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Published on December 27, 2007; 10.1104/pp.107.113738

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Received November 27, 2007
Accepted December 18, 2007

High temperature-induced abscisic acid biosynthesis and its role in the inhibition of gibberellin action in Arabidopsis seeds

Shigeo Toh , Akane Imamura , Asuka Watanabe , Kazumi Nakabayashi , Masanori Okamoto , Yusuke Jikumaru , Atsushi Hanada , Yukie Aso , Kanako Ishiyama , Noriko Tamura , Satoshi Iuchi , Masatomo Kobayashi , Shinjiro Yamaguchi , Yuji Kamiya , Eiji Nambara , and Naoto Kawakami *

Department of Life Sciences, School of Agriculture, Meiji University, Kawasaki, Kanagawa 214-8571, Japan; Growth Regulation Research Group, RIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, Japan; Experimental Plant Division, RIKEN Bioresource Center, Tsukuba, Ibaraki 305-0074, Japan

* Corresponding author; email: kawakami{at}isc.meiji.ac.jp.

Suppression of seed germination at supraoptimal high temperature (thermoinhibiton) during summer is crucial for Arabidopsis (Arabidopsis thaliana (L.) Heynh.) to establish vegetative and reproductive growth in appropriate seasons. Abscisic acid (ABA) and gibberellins (GAs) are well known to be involved in germination control, but it remains unknown how these hormone actions (metabolism and responsiveness) are altered at high temperature. Here we show that ABA levels in imbibed seeds are elevated at high temperature and that this increase is correlated with up-regulation of the zeaxanthin epoxidase gene, ABA1/ZEP and three 9-cis-epoxycarotenoid dioxygenase genes, NCED2, NCED5 and NCED9. Reverse genetic studies show that NCED9 plays a major, and NCED5 and NCED2 play relatively minor roles in high temperature-induced ABA synthesis and germination inhibition. We also show that bioactive GAs stay at low levels at high temperature, presumably through suppression of GA 20-oxidase genes, GA20ox1, GA20ox2, GA20ox3, and GA 3-oxidase genes, GA3ox1 and GA3ox2. Thermoinhibition tolerant germination of loss-of-function mutants of GA negative regulators, SPINDLY (SPY) and RGL2, suggests that repression of GA signaling is required for thermoinibition. Interestingly, ABA-deficient aba2-2 mutant seeds show significant expression of GA synthesis genes and repression of SPY expression even at high temperature. In addition, thermoinhibition resistant germination phenotype of aba2-1 seeds is suppressed by a GA biosynthesis inhibitor, paclobutrazol. We conclude that high temperature stimulates ABA synthesis, and represses GA synthesis and signaling through the action of ABA in Arabidopsis seeds.




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