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Mauricio Canoles, Marisol Soto and Randolph Beaudry

The aldehydes cis-3-hexenal, hexanal, and trans-2-hexenal; the alcohols 1-hexanol, and cis-3-hexenol; and the ketone 1-penten-3-one are produced as a consequence of lipid degradation following tissue disruption and are among the most important volatile compounds in tomato (Lycopersicon esculentum Mill.) aroma. The biosynthesis of cis-3-hexenal and other volatiles noted involves the action of a sequence of enzymes including lipase, lipoxygenase (LOX), hydroperoxide lyase (HPL), isomerase, and alcohol dehydrogenase (ADH) on glycerolipids containing the fatty acids, linoleic acid (18:2) and linolenic acid (18:3), via the LOX pathway. In the current work, the formation and sensory perception of volatile compounds was studied in tomato plant lines where HPL activity was genetically altered. LeHPL co-suppression dramatically reduced the production of lipid-derived C6-volatiles in leaves, but in fruits, only unsaturated C6-volatile production was affected, suggesting LeHPL-independent formation of hexanal occurs in fruits, but not in leaves. Increased production of 5-carbon volatiles is proposed as an alternative way to metabolize 13-hydroperoxy linolenic acid in plants with reduced LeHPL activity. Changes in the volatile profile of leaves and fruits of tomato lines in which LeHPL activity is reduced markedly are readily detected by nontrained sensory panels. The studies demonstrate that a marked reduction in the activity of one of the most critical steps in the LOX pathway can markedly impact sensory perception. Efforts to improve total volatile formation may require the modification of LOX pathway at several steps simultaneously, including precursor formation, and LOX and HPL activities.