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Plant and Cell Physiology Advance Access originally published online on March 11, 2008
Plant and Cell Physiology 2008 49(5):679-690; doi:10.1093/pcp/pcn042
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Gibberellin Homeostasis in Tobacco is Regulated by Gibberellin Metabolism Genes with Different Gibberellin Sensitivity

Lina Gallego-Giraldo1, Susana Ubeda-Tomás1,3, Carmina Gisbert1,4, José L. García-Martínez1, Thomas Moritz2 and Isabel López-Díaz1,*

1Instituto de Biología Molecular y Celular de Plantas, Universidad Politécnica de Valencia-CSIC, Avda. de los Naranjos s/n, 46022 Valencia, Spain
2Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Science, S-90183 Umeå, Sweden

*Corresponding author: E-mail, ilopez{at}ibmcp.upv.es; Fax, +34-963877859.


  Abstract

Gibberellins are phytohormones that regulate growth and development of plants. Gibberellin homeostasis is maintained by feedback regulation of gibberellin metabolism genes. To understand this regulation, we manipulated the gibberellin pathway in tobacco and studied its effects on the morphological phenotype, gibberellin levels and the expression of endogenous gibberellin metabolism genes. The overexpression of a gibberellin 3-oxidase (biosynthesis gene) in tobacco (3ox-OE) induced slight variations in phenotype and active GA1 levels, but we also found an increase in GA8 levels (GA1 inactivation product) and a conspicuous induction of gibberellin 2-oxidases (catabolism genes; NtGA2ox3 and -5), suggesting an important role for these particular genes in the control of gibberellin homeostasis. The effect of simultaneous overexpression of two biosynthesis genes, a gibberellin 3-oxidase and a gibberellin 20-oxidase (20ox/3ox-OE), on phenotype and gibberellin content suggests that gibberellin 3-oxidases are non-limiting enzymes in tobacco, even in a 20ox-OE background. Moreover, the expression analysis of gibberellin metabolism genes in transgenic plants (3ox-OE, 20ox-OE and hybrid 3ox/20ox-OE), and in response to application of different GA1 concentrations, showed genes with different gibberellin sensitivity. Gibberellin biosynthesis genes (NtGA20ox1 and NtGA3ox1) are negatively feedback regulated mainly by high gibberellin levels. In contrast, gibberellin catabolism genes which are subject to positive feedback regulation are sensitive to high (NtGA2ox1) or to low (NtGA2ox3 and -5) gibberellin concentrations. These two last GA2ox genes seem to play a predominant role in gibberellin homeostasis under mild gibberellin variations, but not under large gibberellin changes, where the biosynthesis genes GA20ox and GA3ox may be more important.

Keywords: Feed-back - Gibberellin - Homeostasis - Over-expression - Regulation

Abbreviations: GA2ox, gibberellin 2-oxidase; GA3ox, gibberellin 3-oxidase; GA20ox, gibberellin 20-oxidase; 3ox-OE, gibberellin 3-oxidase overexpressor; 20ox-OE, gibberellin 20-oxidase overexpressor; RT–PCR, reverse transcription–PCR.

3Present address: Centre for Plant Integrative Biology (CPIB), School of Biosciences. University of Nottingham, Loughborough, Leics LE12 5RD, UK.

4Present address: Instituto de Conservación y Mejora de la Agrodiversidad Valenciana (COMAV), Universidad Politécnica de Valencia, CPI E-I8.Camino de Vera s/n, 46022 Valencia, Spain.

(Received December 14, 2007; Accepted March 6, 2008)
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