Abstract
The plant hormone ethylene regulates growth and development as well as responses to biotic and abiotic stresses. Over the last few decades, key elements involved in ethylene signal transduction have been identified through genetic approaches, these elements defining a pathway that extends from initial ethylene perception at the endoplasmic reticulum to changes in transcriptional regulation within the nucleus. Here, we present our current understanding of ethylene signal transduction, focusing on recent developments that support a model with overlapping and non-overlapping roles for members of the ethylene receptor family. We consider the evidence supporting this model for sub-functionalization within the receptor family, and then discuss mechanisms by which such a sub-functionalization may occur. To this end, we consider the importance of receptor interactions in modulating their signal output and how such interactions vary in the receptor family. In addition, we consider evidence indicating that ethylene signal output by the receptors involves both phosphorylation-dependent and phosphorylation-independent mechanisms. We conclude with a current model for signalling by the ethylene receptors placed within the overall context of ethylene signal transduction.
Highlights
Introduction and BackgroundThe gaseous hormone ethylene plays multiple roles in regulating plant growth and development
We conclude with a current model for signalling by the ethylene receptors placed within the overall context of ethylene signal transduction
The subfamily 2 triple mutant etr2 ein4 ers2 has a stronger constitutive ethylene-response phenotype than does the quadruple ers1 etr2 ein4 ers2 mutant. These results demonstrate that ERS1 can act as either a negative or a positive regulator of the ethylene response depending on the genetic background
Summary
The gaseous hormone ethylene plays multiple roles in regulating plant growth and development (reviewed in Abeles et al 1992). Again conflicting with such a model, the plants with low levels of CTR1 suppress ethylene responses more strongly than those with the higher level of CTR1 (Hall et al 2012) These studies point to ETR1 activating CTR1 more effectively than other members of the receptor family, in particular more effectively than the subfamily 2 receptors, and suggest that the kinase activity of ETR1 may play a role in this activation, a point we will return to later in this review. Genomic analysis of the moss P. patens reveals that all members of its ethylene-receptor family contain conserved His kinase domains, suggesting that the ethylene receptors of early land plants rely more upon the two-component signalling pathway than do the evolutionarily younger monocots and dicots (reviewed in Binder et al 2012). What is currently unclear is whether ethylene binding to the receptors activates or inhibits their His kinase activity, in vitro analysis suggesting inhibition but genetic analysis suggesting activation (Voet-vanVormizeele and Groth 2008; Hall et al 2012)
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