Submitted to: Plant Lipid Metabolism
Publication Type: Book / Chapter
Publication Acceptance Date: August 1, 1994
Publication Date: N/A
Interpretive Summary: Lipoxygenase is an enzyme of both plant and animal metabolism that catalyzes the addition of molecular oxygen to certain polyunsaturated fatty acids, resulting in the formation of fatty acid hydroperoxides. In plants these compounds can be further metabolized to other fatty acid metabolites by several enzymatic pathways, which vary according to species. However, there are two major pathways of fatty acid hydroperoxide metabolism that are present in most plant species. One pathway, the hydroperoxide dehydrase pathway, leads to the biosynthesis of jasmonic acid, which is known to have growth regulatory properties and probably serves as a chemical signal in plant defense reactions. The second pathway, the hydroperoxide lyase pathway, leads to the biosynthesis of short-chain aldehyde products that are known to be toxic to certain fungal, bacterial, or insect pests. This report examines the level of expression of these two enzymes in sunflower leaves, cotyledons, hypocotyl, and roots during the first twenty days after germination. The results suggest that there are distinct organ-specific functional roles for each of the two enzymes. Dynamic alterations were observed in the expression of the two enzymes in the cotyledons during sunflower seedling development. In addition, evidence for the presence of two organ-specific isozymes of hydroperoxide lyase is presented.
Technical Abstract: This study demonstrated that fatty acid hydroperoxides have multiple functions within the plant. A striking increase and decrease in the activity of hydroperoxide dehydrase, a jasmonic acid pathway enzyme, occurred in the cotyledons during the first three weeks of growth. The high activity of this enzyme suggests a potential role for jasmonic acid during early seedling growth. Later, as the role of the cotyledon changed from a lipid storage organ to a photosynthetic organ, hydroperoxide lyase activity became the predominant enzyme of fatty acid hydroperoxide metabolism. Lyase activity was very high in leaves, but dehydrase was not detectible there. It is possible that the high activity of lyase masked any dehydrase present. Lyase activity was also present in the roots, but its total activity was less than 1% of that in the leaves. Roots and leaves clearly had different isozymes of hydroperoxide lyase. The root enzyme was less hydrophobic, was lower in molecular weight than the leaf enzyme, and had different substrate specificity. Since hydroperoxide lyase in thought to have a defensive role in plant metabolism, the presence of organ-specific isozymes with differing substrate specificities suggests that hexenal may be a principal defensive metabolite in leaves, while both hexenal and hexanal may be important in root defense.