CHARACTERIZATION OF 3 POTATO LIPOXYGENASES WITH DISTINCT ENZYMATIC-ACTIVITIES AND DIFFERENT ORGAN-SPECIFIC AND WOUND-REGULATED EXPRESSION PATTERNS

Lipoxygenases are ubiquitous enzymes in eukaryotes. In plants, lipoxyg enases are involved in the synthesis of the hormone jasmonic acid that regulates plant responses to wounding and, in addition, is an inducer of tuberization in potato. We have isolated potato lipoxygenase cDNA clones, From their deduced amino acid sequences, three distinct classe s are defined (Lox1, Lox2, and Lox3), They are encoded in gene familie s that display organ-specific expression, lox1 being expressed mostly in tubers and roots, lox2 in leaves, and lox3 in leaves and roots. Con sistent with their organ-specific expression pattern, Lox1 expressed i n bacteria preferentially uses as substrate linoleic acid, abundant in membrane lipids of tubers, whereas linolenic acid, prevalent in leave s, is the preferred substrate for the other two classes of lipoxygenas e. Analyses on reaction products of the enzymes expressed in bacteria reveal that Lox1 primarily produces 9-hydroperoxides. In contrast, the jasmonic acid precursor, 13-hydroperoxylinolenic acid, is the major p roduct, of the action of Lox2 and Lox3 on linolenic acid. Upon woundin g, the levels of Lox2 and Lox3 transcripts rise markedly in leaves. Wh ile Lox3 mRNA accumulation peaks as early as 30 min after wounding, Lo x2 shows a steady increase over a 24-h time course, suggesting differe nt roles for these Lipoxygenase isoforms in the synthesis of the plant hormone jasmonic acid.