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CELL BIOLOGY AND SIGNAL TRANSDUCTION

ABA-Hypersensitive Germination3 Encodes a Protein Phosphatase 2C (AtPP2CA) That Strongly Regulates Abscisic Acid Signaling during Germination among Arabidopsis Protein Phosphatase 2Cs^1,[W]

International Graduate School of Arts and Sciences, Yokohama City University, Tsurumi, Yokohama 230–0045, Japan (T.Y., N.N., T.H.); Plant Molecular Biology, The Institute of Physical and Chemical Research (RIKEN) Tsukuba Institute, Tsukuba, Ibaraki 305–0074, Japan (N.N., T.I., K.S., T.H.); Laboratory of Cellular Biochemistry, RIKEN Wako Institute, Wako, Saitama 351–0198, Japan (N.K., T.A.); and Plant Functional Genomics Research Group, Genomic Sciences Center, RIKEN Yokohama Institute, Tsurumi, Yokohama 230–0045, Japan (T.K., K.S., T.H.)

The phytohormone abscisic acid (ABA) regulates physiologically important developmental processes and stress responses. Previously, we reported on Arabidopsis (Arabidopsis thaliana) L. Heynh. ahg mutants, which are hypersensitive to ABA during germination and early growth. Among them, ABA-hypersensitive germination3 (ahg3) showed the strongest ABA hypersensitivity. In this study, we found that the AHG3 gene is identical to AtPP2CA, which encodes a protein phosphatase 2C (PP2C). Although AtPP2CA has been reported to be involved in the ABA response on the basis of results obtained by reverse-genetics approaches, its physiological relevance in the ABA response has not been clarified yet. We demonstrate in vitro and in vivo that the ahg3-1 missense mutation causes the loss of PP2C activity, providing concrete confirmation that this PP2C functions as a negative regulator in ABA signaling. Furthermore, we compared the effects of disruption mutations of eight structurally related PP2C genes of Arabidopsis, including ABI1, ABI2, HAB1, and HAB2, and found that the disruptant mutant of AHG3/AtPP2CA had the strongest ABA hypersensitivity during germination, but it did not display any significant phenotypes in adult plants. Northern-blot analysis clearly showed that AHG3/AtPP2CA is the most active among those PP2C genes in seeds. These results suggest that AHG3/AtPP2CA plays a major role among PP2Cs in the ABA response in seeds and that the functions of those PP2Cs overlap, but their unique tissue- or development-specific expression confers distinct and indispensable physiological functions in the ABA response.

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: Takashi Hirayama (hirayama{at}gsc.riken.jp).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.070128.

^* Corresponding author; e-mail hirayama{at}gsc.riken.jp; fax 81–45–508–7363.

Received August 24, 2005; returned for revision October 14, 2005; accepted October 23, 2005.

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