Ethylene Signaling

In Arabidopsis, ethylene is perceived by five receptors, which are partially redundant in function. They show homology to bacterial two-component his-tidine kinases and are localized on the endoplasmatic reticulum membrane (Chen et al. 2002; Gao et al. 2003). Ethylene binds to the aminoterminal end of these proteins (Schaller and Bleecker 1995). To be functional, the receptors need copper as a cofactor, which is delivered by RAN1 (responsive to antagonist) (Hirayama et al. 1999; Woeste and Kieber 2000). The ethylene receptors can be divided into two groups. Subfamily 1 contains ETR1 (ethylene resistant) and ERS1 (ethylene response sensor). They have four conserved hydrophobic regions in the amino terminus and their C-terminus strongly resembles the response regulator part of bacterial two-component systems. The second subfamily comprises ETR2, ERS2, and EIN4 (ethylene insensitive). These receptors have only three hydrophobic regions and a C-terminus that may function as a Ser/Thr domain (Hua et al. 1998; Hall and Bleecker 2003). The receptors act as negative regulators of ethylene signaling by activating CTR1 (constitutive triple response) in the absence of the hormone (Hua and Meyerowitz 1998; reviewed by De Paepe and Van Der Straeten (2005)).

CTR1 belongs to the Raf family of Ser/Thr protein kinases, but it is unclear whether it functions as a MAPKKK (mitogen-activated protein kinase kinase kinase). It is supposed to be part of, and a negative regulator of, a MAPK cascade (Kieber et al. 1993). The MAPK cascade acts as a positive regulator of ethylene signaling (Chang 2003; Ouaked et al. 2003), however a direct biochemical connection between CTR1 and SIMKK (the next step in the MAPK cascade) remains to be confirmed. Furthermore, a CTR1-independent pathway is supposed to exist as well, since quadruple loss-of-function receptor mutants show a more severe phenotype than ctrl -null mutants (Hua and Meyerowitz 1998) and ctrl -null mutants still respond to ethylene (Larsen and Chang 2001).

EIN2 is an essential positive regulator in the ethylene signaling pathway. It functions downstream of CTR1 and upstream of EIN3. A loss of function mutation in the gene encoding this protein causes a very severe ethylene insen-sitivity (Hall and Bleecker 2003). EIN2 contains 12 trans-membrane domains in its N-terminal part, which shows substantial similarity to Nramp (natural resistance-associated macrophage protein) proteins. This region is necessary for the regulation of the ethylene signal, possibly through the regulation of the activity of the C-terminus of EIN2, since constitutive expression of this C-terminal part confers a phenotype similar to ctrl (Alonso et al. 1999).

The next components in the ethylene signaling pathway are EIN3 and the EILs (EIN3-like proteins). eil1ein3 double mutants are almost completely ethylene-insensitive and almost indistinguishable from ein2-5 (Alonso et al. 2003). Over-expression of these genes confers a ctrl-like phenotype. This raises questions about the function of the other EILs. The latter proteins were suggested to be important for more specific ethylene responses at particular stages of development (Chao et al. 1997). EIN3 is constitutively synthesized and degraded in a ubiquitin-dependent way. EBF1 (EIN3-binding F-box protein) and EBF2 mediate this breakdown (Guo and Ecker 2003; Potuschak et al. 2003; Gagne et al. 2004). The stability of EIN3 is raised in the presence of ethylene, while other signals (e.g. glucose) destabilize EIN3 (Yanagisawa et al. 2003). This might be caused by EIN5, an exoribonuclease. EIN5 expression is up-regulated by ethylene and the corresponding protein is supposed to act in ethylene signaling by breaking down EBF1/2 mRNA (Olmedo et al. 2006; Potuschak et al. 2006). EIN3 dimers are able to bind the PERE (primary ethylene response element) in the promoter of ERF1 (ethylene response factor; Solano et al. 1998). ERF1 is part of the family of EREBP (ethylene response element binding proteins). These proteins are able to bind the SERE (secondary ethylene-response element), a GCC box in the promoter region of ethylene-regulated genes (Fujimoto et al. 2000).

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