Xylem-specific Overexpression of the GIBBERELLIN ACID 20 OXIDASE Gene (GA20-OXIDASE) from Pine in Hybrid Poplar (Populus tremula L. × P. alba L.) Revealed Reliable Increase in Growth and Biomass Production Just in a Single-copy-line

Abstract

Overexpression of the GA20-OXIDASE gene under control of the constitutive cauliflower mosaic virus 35S promoter in poplar leads to increased shoot growth and biomass production, however, the trees suffer from unstable shoots and poor root growth. Transgenic hybrid poplar (Populus tremula L. × P. alba L.) plants overexpressing the GA20-OXIDASE gene from pine under control of a poplar-xylem-specific DX15-promoter also revealed a superior effect on growth and biomass production but without changing the overall phenotype. We tested seven DX15::GA20-OXIDASE-transgenic lines for growth and biomass production in the glasshouse in 2017, and repeated the experiment in 2018 with the “best-four” lines. Plants from one DX15::GA20-OXIDASE-transgenic line, N457‑4, turned out to be the tallest, with highest biomass, in both years under investigation. In contrast to the other lines tested in 2017 and 2018 carrying two or more copies of the transgene, N457‑4 carries only one copy. We suggest that transcriptional or post-transcriptional transgene silencing in the two- or more copies carrying lines might be responsible for lower GA20-OXIDASE transgene expression and that the single-copy-line N457‑4 has stable GA20-OXIDASE-gene expression.