Mitogen-activated protein kinase cascades and ethylene: signaling, biosynthesis, or both?

Mitogen-activated protein kinase (MAPK) cascades serve as important signaling pathways in all eukaryotic cells, as they link the perception of external stimuli to cellular responses. Since the discovery of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1), a Raf-like MAPK kinase kinase (MAPKKK), there has been intense debate whether MAPK modules are involved in the signal transduction of the plant hormone ethylene. Although the discussion continues, evidence in support of this idea has accumulated. The Arabidopsis (Arabidopsis thaliana) MAPKs MPK3 and MPK6 seem to play a central role in the regulation of the ethylene response pathway by promoting the stabilization of EIN3, a key transcriptional regulator of the ethyleneinduced transcriptional machinery. Other reports, however, place the MPK3/6-containing MAPK module in ethylene biosynthesis rather than in the signaling pathway. In this Update, we discuss the recent findings on MAPK cascades in ethylene signaling and/or biosynthesis.