2007 Volume 24 Issue 2 Pages 209-215
An ATPase associated with various cellular activities (AAA) protein was previously shown to be involved in pathogen response in tobacco plants and designated as NtAAA1. Transgenic tobacco lines in which NtAAA1 was suppressed by the RNA-interference (RNAi) were found to exhibit an elevated resistance to Pseudomonas syringae infection, suggesting that NtAAA1 negatively controlled the defense reaction. To identify genes that were regulated by NtAAA1, differential micro-array screening between NtAAA1-RNAi and wild type plants was performed. Results brought out 330 affected genes, which were classified into functional categories, including transcriptional regulation, signal transduction, secondary metabolism and others. Notably, 43 genes were stress- and defense-related, among which 10 were phytohormone-related. Subsequent examination revealed that, in RNAi transgenic plants, genes related to salicylic acid were up-regulated, whereas those related to jasmonic acid and ethylene were generally down-regulated. When salicylic acid was exogenously applied to leaves, expression of PR-1a, a maker gene of pathogen response, was evidently induced at much higher level in NtAAA1-RNAi transgenic lines than in the control. Simultaneous application of jasmonic acid with salicylic acid markedly cancelled the effect of salicylic acid in the control, but not much in NtAAA1-RNAi transgenic plants. The present findings suggested that NtAAA1 broadly functions in cellular metabolism, and particularly that, responding to jasmonic acid and/or ethylene signals, it might interfere with salicylic acid signaling. This system maintains the defense response at appropriate levels, so that detrimental necrosis is avoided, and therefore NtAAA1 may be regarded as a molecular switch of the salicylic acid signaling pathway.
