The nature of lignin-like compounds in cucumber hypocotyls induced by α-1,4-linked oligogalacturonides

The nature of the phloroglucinol-positive material, which is induced in cucumber ( Cucumis sativus L.) hypocotyls by α-1,4-linked oligogalacturonides, has been investigated by three different experimental approaches and they all suggest that the material may be classified as lignin. Firstly, formation of the material is inhibited by compounds which are known to inhibit the first (phenylalanine ammonia lyase) and the last (peroxidase) step in the biosynthesis of lignin. Secondly, the elicitor enhances incorporation of ¹⁴C]-cinnamic acid derivatives into the hypocotyl cell walls. Thirdly, the analyses of phenols released from cell walls by cupric oxide oxidation show that the walls of oligogalacturonide-treated hypocotyls contain more polyphenolic material than the walls from intact or wounded hypocotyls. The walls from the oligogalac-turonide-treated hypocotyls produce more p -hydroxybenzaldehyde, p -hydroxyace-tophenone, p -hydroxybenzoic acid and p -coumaric acid upon oxidation than the other two wall samples. These results suggest that the lignin formed in cucumber hypocotyls in response to oligogalacturonides is mainly derived from p -coumaryl alcohol.