Plant peroxidases (class III peroxidases) are present in all land plants. They are members of a large multigenic family. Probably due to this high number of isoforms, and to a very heterogeneous regulation of their expression, plant peroxidases are involved in a broad range of physiological processes all along the plant life cycle. Due to two possible catalytic cycles, peroxidative and hydroxylic, peroxidases can generate reactive oxygen species (ROS) ( • OH, HOO• ), polymerise cell wall compounds, and regulate H 2 O 2 levels. By modulating their activity and expression following internal and external stimuli, peroxidases are prevalent at every stage of plant growth, including the demands that the plant meets in stressful conditions. These multifunctional enzymes can build a rigid wall or produce ROS to make it more flexible; they can prevent biological and chemical attacks by raising physical barriers or by counterattacking with a large production of ROS; they can be involved in a more peaceful symbiosis. They are finally present from the first hours of a plant's life until its last moments. Although some functions look paradoxical, the whole process is probably regulated by a fine-tuning that has yet to be elucidated. This review will discuss the factors that can influence this delicate balance.Keywords Evolution . ROS . (abiotic and biotic) stress . Cell wall loosening and cross-linking . Senescence . Fruit ripening . Symbiosis Multigenic family, evolution and homologyHeme peroxidases specific to plants belong to a superfamily that contains three different classes of peroxidases Communicated by P. Kumar

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Two class III peroxidases from Arabidopsis, AtPrx33 and Atprx34, have been studied in this paper. Their encoding genes are mainly expressed in roots; AtPrx33 transcripts were also found in leaves and stems. Light activates the expression of both genes in seedlings. Transformed seedlings producing AtPrx33-GFP or AtPrx34-GFP fusion proteins under the control of the CaMV 35S promoter exhibit fluorescence in the cell walls of roots, showing that the two peroxidases are localized in the apoplast, which is in line with their affinity for the Ca(2+)-pectate structure. The role they can play in cell wall was investigated using (1) insertion mutants that have suppressed or reduced expression of AtPrx33 or AtPrx34 genes, respectively, (2) a double mutant with no AtPrx33 and a reduced level of Atprx34 transcripts, (3) a mutant overexpressing AtPrx34 under the control of the CaMV 35S promoter. The major phenotypic consequences of these genetic manipulations were observed on the variation of the length of seedling roots. Seedlings lacking AtPrx33 transcripts have shorter roots than the wild-type controls and roots are still shorter in the double mutant. Seedlings overexpressing AtPrx34 exhibit significantly longer roots. These modifications of root length are accompanied by corresponding changes of cell length. The results suggest that AtPrx33 and Atprx34, two highly homologous Arabidopsis peroxidases, are involved in the reactions that promote cell elongation and that this occurs most likely within cell walls.