Abstract: The roles of NO and H2O2 in indole-3-butyric acid（IBA）-induced adventitious root formation in marigold plants（Tagetes erecta L.）and the order of action of these molecules within the signal transduction pathway were investigated. IBA treatment was able to induce de novo root organogenesis in marigold explants mimicking the effects of NO and H2O2. Treatment of IBA plus NO or H2O2 promoted adventitious root development compared with explants treated with NO，H2O2 or IBA alone. The IBA-mediated effect was differently reversed by NO scavenger cPTIO and H2O2 scavenger CAT. IBA was able to recover adventitious root development in auxin-depleted explants by NPA，whereas c-PTIO and CAT prevented this effect. Thus，NO and H2O2 were the prerequisite of adventitious root development induced by IBA. IBA treatments enhanced endogenous NO and H2O2 levels in hypocotyls. Therefore，the enhancing effect on the formation of adventitious root of IBA was through the promotion of NO and H2O2 formation. cPTIO inhibited IBA-triggered H2O2 generation. However，CAT failed to depress the NO content induced by IBA. Thus，H2O2 and NO may be the two downstream signal molecules of auxin signaling cascade，and NO may be an upstream signal molecule before H2O2.

Key words: marigold, Tagetes erecta L., NO, H₂O₂, IBA, adventitious root