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Plant and Cell Physiology Advance Access published online on October 30, 2009
Plant and Cell Physiology, doi:10.1093/pcp/pcp147
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Three SnRK2 Protein Kinases are the Main Positive Regulators of Abscisic Acid Signaling in Response to Water Stress in Arabidopsis

Yasunari Fujita1, Kazuo Nakashima1, Takuya Yoshida1,2, Takeshi Katagiri3, Satoshi Kidokoro1,2, Norihito Kanamori1, Taishi Umezawa3, Miki Fujita3, Kyonoshin Maruyama1, Kanako Ishiyama4, Masatomo Kobayashi4, Shoko Nakasone1,2, Kohji Yamada1,2, Takuya Ito3, Kazuo Shinozaki3 and Kazuko Yamaguchi-Shinozaki1,2

1 Biological Resources Division, Japan International Research Center for Agricultural Sciences (JIRCAS), Tsukuba, Ibaraki 305-8686, Japan
2 Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
3 Plant Science Center, RIKEN Yokohama Institute, Yokohama, Kanagawa 230-0045, Japan
4 Experimental Plant Division, Bioresource Center, RIKEN Tsukuba Institute, Tsukuba, Ibaraki 305-0074, Japan.

Corresponding Author: Kazuko Yamaguchi-Shinozaki Biological Resources Division, Japan International Research Center for Agricultural Sciences (JIRCAS), 1-1 Ohwashi, Tsukuba, Ibaraki, 30 -8686 Japan Tel: +81-29-838-6641 Fax: +81-29-838-6643 E-mail: kazukoys{at}jircas.affrc.go.jp


  Abstract

Responses to water stress are thought to be mediated by transcriptional regulation of gene expression via reversible protein phosphorylation events. Previously, we reported that bZIP-type AREB transcription factors are involved in abscisic acid (ABA) signaling under water-stress conditions in Arabidopsis. The AREB1 protein is phosphorylated in vitro by ABA-activated SNF1-related protein kinase 2s (SnRK2s) such as SRK2D/SnRK2.2, SRK2E/SnRK2.6 and SRK2I/SnRK2.3 (SRK2D/E/I). Consistent with this, we now show that SRK2D/E/I and AREB1 colocalize and interact in nuclei in planta. Our results show that unlike srk2d, srk2e and srk2i single and double mutants, srk2d srk2e srk2i triple mutants exhibit greatly reduced tolerance to drought stress and highly enhanced insensitivity to ABA. Under water-stress conditions, ABA- and water stress-dependent gene expressions, including those of transcription factors, are globally and drastically impaired and jasmonic acid (JA)-responsive and flowering genes are upregulated in srk2d/e/i triple mutants, but not in other single and double mutants. The downregulated genes in srk2d/e/i and areb triple mutants largely overlap in ABA-dependent expression, supporting the view that SRK2D/E/I regulate AREBs in ABA signaling in response to water stress. Almost all dehydration-responsive LEA protein genes and group-A PP2C genes are strongly downregulated in the srk2d/e/i triple mutants. Further, our data show that these group-A PP2Cs, such as HAI1 and ABI1, interact with SRK2D. Together, our results indicate that SRK2D/E/I function as main positive regulators, and suggest that ABA signaling is controlled by the dual modulation of SRK2D/E/I and group-A PP2Cs.

Keywords: ABA signaling - Arabidopsis thaliana - Protein phosphatase 2C - SnRK2 protein kinase - water stress

(Received July 21, 2009; Accepted October 18, 2009)
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