Induction of peroxidases in cucumber hypocotyls by wounding and fungal infection

The induction of peroxidases (EC 1.11.1.7) during elicitation of lignification by α‐1,4‐linked oligogalacturonides in cucumber hypocotyl segments (Cucumis sativus L. cv, Wisconsin SMR 58) was investigated. The wounding associated with the preparation of hypocotyl segments induced a 19‐fold increase in peroxidase activity during the following 72 h. The increase was partially due to an increase in activity of a constitutive peroxidase with a pI of 8.9 and partially due to the expression of new peroxidase isozymes with pIs of 3.8, 5.4, 6.2, 9.1 and 9.4. The oligogalacturonides did not induce any peroxidase activity in addition to the wound‐induced activity. These results suggest that either the constitutive peroxidase isozyme (pI 8.9) of intact hypocotyls or some of the wound‐induced peroxidases are involved in the oligogalacturonide‐induced lignification. Induction of the peroxidases by wounding was inhibited by cycloheximide. This indicates that they accumulate as a result of de novo protein synthesis. Actinomycin D caused only a modest inhibition of the wound‐induction peroxidases, indicating that the process is regulated at the level of translation. Peroxidase activity increased more rapidly in resistant than in susceptible cucumber hypocotyls after inoculation with the pathogen Cladosporium cucumerinum Ellis & Arthur. The pattern of isozymes which was induced by fungal infection of resistant hypocotyls was similar to the pattern of isozymes induced by wounding. This suggests that similar induction mechanisms may be involved in the two processes.