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JXB Advance Access published online on July 24, 2008
Journal of Experimental Botany, doi:10.1093/jxb/ern192
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© 2008 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)

RESEARCH PAPER

Germination of photoblastic lettuce seeds is regulated via the control of endogenous physiologically active gibberellin content, rather than of gibberellin responsiveness

Yoshiaki Sawada1,2, Takumi Katsumata3, Jun Kitamura4, Hiroshi Kawaide3, Masatoshi Nakajima4, Tadao Asami4, Kentaro Nakaminami1,2 *, Toshihiro Kurahashi2 {dagger}, Wataru Mitsuhashi1,2, Yasunori Inoue5 and Tomonobu Toyomasu1,2,6,{ddagger}

1Course of the Science of Bioresource, The United Graduate School of Agricultural Science, Iwate University, Morioka, Iwate 020-8550, Japan
2Department of Bioresource Engineering, Yamagata University, Tsuruoka, Yamagata 997-8555, Japan
3Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, 183-8509, Japan
4Department of Applied Biological Chemistry, University of Tokyo, Bunkyo-ku, Tokyo, 113-8657, Japan
5Faculty of Science and Technology, Tokyo University of Sciece, Noda, Chiba 278-8510, Japan
6Department of Agriculture, Yamagata University, Tsuruoka, 997-8555, Japan

{ddagger} To whom correspondence should be addressed. E-mail: toyomasu{at}tds1.tr.yamagata-u.ac.jp

Phytochrome regulates lettuce (Lactuca sativa L. cv. Grand Rapids) seed germination via the control of the endogenous level of bioactive gibberellin (GA). In addition to the previously identified LsGA20ox1, LsGA20ox2, LsGA3ox1, LsGA3ox2, LsGA2ox1, and LsGA2ox2, five cDNAs were isolated from lettuce seeds: LsCPS, LsKS, LsKO1, LsKO2, and LsKAO. Using an Escherichia coli expression system and functional assays, it is shown that LsCPS and LsKS encode ent-copalyl diphosphate synthase and ent-kaurene synthase, respectively. Using a Pichia pastoris system, it was found that LsKO1 and LsKO2 encode ent-kaurene oxidases and LsKAO encodes ent-kaurenoic acid oxidase. A comprehensive expression analysis of GA metabolism genes using the quantitative reverse transcription polymerase chain reaction suggested that transcripts of LsGA3ox1 and LsGA3ox2, both of which encode GA 3-oxidase for GA activation, were primarily expressed in the hypocotyl end of lettuce seeds, were expressed at much lower levels than the other genes tested, and were potently up-regulated by phytochrome. Furthermore, LsDELLA1 and LsDELLA2 cDNAs that encode DELLA proteins, which act as negative regulators in the GA signalling pathway, were isolated from lettuce seeds. The transcript levels of these two genes were little affected by light. Lettuce seeds in which de novo GA biosynthesis was suppressed responded almost identically to exogenously applied GA, irrespective of the light conditions, suggesting that GA responsiveness is not significantly affected by light in lettuce seeds. It is proposed that lettuce seed germination is regulated mainly via the control of the endogenous content of bioactive GA, rather than the control of GA responsiveness.

Key words: Germination, gibberellin metabolism, gibberellin signalling, lettuce

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

{dagger} Present address: National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892, USA.

Received 30 March 2008; Revised 4 June 2008 Accepted 24 June 2008


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