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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Summary• The respective distribution of superoxide (O 2• -) and hydrogen peroxide (H 2 O 2 ), two reactive oxygen species (ROS) involved in root growth and differentiation, was determined within the Arabidopsis root tip. We investigated the effect of changing the levels of these ROS on root development and the possible interactions with peroxidases.• H 2 O 2 was detected by confocal laser-scanning microscopy using hydroxyphenyl fluorescein (HPF). Both O 2• -accumulation and peroxidase distribution were assessed by light microscopy, using nitroblue tetrazolium (NBT) and o -dianisidine, respectively. Root length and root hair length and density were also quantified following ROS scavenging.• O 2• -was predominantly located in the apoplast of cell elongation zone, whereas H 2 O 2 accumulated in the differentiation zone and the cell wall of root hairs in formation. Treatments that decrease O 2• -concentration reduced root elongation and root hair formation, while scavenging H 2 O 2 promoted root elongation and suppressed root hair formation.• The results allow to precise the respective role of O 2• -and H 2 O 2 in root growth and development. The consequences of their distinct accumulation sites within the root tip are discussed, especially in relation to peroxidases.

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ABSTRACrThe effect of ozone was studied on the peroxidase activity from various compartments of Sedam album leaves (epidermis, intercellular fluid, residual cell material, and total cell material). The greatest increase following a 2-hour ozone exposure (OA microliters 03 per liter) was observed in extracellular peroxidases. Most of the main bands of peroxidase activity separated by isoelectric focusing exhibited an increase upon exposure to ozone. Incubation experiments with isolated peeled or unpeeled leaves showed that leaves from ozone-treated plants release much more peroxidases in the medium than untreated leaves. The withdrawal of Ca" ions reduced the level of extracellular peroxidase activity either in whole plants or in incubation experiments. This reduction and the activation obtained after addition of Ca2" resulted from a direct requirement of Ca2" by the enzyme and from an effect of Ca"2 on peroxidase secretion. The ionophore A23187 promoted an increase of extracellular peroxidase activity only in untreated plants. The release of peroxidases by untreated and ozone-treated leaves is considerably lowered by metabolic inhibitors (3-(3,4-dichlorophenyl)-1,1-dimethylurea and sodium azide) and by puromycin.Peroxidase activity increases in plants in response to a great variety ofstresses, including viral, microbial, or fungal infections, salt stress, wounding, or air pollution (9). Several pollutants such as ozone (5,6,22), SO2 (14, 16), or NO2 (13) are known to induce an enhancement ofthe total peroxidase activity of plants. The peroxidase increase following an exposure to ozone is different in different species and is a function of the resistance of the plant to ozone. Ozone-tolerant and ozone-sensitive cultivars have been described, the peroxidase activity of the former being less affected by ozone (5).In a previous work (4), it was shown that there is a parallelism between the level of air pollution and the peroxidase activity measured in Sedum album leaves. This was demonstrated in plants grown in diversely polluted areas. It appeared that S. album is a suitable plant material for the study ofthe mechanism leading plants exposed to a pollutant to increase their peroxidase activity.The present work was performed in the laboratory, under controlled environmental conditions and using standardized amounts of ozone as air pollutant. The dose applied to S. album (0.4 ,ul 1-' for 2 h) is known to increase the total peroxidase activity in many plant species (21,22). Peroxidase activity was measured in several leaf compartments. The data obtained showed that after an exposure to ozone the most significant increase of peroxidase activity occurred in the extracellular compartment. As the release of peroxidase was reported to be a calcium-dependent process in spinach cell suspensions (18), the possible involvement ofthis ion in the control ofthe extracellular peroxidases of S. album leaves was also investigated in relation with the response to ozone. MATERIALS AND METHODSPlant Material and Growing Conditions. Sedu...

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