Progress report: ethylene signaling and responses

Ethylene regulates many cellular and developmental processes in plants through a signaling pathway con served in monocots and dicots (reviewed in Chen et al. 2005). The current model for the ethylene-signaling pathway, based on genetic analysis in Arabidopsis tha liana, is shown in Fig. la. Ethylene is perceived by a family of receptors related to bacterial histidine kinases (O'Malley et al. 2005) that modulate the activity of the Raf-like kinase CTR1, a negative regulator of the pathway. Functioning downstream of CTR1 is EIN2, a protein with similarities to Nramp metal-ion transport ers, followed by members of the EIN3/EIN3-like (EIL) family of transcription factors, which regulate the tran scription of primary ethylene-response genes. Additional components of the signal transduction pathway have been proposed, but their contribution to signaling is not yet resolved. Due to the similarity of CTR1 to Raf, a mitogen-activated protein kinase kinase kinase (MAPKKK), a MAP-kinase (MAPK) cascade has been proposed to act downstream of CTR1. In 2003, a putative MAPK module was implicated in ethylene signal transduction (Ouaked et al. 2003), but additional research indicates that the primary role of the MAPK in question (MPK6 in A. thaliana) is in the regulation of ethylene biosynthesis, not signaling (Ecker 2004; Liu and Zhang 2004). Because the ethylene receptors are related to histidine kinases, researchers have also wondered if