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J. Biol. Chem., Vol. 281, Issue 8, 5310-5318, February 24, 2006

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The Regulatory Domain of SRK2E/OST1/SnRK2.6 Interacts with ABI1 and Integrates Abscisic Acid (ABA) and Osmotic Stress Signals Controlling Stomatal Closure in Arabidopsis^*

Riichiro Yoshida, Taishi Umezawa, Tsuyoshi Mizoguchi^¶, Seiji Takahashi¹, Fuminori Takahashi^¶, and Kazuo Shinozaki^¶||2

From the Laboratory of Plant Molecular Biology, RIKEN Tsukuba Institute, 3-1-1, Koyadai, Tsukuba, Ibaraki 305-0074, Japan, Plant Mutation Exploration Team, Plant Functional Genomics Research Group, RIKEN Genomic Sciences Center, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan, ^¶Institute of Biological Sciences, Tsukuba University, Tennodai, Tsukuba, Ibaraki, 305-8572, Japan, and ^||Gene Discovery Research Group, RIKEN Plant Science Center, 1-7-2 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan

ABI1 and ABI2 encode PP2C-type protein phosphatases and are thought to negatively regulate many aspects of abscisic acid (ABA) signaling, including stomatal closure in Arabidopsis. In contrast, SRK2E/OST1/SnRK2.6 encodes an Arabidopsis SnRK2 protein kinase and acts as a positive regulator in the ABA-induced stomatal closure. SRK2E/OST1 is activated by osmotic stress as well as by ABA, but the independence of the two activation processes has not yet been determined. Additionally, interaction between SRK2E/OST1 and PP2C-type phosphatases (ABI1 and ABI2) is not understood. In the present study, we demonstrated that the abi1-1 mutation, but not the abi2-1 mutation, strongly inhibited ABA-dependent SRK2E/OST1 activation. In contrast, osmotic stress activated SRK2E/OST1 even in abi1-1 and aba2-1 plants. The C-terminal regulatory domain of SRK2E/OST1 was required for its activation by both ABA and osmotic stress in Arabidopsis. The C-terminal domain was functionally divided into Domains I and II. Domain II was required only for the ABA-dependent activation of SRK2E/OST1, whereas Domain I was responsible for the ABA-independent activation. Full-length SRK2E/OST1 completely complemented the wilty phenotype of the srk2e mutant, but SRK2E/OST1 lacking Domain II did not. Domain II interacted with the ABI1 protein in a yeast two-hybrid assay. Our results suggested that the direct interaction between SRK2E/OST1 and ABI1 through Domain II plays a critical role in the control of stomatal closure.

Received for publication, September 7, 2005 , and in revised form, December 2, 2005.

^* This work was supported in part by the Program for Promotion of Basic Research Activities for Innovative Biosciences (BRAIN) and grants-in-aid for scientific research from the Ministry of Education, Science, Sports, and Culture of Japan (14740451 and 16780078) (to R. 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.

¹ Present address: Inst. of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.

² To whom correspondence should be addressed. Tel.: 81-29-836-4359; Fax: 81-29-836-9060; E-mail: sinozaki{at}rtc.riken.jp.

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