Herbivore-induced allene oxide synthase transcripts and jasmonic acid in Nicotiana attenuata

Exogenous jasmonate treatment of Nicotiana attenuata Torr. ex Wats. plants elicits durable resistance against herbivores and attack from its specialis t herbivore, Manduca sexta, results in an amplification of the transient wo und-induced increase in endogenous jasmonic acid levels (JA). To understand whether this "JA burst" is under transcriptional control, we cloned allene oxide synthase (AOS, EC 4.2.1.92), the enzyme that catalyzes the dehydrati on of 13(S)-hydroperoxy octadecatrienoic acid to an allene oxide, the first specific reaction in JA biosynthesis. An AOS cDNA coding for a 520 aa prot ein (58.6 kDa) with an isoelectric point of 8.74 was overexpressed in bacte ria and determined to be a functional AOS. Southern blot analysis indicated the presence of more than one gene and AOS transcripts were detected in al l organs, with the highest levels in stems, stem leaves and flowers. Attack by M. sexta larvae resulted in a sustained JA burst producing an endogenou s JA amount 9-fold above control levels and 3-fold above maximum wound-indu ced levels, a response which could be mimicked by the addition of Manduca o ral secretion and regurgitant to puncture wounds. M. sexta attack, wounding and regurgitant treatment transiently increased AOS transcript in the woun ded leaf, but increases were not proportional to the JA response. Moreover, transcript accumulation lagged behind JA accumulation. Systemic wound-indu ced increases in AOS transcript, AOS activity or JA accumulation could not be detected. We conclude that increase in AOS transcript does not contribut e to the initial increase in endogenous JA, but may contribute to sustainin g the JA burst. (C) 2001 Elsevier Science Ltd. All rights reserved.