ACQUIRED-RESISTANCE SIGNAL-TRANSDUCTION IN ARABIDOPSIS IS ETHYLENE INDEPENDENT

To clarify the role of ethylene in systemic acquired resistance (SAR), we conducted experiments using Arabidopsis ethylene response mutants. Plants that are nonresponsive to ethylene (i.e., etr1 and ein2) showe d normal sensitivity to the SAR-inducing chemicals salicylic acid (SA) and 2,6-dichloroisonicotinic acid with respect to SAR gene induction and pathogen resistance. This indicated that chemically induced SAR is not an ethylene-dependent process in Arabidopsis. Ethephon, an ethyle ne-releasing chemical, induced SAR gene expression in both the wild ty pe and ethylene mutants, whereas ethylene alone did not, suggesting th at induction of these genes by ethephon is not due to the action of et hylene. Furthermore, transgenic plants expressing salicylate hydroxyla se, a bacterial enzyme that degrades SA to catechol, did not accumulat e SAR mRNAs in response to ethephon. Thus, SAR gene induction by ethep hon appears to be mediated through SA. Other experiments suggested tha t ethylene may play a role in SAR by enhancing tissue sensitivity to t he action of SA.