Growth of Mixoploid GIBBERELLIC ACID 20 OXIDASE (GA20-OXIDASE) Overexpressing Transgenic Populus

Biomass production is traditionally an important goal in forest tree breeding programs. However, progress in classical tree breeding is limited because of long generation cycles of many forest tree species. Due to this limitation, genetic engineering, and also the quite recently developed genome editing technique, are very attractive tools to improve tree characteristics in a relatively short time. As a proof-of-concept approach, the GIBBERELLIC ACID 20 OXIDASE (GA20-OXIDASE) gene was constitutively overexpressed in poplar leading to significantly higher plants. Unfortunately, deleterious side-effects were also observed, like longer and thinner shoots leading to plants with unstable stems unable to grow without external stabilization. In the literature, polyploidization was reported to increase stem thickness relatively to plant height in several plant species. Therefore, polyploidization of GA20-OXIDASE overexpressing transgenic poplar lines was attempted. Mixoploid (ploidy chimeric: diploid and tetraploid nuclei) plants were obtained and assessed for several years for ploidy status and growth performance (stem height and diameter, bud set) in climate chamber and glasshouse. The results indicate that mixoploid GA20-OXIDASE overexpressing transgenic poplar still reveal increased growth but also showed a lower height-to-stem ratio compared to the diploid GA20-OXIDASE transgenic poplar, enabling the mixoploid transgenic plants to grow without external stabilization.