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DEVELOPMENT AND HORMONE ACTION

Gibberellin Biosynthesis in Developing Pumpkin Seedlings^1,2

Institut für Pflanzenbiologie der Technischen Universität Braunschweig, D–38106 Braunschweig, Germany

A gibberellin (GA) biosynthetic pathway was discovered operating in root tips of 7-d-old pumpkin (Cucurbita maxima) seedlings. Stepwise analysis of GA metabolism in cell-free systems revealed the conversion of GA₁₂-aldehyde to bioactive GA₄ and inactive GA₃₄. Highest levels of endogenous GA₄ and GA₃₄ were found in hypocotyls and root tips of 3-d-old seedlings. cDNA molecules encoding two GA oxidases, CmGA20ox3 and CmGA3ox3, were isolated from root tips of 7-d-old LAB150978-treated seedlings. Recombinant CmGA20ox3 fusion protein converted GA₁₂ to GA₉, GA₂₄ to GA₉, GA₁₄ to GA₄, and, less efficiently, GA₅₃ to GA₂₀, and recombinant CmGA3ox3 protein oxidized GA₉ to GA₄. Transcript profiles were determined for four GA oxidase genes from pumpkin revealing relatively high transcript levels for CmGA7ox in shoot tips and cotyledons, for CmGA20ox3 in shoot tips and hypocotyls, and for CmGA3ox3 in hypocotyls and roots of 3-d-old seedlings. Transcripts of CmGA2ox1 were mainly found in roots of 7-d-old seedlings. In roots of 7-d-old seedlings, transcripts of CmGA7ox, CmGA20ox3, and CmGA3ox3 were localized in the cap and the rhizodermis by in situ hybridization. We conclude that hypocotyls and root tips are important sites of GA biosynthesis in the developing pumpkin seedling.

² This paper is dedicated to Jan E. Graebe on the occasion of his 75th birthday.

³ Present address: Schleicher & Schuell BioScience GmbH, Postfach 1160, D–37582 Dassel, Germany.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.064162.

^* Corresponding author; e-mail theo.lange{at}tu-bs.de; fax 49–531–3918180.

Received April 25, 2005; returned for revision June 13, 2005; accepted July 10, 2005.

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