Georgia 30602-471 2 (M.G.H.)To study H,O, production, the epidermal surfaces of hypocotyl segments from etiolated seedlings of cucumber (Cucumis safivus L.) were gently abraded. Freshly abraded segments were not constitutively competent for rapid H,O, elicitation. This capacity developed subsequent to abrasion in a time-dependent process that was greatly enhanced in segments exhibiting an acquired resistance to penetration of their epidermal cell walls by Collefofricbum lagenarium, because of root pretreatment of the respective seedlings with 2,6-dichloroisonicotinic acid. When this compound or salicylic acid was applied t o abraded segments, it also greatly enhanced the induction of competence for H,O, elicitation. This process was fully inhibited by 5 PM cycloheximide or 200 PM puromycin, suggesting a requirement for translational protein synthesis. Both a crude elicitor preparation and a partially purified oligoglucan mixture from Pbyfophfbora sojae also induced, in addition t o H,O, production, a refractory state, which explains the transient nature of H,O, elicitation. Taken together, these results suggest that the cucumber hypocotyl epidermis becomes conditioned for competence t o produce H,O, i n response to elicitors by a stimulus resulting from breaching the cuticle and/or cutting segments. This conditioning process is associated with protein synthesis and is greatly enhanced when substances able t o induce systemic acquired resistance are present in the tissue.The interaction of plant cells with potentially pathogenic microorganisms is associated with rather complex biochemical and physiological events. Experiments on these processes are, therefore, often performed with simplified systems in which the pathogen is reduced to "elicitors" and the plant to a mechanically wounded tissue surface or to cell-suspension cultures. These models have already contributed many insights into the mechanisms of plant responses presumably related to defense but still require refinement to better cover the diverse biological features of host/pathogen interactions. It should be considered, for instance, that severa1 signals may act subsequently in the course of pathogenesis and that the nutritional status, age, or a previous infection of a plant can influence the effectiveness of defense responses.

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 H(2)O(2) 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 H(2)O(2) production involves protein phosphorylation, Ca(2+) influx, and NAD(P)H oxidase. In contrast, a novel cucumber cutin monomer, dodecan-1-ol, also elicited H(2)O(2) in freshly abraded hypocotyls without previous competence induction. This finding suggests the presence of an additional H(2)O(2)-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 H(2)O(2)-generating system becomes effective.