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Published on January 9, 2008; 10.1104/pp.107.115162

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Received December 19, 2007
Accepted January 3, 2008

Phytochrome- and Gibberellin-Mediated Regulation of Abscisic Acid Metabolism during Germination of Photoblastic Lettuce Seeds

Yoshiaki Sawada , Miki Aoki , Kentaro Nakaminami , Wataru Mitsuhashi , Kiyoshi Tatematsu , Tetsuo Kushiro , Tomokazu Koshiba , Yuji Kamiya , Yasunori Inoue , Eiji Nambara , and Tomonobu Toyomasu *

Course of the Science of Bioresource, The United Graduate School of Agricultural Science, Iwate University, Morioka, Iwate 020-8550, Japan; Department of Bioresource Engineering, Yamagata University, Tsuruoka,Yamagata 997-8555, Japan; RIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, Japan; Department of Biological Sciences, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan; Faculty of Science and Technology, Tokyo University of Sciece, Noda, Chiba 278-8510, Japan

* Corresponding author; email: toyomasu{at}tds1.tr.yamagata-u.ac.jp.

Germination of lettuce (Lactuca sativa L. cv. Grand Rapids) seeds is regulated by phytochrome. The action of phytochrome includes alterations in the levels of gibberellins (GAs) 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 to LsNCED4) and four cDNAs for ABA 8'-hydroxylase (catabolism; LsABA8ox1 to 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 PCR 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.




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