Metabolite-based Defense Mechanisms

Lipoxygenases catalyze the hydroperoxidation of polyunsaturated fatty acids and thus the first step in the synthesis of fatty acid metabolites in plants. Products of the LOX pathway have multiple functions as growth regulators, antimicrobial compounds, flavours and odours as well as signal molecules. Based on the effects of LOX products or on the correlation of increases in LOX protein and the onset of specific processes, a physiological function for LOXs has been proposed for growth and development and for the plant response to patho­gen infection and wound stress.

Lipoxygenases are ubiquitous enzymes in eukaryotes. In plants, lipoxygenases 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 classes are defined (Lox1, Lox2, and Lox3). They are encoded in gene families that display organ-specific expression, lox1 being expressed mostly in tubers and roots, lox2 in leaves, and lox3 in leaves and roots.Consistent with their organ-specific expression pattern, Lox1 expressed in bacteria preferentially uses as substrate linoleic acid, abundant in membrane lipids of tubers, whereas linolenic acid, prevalent in leaves, is the preferred substrate for the other two classes of lipoxygenase. 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 product of the action of Lox2 and Lox3 on linolenic acid. Upon wounding, the levels of Lox2 and Lox3 transcripts rise markedly in leaves. While Lox3 mRNA accumulation peaks as early as 30 min after wounding, Lox2 shows a steady increase over a 24-h time course, suggesting different roles for these lipoxygenase isoforms in the synthesis of the plant hormone jasmonic acid.