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. 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 (Xu et al. 2008).