DEFENSE RESPONSES IN INFECTED AND ELICITED CUCUMBER (CUCUMIS-SATIVUS L) HYPOCOTYL SEGMENTS EXHIBITING ACQUIRED-RESISTANCE

Segments from dark-grown cucumber (Cucumis sativus L.) hypocotyls were used to study defense reactions occurring upon fungal infection and i nduced by elicitors in the same tissue. The segments were rendered res istant to infection by Colletotrichum lagenarium either by growing the seedlings in the presence of dichloroisonicotinic acid (DCIA) or by p reincubation 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 resis tant hypocotyl segments, the fungus induced, at the time of attempted penetration, an increased deposition of phenolics, which were visualiz ed by autofluorescence. These phenolics were located mainly in the epi dermal cell wall around and in the emerging papillae below appressoria and were quantified either as lignin-like polymers by the thioglycoli c acid method or as 4-OH-benzaldehyde, 4-OH-benzoic, or 4-coumaric aci d liberated upon treatment with alkali at room temperature. Pretreatme nt with DCIA, SA, and 5CSA induced little chitinase activity, but this activity greatly increased in resistant tissues upon subsequent infec tion. These observations indicate that resistance is associated with a n improved perception of the pathogen stimulus resulting in the enhanc ed induction of diverse defense reactions. When the cut segments were pretreated with DCIA, SA, or 5CSA and then split and incubated with ch itosan fragments, the deposition of cell wall phenolics was also enhan ced. These pretreated and split segments also exhibited an increase in the rapid production of activated oxygen species induced by an elicit or preparation from Phytophthora megasperma f. sp. Glya. Pretreatment of the segments with methyl jasmonate neither induced resistance nor e nhanced induction of cell wall phenolics upon fungal infection, althou gh we observed in the corresponding split segments some increase in ch itosan-induced cell wall phenolics and in elicitor-induced rapid produ ction of activated oxygen species.