Allene oxide synthase and hydroperoxide lyase product accumulation in Artemisia species

Jasmonic acid (JA) and some of its precursors and derivatives are important signaling molecules in plants, especially in defense reactions. Prior reports indicated that Artemisia species might accumulate high levels of JA and methyl jasmonate (MJ), but these levels have not been quantified. We surveyed representative species of Artemisia from around the world and found several species of Artemisia to accumulate unusually high basal levels of JA and MJ. The pool of free linolenic acid was in excess of the amount needed to support the synthesis of JA and MJ. Basal levels of MJ were comparable to or higher than those of JA, and wounding increased only MJ levels. Surveyed species of Artemisia accumulated JA and MJ in the range of 0.6-10 and 2-39 nmol/g FW, with A. tridentata subspecies vaseyana accumulating 20-48 nmol/g FW of JA and MJ depending on growth conditions. Some Artemisia species had two- to eight-fold higher JA when grown outside compared to growth in a greenhouse. Products of the hydroperoxide lyase (HPL) pathway, (Z)-3-hexenal, (E)-2-hexenal and hexanal, were only synthesized de novo upon wounding. The data suggest that wounding was needed to activate HPL, and/or bring the enzyme in close proximity to its substrate. Conversely, enzymes of the JA pathway were functioning near their maximum in unwounded leaves of Artemisia.