Wound-induced apoplastic peroxidase activities: their roles in the production and detoxification of reactive oxygen species

Production of reactive oxygen species (ROS) is a widely reported response of plants to wounding. However, the nature of enzymes responsible for ROS production and metabolism in the apoplast is still an open question. We identified and characterized the proteins responsible for the wound-induced production and detoxification of ROS in the apoplast of wheat roots ( Triticum aestivum L.). Compared to intact roots, excised roots and leachates derived from them produced twice the amount of superoxide (O₂^•−). Wounding also induced extracellular peroxidase (ECPOX) activity mainly caused by the release of soluble peroxidases with molecular masses of 37, 40 and 136 kD. Peptide mass analysis by electrospray ionization–quadrupole time-of-flight–tandem mass spectrometry (ESI–QTOF–MS/MS) following lectin affinity chromatography of leachates showed the presence of peroxidases in unbound (37 kD) and bound (40 kD) fractions. High sensitivity of O₂^•−-producing activity to peroxidase inhibitors and production of O₂^•− by purified peroxidases in vitro provided evidence for the involvement of ECPOXs in O₂^•− production in the apoplast. Our results present new insights into the rapid response of roots to wounding. An important component of this response is mediated by peroxidases that are released from the cell surface into the apoplast where they can display both oxidative and peroxidative activities.