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Plant and Cell Physiology Advance Access published online on March 11, 2008
Plant and Cell Physiology, 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 e-mail: journals.permissions@oxfordjournals.org

GIBBERELLIN HOMEOSTASIS IN TOBACCO IS REGULATED BY GA METABOLISM GENES WITH DIFFERENT GA SENSITIVITY

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

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

Corresponding author: Dr. Isabel López-Díaz, Instituto de Biología Molecular y Celular de Plantas, CSIC, Universidad Politecnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain, telephone: + 34 963877866; fax: + 34 963877859. E-mail: ilopez{at}ibmcp.upv.es


  Abstract

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

Present address of Carmina Gisbert is 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

Present address of Susana Ubeda-Tomás is Centre for Plant Integrative Biology (CPIB) School of Biosciences. University of Nottingham. Loughborough Leics LE125RD. UK

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