, in some cases, the roots of the mutants. Regarding the growth of roots, both the length of primary roots and/or the number of lateral roots were affected by the mutation of <i>AtGPXLs</i>. A strong negative correlation was observed between the ROS level of wild type shoots and the development of lateral roots, but it was altered in mutants, while in the case of <i>Atgpxl1</i>, <i>Atgpxl5</i>, and <i>Atgpxl7</i> seedlings, it disappeared; in other mutants (<i>Atgpxl4</i>, <i>Atgpxl6</i>, and <i>Atgpxl8</i>), the correlation became stronger. Our analysis underpins the discrete role of AtGPXL enzymes in controlling the growth and development of plants by fine tuning the ROS contents and redox status in an organ-specific way. Differences in root phenotype and metabolic activity between <i>Atgpxl</i> mutants and wild type plants highlight the essential role of AtGPXLs in ROS processing to support growth, which is particularly evident when one GPXL isoenzyme is absent or its activity is reduced, both under normal and abiotic stress conditions.