Cucumber Hypocotyls Respond to Cutin Monomers via Both an Inducible and a Constitutive H2O2-Generating System1

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Abstract
Hypocotyls from etiolated cucumber (Cucumis sativa L.) seedlings were gently abraded at their surface to allow permeation of elicitors. Segments from freshly abraded hypocotyls were only barely competent for H2O2 elicitation with fungal elicitor or hydroxy fatty acids (classical cutin monomers). However, elicitation competence developed subsequent to abrasion, reaching an optimum after about 4 h. This process was potentiated in seedlings displaying acquired resistance to Colletotrichum lagenarium due to root pretreatment with 2,6-dichloroisonicotinic acid or a benzothiadiazole. Induction of competence depended on protein synthesis and could be effected not only by surface abrasion, but also by fungal spore germination on the epidermal surface or by rotating the seedlings in buffer. Inhibitor studies indicated that the inducible mechanism for H2O2 production involves protein phosphorylation, Ca2+ influx, and NAD(P)H oxidase. In contrast, a novel cucumber cutin monomer, dodecan-1-ol, also elicited H2O2 in freshly abraded hypocotyls without previous competence induction. This finding suggests the presence of an additional H2O2-generating system that is constitutive. It is insensitive to inhibitors and has, in addition, a different specificity for alkanols. Thus, dodecan-1-ol might initiate defense before the inducible H2O2-generating system becomes effective.