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First published online April 15, 2009; 10.1104/pp.109.138461 Plant Physiology 150:547-551 (2009) © 2009 American Society of Plant Biologists
ETR1-Specific Mutations Distinguish ETR1 from Other Arabidopsis Ethylene Receptors as Revealed by Genetic Interaction with RTE11,[C],[W]Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742
The plant growth regulator ethylene is perceived by a family of homologous receptors that negatively regulate ethylene responses. It is well established that dominant missense mutations within the ethylene-binding domain of any of these receptors result in ethylene insensitivity (Chang et al., 1993
Ethylene receptors have a membrane-bound N-terminal ethylene-binding domain followed by a cytosolic GAF domain (Hall et al., 2007
Arabidopsis has five ethylene receptors, and for each there exist dominant mutant forms conferring ethylene insensitivity (Chang et al., 1993
The Arabidopsis REVERSION-TO-ETHYLENE SENSITIVITY1 (RTE1) gene is a positive regulator of ETR1 that was identified in a genetic screen for suppressors of the dominant ethylene-insensitive mutation etr1-2 (Resnick et al., 2006
In the work presented here, we wanted to determine more definitively whether RTE1 action is specific to ETR1 or whether RTE1 could similarly affect other ethylene receptors in Arabidopsis. Our approach was to test rte1-2 for the ability to suppress dominant mutations in ethylene receptor genes other than ETR1. Previously, we had tested single gain-of-function alleles for each of the other Arabidopsis ethylene receptor genes (ers1-10, ein4-1, etr2-1, and ers2-2) and found that they were not suppressed by rte1-2 (Resnick et al., 2006 We focused on ERS1, the only other Arabidopsis ethylene receptor in the same subfamily as ETR1 (subfamily I). Among Arabidopsis ethylene receptors, ERS1 has the most closely related ethylene-binding domain to that of ETR1 (75% identity, 83% similarity). We tested five amino acid substitutions that are known to confer dominant ethylene insensitivity when present in the ETR1 receptor (Table I ); four (Y32A, E38A, F58A, and A102T) are dependent on RTE1 for ethylene insensitivity, and one (C65Y) is RTE1 independent. Each of the substitutions lies within a highly conserved region in one of the three predicted transmembrane domains of the ethylene-binding domain (Fig. 1 ).
Mutations encoding these amino acid substitutions were introduced to the corresponding conserved positions in the ERS1 coding sequence using in vitro site-directed mutagenesis (Supplemental Materials and Methods S1). In order to control for possible differences in expression between ETR1 and ERS1, two separate ers1 transgene (Ters1) constructs were created for each mutation, one driven by the native ERS1 promoter and the other driven by the native ETR1 promoter. Each construct was stably transformed into wild-type and rte1-2 mutant plants, and six transformed lines were analyzed for each construct in each genetic background. Ethylene insensitivity was assessed using the classic triple-response assay in dark-grown seedlings (Guzmán and Ecker, 1990
We next tested EIN4, a subfamily II ethylene receptor. EIN4, like ETR1, possesses a C-terminal receiver domain, which ERS1 lacks. We introduced a mutation coding for A125T (equivalent to ETR1 A102T) into the EIN4 cDNA sequence driven by the cauliflower mosaic virus 35S promoter (Supplemental Materials and Methods S1). Consistent with the results for ERS1, wild-type plants transformed with the Tein4 (A125T) transgene failed to display ethylene insensitivity (Fig. 2).
These findings suggest that etr1 missense mutations requiring RTE1 to confer ethylene insensitivity do not confer the same insensitivity when introduced into the corresponding conserved positions in other Arabidopsis ethylene receptor genes. If this hypothesis is correct, then existing ethylene-insensitive mutations in the four other ethylene receptor genes should not be suppressed by rte1 when carried by the etr1 gene. We tested this using two amino acid substitutions, I62F and P36L (numbered here based on the ETR1 sequence). The corresponding I62F substitution is encoded by an existing mutant allele in four different ethylene receptor genes: etr1-4 (Chang et al., 1993
In conclusion, not all dominant ethylene-insensitive alleles of etr1 are transferable to other ethylene receptor genes. The data suggest that dominant etr1 mutations that require RTE1 to confer ethylene insensitivity are essentially silent when transferred to the identical conserved positions in other ethylene receptor genes. It remains to be seen whether this extends to all of the subfamily II receptors and whether other known RTE1-dependent etr1 alleles behave similarly. Nevertheless, our results are unexpected given the strong sequence conservation among the ethylene receptors, particularly within the ethylene-binding domain. Moreover, several dominant missense mutations that confer ethylene insensitivity have been previously transferred to or identified in the corresponding positions in other ethylene receptor isoforms even between different species. For example, this is how the Arabidopsis ERS1 and ERS2 genes were shown to encode functional ethylene receptors in the absence of endogenous mutant alleles for these genes: the etr1-4 (I62F) mutation was introduced to an ERS1 transgene (Hua et al., 1995
Our findings also demonstrate that RTE1 is specific for ETR1. It is unknown why the RTE1 mechanism that promotes signaling in ETR1 lacks this role for ERS1 (and probably for the three other ethylene receptors as well). The specificity of RTE1 does not appear to be based on differences between ETR1 and ERS1 expression patterns, as the same results were obtained using either the ERS1 or ETR1 promoter to drive the expression of the mutant transgenes. The RTE1 protein may act to maintain or stabilize active conformations of certain ETR1 dominant mutant forms (Resnick et al., 2008 The ETR1-RTE1 genetic interaction may have coevolved to distinguish ETR1 from the other ethylene receptors. The inability of the ers1 and ein4 mutant transgenes to confer insensitivity suggests that ERS1 and EIN4 signaling is not only independent of RTE1 but independent of an RTE1-like protein. If a protein similar to RTE1 were acting on ERS1 and EIN4, then we should have seen ethylene insensitivity for all the dominant mutations carried by Ters1 and Tein4. The Arabidopsis genome carries a second copy of RTE1, called RTH, which does not appear to play a role in ethylene signaling (M. Rivarola and C. Chang, unpublished data). In plants other than Arabidopsis, it might be possible to determine which ethylene receptors, if any, are dependent on RTE1 orthologs by introducing RTE1-dependent mutations into the receptors and testing them for the ability to confer ethylene insensitivity, as in this paper. These findings have implications for practical applications that involve the engineering of dominant mutations into heterologous ethylene receptor genes for the purpose of generating ethylene insensitivity in plants.
The following materials are available in the online version of this article.
We are grateful to Wuyi Wang and Brad Binder for providing the ERS1 cDNA clone and the Tetr1 (P36L) clone, G. Eric Schaller for providing the ERS1 genomic DNA clone, and Franklin T. Johnson and David Lee for assistance with mutagenesis and transformation of Tein4 (A125T). We thank Zhongchi Liu and Mandy Kendrick for comments on the manuscript. Received March 19, 2009; accepted April 13, 2009; published April 17, 2009.
1 This work was supported by the National Institutes of Health (grant no. 1R01GM071855). C.C. was partially supported by the University of Maryland Agricultural Experiment Station, and M.R. was partially supported by the Bamford Fellowship from the College of Chemical and Life Sciences at the University of Maryland, College Park. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Caren Chang (carenc{at}umd.edu).
[C] Some figures in this article are displayed in color online but in black and white in the print edition.
[W] The online version of this article contains Web-only data. www.plantphysiol.org/cgi/doi/10.1104/pp.109.138461 * Corresponding author; e-mail carenc{at}umd.edu.
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