Eicosanoids in the Phytophthora infestans-potato interaction: lipoxygenase metabolism of arachidonic acid and biological activities of selected lipoxygenase products

Abstract

Potato tuber and leaf discs were treated with the fungal elicitor arachidonic acid. Hydroperoxy-eicosatetraenoic acids (HpETEs), produced by reaction of lipoxygenase with arachidonic acid, were then extracted from the tissues and analysed. 5-, 8-, 11-, 12-, and 15-HpETE were formed rapidly by both leaf and tuber tissues. 5-HpETE was the predominant lipoxygenase product in treated tuber discs, whereas 12- and 15-HpETE were the principal products produced by leaf discs. HpETEs were formed in tuber discs within 10 min after addition of the elicitor and increased during the first 5 h after treatment. The quantity of HpETEs, however, was inversely related to the amount of arachidonic acid initially applied to the discs and to the sesquiterpene phytoalexin levels induced. Treatments that enhance ( β -glucans from Phytophthora infestans ) or inhibit (abscisic acid) elicitor activity generally did not have a corresponding effect on the levels of HpETEs formed in tuber tissues, suggesting that these treatments primarily affect the response at sites different from the initial lipoxygenase reaction with arachidonic acid. Eicosatetraynoic acid, an acetylenic analogue of arachidonic acid and irreversible inhibitor of lipoxygenases, suppressed the elicitor-induced hypersensitive response in tuber discs and inhibited formation of HpETEs from arachidonic acid by microsomes from tuber discs. A collection of purified lipoxygenase products of arachidonic acid were tested for elicitor activity in potato and fungitoxicity to P. infestans and P. capsici . None of these possessed elicitor activity but several HpETEs and hydroxy-acids were inhibitory to cystospore germination. The results are discussed in relation to the proposed role of lipoxygenase and lipoxygenase products of arachidonic acid in hypersensitivity expression and disease resistance in potato.