When plants are exposed to stressful environmental conditions, the production of Reactive Oxygen Species (ROS) increases and can cause significant damage to the cells. Antioxidant defenses, which can detoxify ROS, are present in plants. A major hydrogen peroxide detoxifying system in plant cells is the ascorbate-glutathione cycle, in which, ascorbate peroxidase (APX) enzymes play a key role catalyzing the conversion of H(2)O(2) into H(2)O, using ascorbate as a specific electron donor. Different APX isoforms are present in distinct subcellular compartments, such as chloroplasts, mitochondria, peroxisome, and cytosol. The expression of APX genes is regulated in response to biotic and abiotic stresses as well as during plant development. The APX responses are directly involved in the protection of plant cells against adverse environmental conditions. Furthermore, mutant plants APX genes showed alterations in growth, physiology and antioxidant metabolism revealing those enzymes involvement in the normal plant development.

• Peroxidases are involved in several important processes, such as development and responses to environmental cues. In higher plants, most peroxidases are encoded by large, multigenic families that mainly originated from gene and chromosomal duplications. • Using phylogenetic, genomic and functional analyses, we have identified and characterized a new class of putative heme peroxidases, called ascorbate peroxidase‐related (APx‐R), which arose specifically in the lineage of plants. • The APx‐R protein is structurally related to the ascorbate peroxidases, although the active site contains many conserved substitutions. Unlike all other plant peroxidases, which are encoded by gene families, APx‐R is encoded by a single‐copy gene in virtually all the species analyzed. APx‐R proteins are targeted to the chloroplast and can physically interact with chloroplastic APx proteins. APx‐R‐knockdown rice (Oryza sativa) plants presented delayed development and a disturbed steady state of the antioxidant system compared with wild type. Moreover, the accumulation of APx‐R transcripts was modulated by drought, UV irradiation, cold, and aluminum exposure in rice, suggesting the involvement of APx‐R in the environmental stress response. • Our results reveal the existence of a new class of heme peroxidase which seems to play a role in the antioxidant system in plants, probably by modulating the activity of chloroplastic APx proteins.

There is large diversity among glutathione peroxidase (GPx) enzymes regarding their function, structure, presence of the highly reactive selenocysteine (SeCys) residue, substrate usage, and reducing agent preference. Moreover, most vertebrate GPxs are very distinct from non-animal GPxs, and it is still unclear if they came from a common GPx ancestor. In this study, we aimed to unveil how GPx evolved throughout different phyla. Based on our phylogenetic trees and sequence analyses, we propose that all GPx encoding genes share a monomeric common ancestor and that the SeCys amino acid was incorporated early in the evolution of the metazoan kingdom. In addition, classical GPx and the cysteine-exclusive GPx07 have been present since non-bilaterian animals, but they seem to have been lost throughout evolution in different phyla. Therefore, the birth-and-death of GPx family members (like in other oxidoreductase families) seems to be an ongoing process, occurring independently across different kingdoms and phyla.