Gallego-Giraldo L, Gisbert C, Garcia-Martinez JL, Moritz T, Lopez-Diaz I
Gibberellin homeostasis in tobacco is regulated by gibberelllin metabolism genes with different gibberellin sensitivity
Plant and Cell Physiology: 2008 49:679-690

Abstract
Gibberellins are phytohormones that regulate growth and developmentof plants. Gibberellin homeostasis is maintained by feedbackregulation of gibberellin metabolism genes. To understand thisregulation, we manipulated the gibberellin pathway in tobaccoand studied its effects on the morphological phenotype, gibberellinlevels and the expression of endogenous gibberellin metabolismgenes. The overexpression of a gibberellin 3-oxidase (biosynthesisgene) in tobacco (3ox-OE) induced slight variations in phenotypeand active GA1 levels, but we also found an increase in GA8levels (GA1 inactivation product) and a conspicuous inductionof gibberellin 2-oxidases (catabolism genes; NtGA2ox3 and -5),suggesting an important role for these particular genes in thecontrol of gibberellin homeostasis. The effect of simultaneousoverexpression of two biosynthesis genes, a gibberellin 3-oxidaseand a gibberellin 20-oxidase (20ox/3ox-OE), on phenotype andgibberellin content suggests that gibberellin 3-oxidases arenon-limiting enzymes in tobacco, even in a 20ox-OE background.Moreover, the expression analysis of gibberellin metabolismgenes 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. Gibberellinbiosynthesis genes (NtGA20ox1 and NtGA3ox1) are negatively feedbackregulated mainly by high gibberellin levels. In contrast, gibberellincatabolism genes which are subject to positive feedback regulationare sensitive to high (NtGA2ox1) or to low (NtGA2ox3 and -5)gibberellin concentrations. These two last GA2ox genes seemto play a predominant role in gibberellin homeostasis undermild gibberellin variations, but not under large gibberellinchanges, where the biosynthesis genes GA20ox and GA3ox may bemore important.

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