Segments from dark-grown cucumber (Cucumis sativus L.) hypocotyls were used to study defense reactions occurring upon
fungal infection and induced by elicitors in the same tissue. The segments were rendered resistant to
infection by Colletotrichum lagenarium either by growing the seedlings in the presence of dichloroisonicotinic
acid (DCIA) or by preincubation of the cut segments with DCIA,
salicylic acid (SA), or
5-chlorosalicylic acid (5CSA). This resistance appears to be due mainly to inhibition of fungal penetration into epidermal cells. In the resistant hypocotyl segments, the fungus induced, at the time of attempted penetration, an increased deposition of phenolics, which were visualized by autofluorescence. These phenolics were located mainly in the epidermal cell wall around and in the emerging papillae below appressoria and were quantified either as
lignin-like
polymers by the
thioglycolic acid method or as 4-OH-benzaldehyde, 4-OH-benzoic, or
4-coumaric acid liberated upon treatment with
alkali at room temperature. Pretreatment with DCIA, SA, and 5CSA induced little
chitinase activity, but this activity greatly increased in resistant tissues upon subsequent
infection. These observations indicate that resistance is associated with an improved perception of the pathogen stimulus resulting in the enhanced induction of diverse defense reactions. When the cut segments were pretreated with DCIA, SA, or 5CSA and then split and incubated with
chitosan fragments, the deposition of cell wall phenolics was also enhanced. These pretreated and split segments also exhibited an increase in the rapid production of activated
oxygen species induced by an elicitor preparation from Phytophthora megasperma f. sp. Glya. Pretreatment of the segments with
methyl jasmonate neither induced resistance nor enhanced induction of cell wall phenolics upon
fungal infection, although we observed in the corresponding split segments some increase in
chitosan-induced cell wall phenolics and in elicitor-induced rapid production of activated
oxygen species.