Lipoxygenase (LOX) is involved in the synthesis of jasmonic acid, methyl jasmonate, traumatin, and C-6 volatile compounds that serve hormone-like regulatory and defense-related roles in plants. To study possible lipoxygenase involvement in potato tuberization and defense responses against pests, we have isolated, sequenced, and characterized three lipoxygenase cDNA clones that represent distinct LOX genes. Two of these genes, designated POTLX-1 and POTLX-2, have been isolated from an early tuber cDNA library. Northern hybridization studies indicated that accumulation of POTLX-1 and POTLX-2 mRNA was specific to developing tubers and roots and correlated positively with tuber growth. Hormonal treatment, pathogen infection, or wounding did not induce POTLX-1 and POTLX-2 mRNA accumulation in leaves. In situ hybridization showed that POTLX-1 transcripts accumulated in the apical and subapical regions of the new tuber and in the vascular tissue of the most actively growing part of the developing tuber. Thus, POTLX-1 transcripts were localized within the sites of the most active cell division that occurs during tuber initiation. Our results indicate that POTLX-1 and/or POTLX-2 may have an important function in controlling tuber initiation and growth. The third lipoxygenase cDNA clone, designated POTLX-3, was isolated from an ABA-induced potato leaf cDNA library. This is a novel potato LOX gene as it shares relatively low sequence identity with other classes of potato LOX genes. POTLX-3 transcript accumulation was not detected in untreated, healthy potato organs, but it was induced in leaves treated with ethylene or methyl jasmonate. Neither wounding nor treatment of leaves with ABA, cytokinin, auxin, or gibberellic acid induced POTLX-3 transcript accumulation. However, POTLX-3 mRNA accumulation was induced in potato leaves infected with virulent or avirulent strains of Phytophthora infestans. During the resistance response, POTLX-3 mRNA was induced within 6 h and continued to accumulate for a week postinoculation. During the compatible interaction, mRNA accumulation was induced 3 to 6 days postinoculation. POTLX-3 also was induced during hypersensitive response development caused by Pseudomanas syringae pv. phaseolicola, and this indicates that POTLX-3 expression may be associated with the hypersensitive response. Thus, POTLX-3 may be involved specifically in defense responses against pathogens.