Overexpression of Arabidopsis thaliana Gibberellin 2-Oxidase 2 Gene (AtGA2ox2) Leads to Dwarfism in Populus × berolinensis: An In Vitro Study

This study describes the creation of transgenic Berlin poplar (Populus × berolinensis K. Koch) expressing the Arabidopsis thaliana AtGA2ox2 gene, which encodes gibberellin 2-oxidase, and provides an initial evaluation of the resulting phenotypic effects. Genetic transformation of poplar was performed via an Agrobacterium-mediated technique using the pBI121 plasmid, where the AtGA2ox2 gene was placed under the control of the constitutive promoter of the cauliflower mosaic virus 35S RNA. The constitutive expression of AtGA2ox2 resulted in a distinct dwarf phenotype of poplar cultured in vitro. Compared to control plants, this transgenic poplar exhibited extremely reduced internode length and altered leaves morphology with a shape of inverted boat or spoon-shaped similar to those forming after satsuma dwarf virus infection in citrus trees. This dwarf phenotype precludes conventional microclonal vegetative propagation via apical and lateral buds. Furthermore, micropropagation via morphogenesis induction from callus is hindered by an altered hormonal balance within the transgenic cells, which exhibit low morphogenic capacity and tend to remain in a callus state. Regenerants derived from callus culture are difficult to root, and those that do root often fail to survive. The few successfully rooted plants exhibit extremely slow growth, with biomass accumulation restricted primarily to the formation of new rosette leaves. Furthermore, transferring these plantlets to ex vitro conditions is challenging due to the inherent fragility of both the shoots and the root systems. Consequently, the transgenic line is currently maintained in vitro as a callus culture and transferring of regenerated plants to ex vitro conditions requires further optimization to address their slow growth and poor vigor. Finally, while exogenous application of gibberellic acid to the shoot apices restores the wild-type phenotype, this effect is only transient.