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J. Biol. Chem., Vol. 283, Issue 40, 26996-27006, October 3, 2008

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Activation of MAPK Kinase 9 Induces Ethylene and Camalexin Biosynthesis and Enhances Sensitivity to Salt Stress in Arabidopsis^*

From the State Key Laboratory of Plant Physiology and Biochemistry, College of Biological Sciences, China Agricultural University, Beijing 100094, China

Mitogen-activated protein kinase (MAPK) cascades play important roles in regulating plant growth, development, and responses to various environmental stimuli. We demonstrate that MKK9, an MKK, is an upstream activator of the MPKs MPK3 and MPK6 both in vitro and in planta. Expression of active MKK9 protein in transgenic plants induces the synthesis of ethylene and camalexin through the activation of the endogenous MPK3 and MPK6 kinases. As a consequence, transcription of multiple genes responsible for ethylene biosynthesis, ethylene responses, and camalexin biosynthesis is coordinately up-regulated. The activation of MKK9 inhibits hypocotyl elongation in the etiolated seedlings. MKK9-mediated effects on hypocotyl elongation were blocked by the ethylene biosynthesis inhibitor, aminoethoxyvinylglycine, and ethylene receptor antagonist, Ag⁺. Expression of active MKK9 protein enhances the sensitivity of transgenic seedlings to salt stress, whereas loss of MKK9 activity reduces salt sensitivity indicating a role for MKK9 in the salt stress response. The results reported here reveal that the MKK9-MPK3/MPK6 cascade participates in the regulation of the biosynthesis of ethylene and camalexin and may be an important axis in the stress responses of Arabidopsis.

Received for publication, February 21, 2008 , and in revised form, July 14, 2008.

^* This work was supported by National Natural Science Foundation of China Grants 30421002 and 30770203, New Century Excellence Talents in University Grant 04-0131 (to D. R.), and National Natural Science Foundation of China Grant 30771124 (to H. Y.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S4.

¹ Both authors contributed equally to this work.

² To whom correspondence should be addressed: 2 West Yuanmingyuan Rd., Haidian District, Beijing 100094, China. Fax: 86-10-62731332; E-mail: ren{at}cau.edu.cn.

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