Ectopic expression of pumpkin gibberellin oxidases alters gibberellin biosynthesis and development of transgenic Arabidopsis plants

Abstract

Immature pumpkin (Cucurbita maxima) seeds contain gibberellin (GA) oxidases with unique catalytic properties resulting in GAs of unknown function for plant growth and development. Overexpression of pumpkin GA 7-oxidase (CmGA7ox) in Arabidopsis (Arabidopsis thaliana) resulted in seedlings with elongated roots, taller plants that flower earlier with only a little increase in bioactive GA4 levels compared to control plants. In the same way, overexpression of the pumpkin GA 3-oxidase1 (CmGA3ox1) resulted in a GA overdose phenotype with increased levels of endogenous GA4. This indicates that, in Arabidopsis, 7-oxidation and 3-oxidation are rate-limiting steps in GA plant hormone biosynthesis that control plant development. With an opposite effect, overexpression of pumpkin seed-specific GA 20-oxidase1 (CmGA20ox1) in Arabidopsis resulted in dwarfed plants that flower late with reduced levels of GA4 and increased levels of physiological inactive GA17 and GA25 and unexpected GA34 levels. Severe dwarfed plants were obtained by overexpression of the pumpkin GA 2-oxidase1 (CmGA2ox1) in Arabidopsis. This dramatic change in phenotype was accompanied by a considerable decrease in the levels of bioactive GA4 and an increase in the corresponding inactivation product GA34 in comparison to control plants. In this study, we demonstrate the potential of four pumpkin GA oxidase-encoding genes to modulate the GA plant hormone pool and alter plant stature and development.

Figure 1.

The third part of the GA biosynthetic pathway leading to the formation of bioactive GAs in Arabidopsis. Biosynthetic steps resulting from the overexpression of pumpkin GA oxidases. a, 7-Oxidase (CmGA7ox). b, Seed-specific 20-oxidase1 (CmGA20ox1). c, Seed-specific 3-oxidase1 (CmGA3ox1). d, 2-Oxidase1 (CmGA2ox1). Main pathways are indicated by thick arrows. The boxed GAs indicate bioactive GAs. Metabolic relationships are discussed in the text.

Figure 2.

Overexpression of pumpkin GA oxidases in Arabidopsis: effects on plant development. A, Phenotypes of 14-d-old seedlings grown in Murashige and Skoog medium. Wild-type seedlings (left) compared to seedlings expressing S copies of CmGA7ox (line S12.8, middle) or expressing S copies of CmGA3ox1 (line S17.7, right). Bar = 1 cm. B, Phenotypes of 7-week-old plants transferred to soil after 28 d in Murashige and Skoog medium. Nontransformed plants (wild type, left) compared to AS and S lines transformed with CmGA7ox (middle) or CmGA3ox1 (right). C, Phenotypes of 7-week-old plants transferred to soil after 28 d in Murashige and Skoog medium containing 10⁻⁶ m GA₃. Nontransformed plants (wild type, left) compared to different AS and S lines transformed with CmGA20ox1 (middle) or CmGA2ox1 (right).