Tissue-specific oxylipin signature of tomato flowers: allene oxide cyclaseis highly expressed in distinct flower organs and vascular bundles

A crucial step in the biosynthesis of jasmonic acid (JA) is the formation o f its correct stereoisomeric precursor, cis(+)12-oxophytodienoic acid (OPDA ). This step is catalysed by allene oxide cyclase (AOC), which has been rec ently cloned from tomato. In stems, young leaves and young flowers, AOC mRN A accumulates to a low level, contrasting with a high accumulation in flowe r buds, flower stalks and roots. The high levels of AOC mRNA and AOC protei n in distinct flower organs correlate with high AOC activity, and with elev ated levels of JA, OPDA and JA isoleucine conjugate. These compounds accumu late in flowers to levels of about 20 nmol g(-1) fresh weight, which is two orders of magnitude higher than in leaves. In pistils, the level of OPDA i s much higher than that of JA, whereas in flower stalks, the level of JA ex ceeds that of OPDA. In other flower tissues, the ratios among JA, OPDA and JA isoleucine conjugate differ remarkably, suggesting a tissue-specific oxy lipin signature. Immunocytochemical analysis revealed the specific occurren ce of the AOC protein in ovules, the transmission tissue of the style and i n vascular bundles of receptacles, flower stalks, stems, petioles and roots . Based on the tissue-specific AOC expression and formation of JA, OPDA and JA amino acid conjugates, a possible role for these compounds in flower de velopment is discussed in terms of their effect on sink-source relationship s and plant defence reactions. Furthermore, the AOC expression in vascular bundles might play a role in the systemin-mediated wound response of tomato .