Old methods rediscovered: application and improvement of two direct transformation methods to hybrid poplar (Populus tremula × P. alba)

Two direct DNA transfer methods, biolistic transformation and a protoplast transformation approach using the INRA-clone 717 1B4 (Populus tremula × P. alba), are applied to poplars and compared. Both the in vitro culture and the transformation parameters were optimized to receive a maximum quantity of transformed cells to achieve a stable transformation. For the first time, the stable integration of gfp and dsred in the poplar genome and their expression as visual reporter genes in regenerated plantlets can be shown. For biolistic transformation, stem segments cut lengthwise and incubated for 10 days on a callus induction medium revealed the highest number of transient Gfp- and dsRed signals. After optimization of the in vitro culture parameter, Gfp and dsRed-expressing transgenic poplars were regenerated, proven by PCR and Southern blot analysis. For protoplast transformation, the focus was initially on the development of a highly efficient protoplast isolation and plant regeneration system. Using an enzyme solution consisting of 1.0% cellulase R10 and 0.24% macerozyme, 1 × 107 protoplasts were obtained from 1 g fresh weight leaves. Following incubation of the protoplasts in 600 mOsm culture medium, a high number of microcalli were obtained, from which plantlets were regenerated. The parameters for isolation and regeneration were then complemented by an efficient protoplast transformation protocol with 40% PEG1500. The results of this study confirm that both the biolistic and the protoplast transformation methods can be considered suitable for transferring cisgenes directly into poplar.