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Plant Physiology 143:612-616 (2007) © 2007 American Society of Plant Biologists ETHYLENE RESPONSE 1 Histidine Kinase Activity of Arabidopsis Promotes Plant Growth[C],[W]Department of Molecular Biology, Massachusetts General Hospital, Department of Genetics, Harvard Medical School, Boston, Massachusetts 02114
ETHYLENE RESPONSE 1 (ETR1) is one of five receptors for the plant hormone ethylene (Bleecker et al., 1988
Biochemical analyses have demonstrated functional HK activity (Gamble et al., 1998
Thus far, the biological function or an in vivo signaling mechanism for ETR1 HK activity remains unknown. Only recently, the HK activity was shown to be important in the rate of growth recovery after ethylene removal (Binder et al., 2004
To examine ETR1 HK function in vivo, we generated transgenic lines of etr1-7, an ETR1 null mutant, that contained either a genomic transgene of ETR1 (gETR1) or a cDNA of ETR1 (cETR1) under the control of native ETR1 promoter (2.2 kb). We also generated etr1-7 lines that were transformed with the cDNA of ETR1H353Q (cETR1H353Q), in which Gln (Q) replaces the only phosphorylatable His (H) 353 (Moussatche and Klee, 2004
First, we reassessed whether ETR1 HK activity is involved in ethylene signaling by observing etiolated seedlings growing on the Murashige and Skoog media containing 1% (w/v) Suc and 10 µM of 1-aminocylopropane-1-carboxylic acid (ACC), which is the immediate precursor of ethylene. Wild-type, etr1-7, and transgenic gETR1, cETR1H353Q, and cETR1 seedlings displayed a typical triple response: inhibition of hypocotyl and root growth, exaggeration of apical hook formation, and hypocotyl thickening (Fig. 1, C and D). As expected, the etr1-1 and etr1-2 alleles showed strong and weak insensitivity to ACC, respectively (Bleecker et al., 1988
An indifferent response among the transgenic lines at a saturating dose of ACC (10 µM) could be caused by the functional redundancy of receptors in ethylene signaling (Hua and Meyerowitz, 1998
Next, we evaluated the role of ETR1 HK activity in response to ambient and/or endogenous ethylene. Wild-type, transgenic gETR1, and cETR1 etiolated seedlings growing without ACC addition displayed normal hypocotyl elongation (Fig. 1, C and D). However, dark-grown seedlings appear to be sensitive to the ambient/endogenous ethylene, because seedlings exhibited enhanced hypocotyl elongation in the presence of silver. Silver has been shown to competitively inhibit ethylene receptor function (Rodríguez et al., 1999
We note that etr1-7, in comparison to wild type, displayed relatively slow hypocotyl elongation in the absence of silver with statistic confidence (*P < 0.05) and fully enhanced hypocotyl elongation in the presence of silver (Fig. 1, C and D). This observation may implicate a role for ETR1 in plant growth that is independent of ethylene signaling (Hua and Meyerowitz, 1998 Interestingly, transgenic cETR1H353Q seedlings exhibited even slower hypocotyl elongation than etr1-7 without silver and lacked further hypocotyl elongation with silver. This indicates that the growth defect caused by the loss of ETR1 HK activity is not because of the increased ethylene production or ethylene responsiveness. Because plant growth in either the light or darkness is mechanically distinct, we decided to also observe growth of transgenic plants under light. With the exception of cETR1H353Q, all other lines were indistinguishable from wild type under normal growth conditions (the cycle of 16-h light and 8-h dark with the light intensity of 75 µE m2 s1). The cETR1H353Q lines showed a minor reduction in rosette size (Fig. 2, A [top] and B). In growth-promoting light conditions (the light intensity of 250 µE m2 s1), wild-type, etr1-7, and transgenic cETR1 and gETR1 plants exhibited relatively bigger rosette size (Fig. 2, A [bottom] and B). However, transgenic cETR1H353Q plants failed to promote such enhanced rosette growth, indicating that ETR1 HK activity is also involved in growth promotion under light. It will be interesting to further test if the light signaling-implicated GAF domain of ETR1 plays an additional role in the high light-driven growth promotion.
Because ethylene receptors have at least partial functional redundancy, the growth defects of cETR1H353Q suggest that ETR1H353Q has a dominant negative effect on the other ethylene receptors. This may indicate the existence of heteromeric receptor complexes. Currently, a receptor complex containing an ETR1 homodimer and CONSTITUTIVE TRIPLE RESPONSE 1 (CTR1) has been biochemically characterized; however, other partners remain to be identified (Schaller et al., 1995
To gain insight into the molecular mechanism(s) underlying the growth defects associated with the loss of ETR1 HK activity, we monitored gene expression in etiolated seedlings using quantitative reverse transcription-PCR. Typically, a receptor HK is a signal input module of a two-component system (Hwang et al., 2002
The gene expression of the A-type ARR5 is a hallmark of two-component system activity (D'Agostino et al., 2000
We also detected a high level of the B-type ARR1 expression in etr1-1 (see Supplemental Fig. S2). Currently, the function of increased ARR1 gene expression is unknown; however, it may balance the constitutive A-type ARR activity, which plays a repressor role in the two-component system (Hwang et al., 2002
To substantiate the downstream molecular mechanism of ETR1 HK activity, we examined whether the kinase activity could modulate two-component system by taking advantage of the well-established ARR6 promoter fused with luciferase reporter (ARR6-LUC; Hwang and Sheen, 2001
Because ETR1WT was as potent as D659E for reporter induction, the ETR1WT appeared to be activated upon its expression. This is consistent with the previous observation that ETR1 is autophosphorylated without its ligand binding in vitro (Gamble et al., 1998 Here we have shown that ETR1 HK activity triggers the two-component system and promotes plant growth in Arabidopsis. It remains to be determined if the two-component system activated by ETR1 HK activity contributes to the plant growth promotion. It would also be interesting to know if ETR1 HK activity switches on and off in the absence and presence of ethylene, respectively, to coordinate plant growth promotion and inhibition. Further elucidation of molecular mechanisms underlying ETR1 action will improve our understanding of ethylene functions in plant growth and developments.
The following materials are available in the online version of this article.
We thank the late Dr. Tony Bleecker for sharing etr1-7 seeds and his excellent contribution in the ethylene-signaling field. We appreciate Dr. Meyerowitz for generating the etr1-7 line, Ms. Mandy Reading and Dr. Caren Chang for sharing etr1-2 seeds, Dr. Bruno Mueller for sharing the ARR10 construct and helping with the manuscript preparation, and Drs. Michael McManus and Andrew Diener for providing many comments on the manuscript preparation. We also express our sincere gratitude to Dr. Jen Sheen for her support during our research tenure as postdoctoral fellows in Massachusetts General Hospital/Harvard Medical School. Received October 18, 2006; accepted December 13, 2006; published February 7, 2007.
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: Sang-Dong Yoo (yoo{at}molbio.mgh.harvard.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.106.091504 * Corresponding author; e-mail yoo{at}molbio.mgh.harvard.edu; fax 6176433050.
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