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DEVELOPMENT AND HORMONE ACTION

High Humidity Induces Abscisic Acid 8'-Hydroxylase in Stomata and Vasculature to Regulate Local and Systemic Abscisic Acid Responses in Arabidopsis^1,[OA]

RIKEN Plant Science Center, Yokohama, Kanagawa 230–0045, Japan (M.O., Y.T., Y.K., M.S., E.N.); Department of Cell and Systems Biology, University of Toronto, Toronto, Ontario M5S 3B2, Canada (E.N.); The Centre for the Analysis of Genome Evolution and Function, University of Toronto, Toronto, Ontario M5S 3B2, Canada (E.N.); and Plant Biotechnology Institute, National Research Council of Canada, Saskatoon, Saskatchewan S7N 0W9, Canada (S.R.A.)

Levels of endogenous abscisic acid (ABA) are changed dynamically in response to environmental conditions. The ABA 8'-hydroxylase is a key enzyme in ABA catabolism and is encoded by CYP707A genes. In this study, we examined physiological roles of Arabidopsis (Arabidopsis thaliana) CYP707As in the plant's response to changes in humidity. The cyp707a1 and cyp707a3 mutants displayed lower stomatal conductance under turgid conditions (relative humidity 60%) than the wild type. When wild-type plants were transferred to high-humidity conditions (relative humidity 90%), CYP707A1 and CYP707A3 transcript levels increased, followed by the reduction of ABA levels. The cyp707a3 mutant exhibited high ABA levels even after transferring to high-humidity conditions, whereas, under similar conditions, the cyp707a1 mutant exhibited low ABA levels comparable to the wild type. Analysis of spatial expression patterns by using transgenic plants harboring a promoter::β-glucuronidase gene indicated that high-humidity-induced expression of CYP707A1 and CYP707A3 occurred primarily in guard cells and vascular tissues, respectively. Furthermore, stomatal closure of the cyp707a1 mutant, but not cyp707a3 mutant, was ABA hypersensitive when epidermal peel was treated with exogenous ABA, suggesting that CYP707A1 is essential for ABA catabolism inside the guard cells. These results implicate that CYP707A3 reduces the amount of mobile ABA in vascular tissues in response to high humidity, whereas CYP707A1 inactivates local ABA pools inside the guard cells. Taken together, ABA catabolism in both vascular tissues and guard cells participates in the systemic ABA action that controls stomatal movement in response to high humidity.

The author responsible for the 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: Eiji Nambara (eiji.nambara{at}utoronto.ca).

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.130823

^* Corresponding author; e-mail eiji.nambara{at}utoronto.ca.

Received October 3, 2008; accepted November 23, 2008; published November 26, 2008.

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