The oxidative burst, the generation of reactive oxygen species (ROS) in response to microbial pathogen attack, is a ubiquitous early part of the resistance mechanisms of plant cells. It has also become apparent from the study of a number of plant–pathogen interactions and those modelled by elicitor treatment of cultured cells that there may be more than one mechanism operating. However, one mechanism may be dominant in any given species. NADPH oxidases have been implicated in a number of systems and have been cloned and characterized. However, the enzyme system which is the major source of ROS in French bean (Phaseolus vulgaris) cells treated with a cell wall elicitor from Colletotrichum lindemuthianum, appears to be dependent on an exocellular peroxidase. The second component, the extracellular alkalinization, occurs as a result of the Ca²⁺ and proton influxes and the K⁺ efflux common to most elicitation systems as one of the earliest responses. The third component, the actual reductant/substrate, has remained elusive. The low molecular weight compound composition of apoplastic fluid was compared before and after elicitation. The substrate only becomes available some min after elicitation and can be extracted, so that by comparing the profiles by LC‐MS it has been possible to identify possible substrates. The mechanism has proved to be complex and may involve a number of low molecular weight components. Stimulation of H₂O₂ production was observed with saturated fatty acids such as palmitate and stearate without concomitant oxylipin production. This biochemical evidence is supported by immunolocalization studies on papillae forming at bacterial infection sites that show the peroxidase isoform present at sites of H₂O₂ production revealed by cerium chloride staining together with the cross‐linked wall proteins and callose and callose synthase. The peroxidase has been cloned and expressed in Pichia pastoris and has been shown to catalyse the oxidation reaction with the same kinetics as the purified enzyme. Furthermore, Arabidopsis plants transformed heterologously using the French bean peroxidase in antisense orientation have proved to be highly susceptible to bacterial and fungal pathogens. Thus it is possible that Arabidopsis is another species with the potential to mount an apoplastic oxidative burst and these transformed plant lines may be useful to identify the peroxidase that is responsible.

Sterols, which are isoprenoid derivatives, are structural components of biological membranes. Special attention is now being given not only to their structure and function, but also to their...

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.