Mitogen-activated protein kinase (MAPK) cascades play important roles in regulating plant growth, development, and responses to various environmental stimuli. We demonstrate that MKK9, an MKK, is an upstream activator of the MPKs MPK3 and MPK6 both in vitro and in planta. Expression of active MKK9 protein in transgenic plants induces the synthesis of ethylene and camalexin through the activation of the endogenous MPK3 and MPK6 kinases. As a consequence, transcription of multiple genes responsible for ethylene biosynthesis, ethylene responses, and camalexin biosynthesis is coordinately up-regulated. The activation of MKK9 inhibits hypocotyl elongation in the etiolated seedlings. MKK9-mediated effects on hypocotyl elongation were blocked by the ethylene biosynthesis inhibitor, aminoethoxyvinylglycine, and ethylene receptor antagonist, Ag(+). Expression of active MKK9 protein enhances the sensitivity of transgenic seedlings to salt stress, whereas loss of MKK9 activity reduces salt sensitivity indicating a role for MKK9 in the salt stress response. The results reported here reveal that the MKK9-MPK3/MPK6 cascade participates in the regulation of the biosynthesis of ethylene and camalexin and may be an important axis in the stress responses of Arabidopsis.