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DEVELOPMENT AND HORMONE ACTION

Phytochrome- and Gibberellin-Mediated Regulation of Abscisic Acid Metabolism during Germination of Photoblastic Lettuce Seeds^1,[OA]

Course of the Science of Bioresource, United Graduate School of Agricultural Science, Iwate University, Morioka, Iwate 020–8550, Japan (Y.S., K.N., W.M., T.T.); Department of Bioresource Engineering, Yamagata University, Tsuruoka, Yamagata 997–8555, Japan (M.A., W.M., T.T.); RIKEN Plant Science Center, Yokohama, Kanagawa 230–0045, Japan (K.T., T. Kushiro, Y.K., E.N.); Department of Biological Sciences, Tokyo Metropolitan University, Hachioji, Tokyo 192–0397, Japan (T. Koshiba); and Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278–8510, Japan (Y.I.)

Germination of lettuce (Lactuca sativa) ‘Grand Rapids’ seeds is regulated by phytochrome. The action of phytochrome includes alterations in the levels of gibberellin (GA) and abscisic acid (ABA). To determine the molecular mechanism of phytochrome regulation of ABA metabolism, we isolated four lettuce cDNAs encoding 9-cis-epoxycarotenoid dioxygenase (biosynthesis; LsNCED1–LsNCED4) and four cDNAs for ABA 8'-hydroxylase (catabolism; LsABA8ox1–LsABA8ox4). Measurements of ABA and its catabolites showed that a decrease in ABA level coincided with a slight increase in the level of the ABA catabolite phaseic acid after red light treatment. Quantitative reverse transcription-polymerase chain reaction analysis indicated that ABA levels are controlled by phytochrome through down-regulation of LsNCED2 and LsNCED4 expression and up-regulation of LsABA8ox4 expression in lettuce seeds. Furthermore, the expression levels of LsNCED4 decreased after GA₁ treatment, whereas the levels of expression of the other two genes were unaffected. The LsNCED4 expression was also down-regulated by red light in lettuce seeds in which GA biosynthesis was suppressed by AMO-1618, a specific GA biosynthesis inhibitor. These results indicate that phytochrome regulation of ABA metabolism is mediated by both GA-dependent and -independent mechanisms. Spatial analysis showed that after red light treatment, the ABA decrease on the hypocotyl side was greater than that on the cotyledon side of lettuce seeds. Moreover, phytochrome-regulated expression of ABA and GA biosynthesis genes was observed on the hypocotyl side, rather than the cotyledon side, suggesting that this regulation occurs near the photoperceptive site.

² Present address: Division of Plant and Soil Sciences, Davis College of Agriculture, West Virginia University, Morgantown, WV 26506–6108.

³ Present address: Faculty of Pharmaceutical Sciences, University of Tokyo, Bunkyo-ku, Tokyo 113–0033, Japan.

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: Tomonobu Toyomasu (toyomasu{at}tds1.tr.yamagata-u.ac.jp).

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.115162

^* Corresponding author; e-mail toyomasu{at}tds1.tr.yamagata-u.ac.jp.

Received December 19, 2007; accepted January 3, 2008; published January 9, 2008.

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