Vegetative and generative dispersal capacity of field released transgenic aspen trees

Transfer of genes by pollen or wind-dispersed seed is considered a main potential risk when field release experiments with transgenic trees are initiated. In Germany, the first release experiment with genetically transformed trees was initiated in 1996. To ensure that the transgenic trees remained in the vegetative phase, the duration of the experiment was limited to 5 years. In total, 457 1-year-old trees including eight transgenic aspen lines carrying either the 35S-rolC or the rbcS-rolC gene construct, and three control clones were transferred to the field. In 1998 and 2000, 12 plants of transgenic lines all carrying the 35S-rolC gene construct formed female flower buds. Furthermore, one young aspen plant identified as a root sucker was observed in 1999 followed by an increasing number of root suckers derived from transgenic and non-transgenic trees in 2000 and 2001. In 2001, the last year of the field trial, 15 root suckers were detected outside the field. In total, 234 root suckers were harvested in 2000 and 2001 and analysed for their transgenic status. More than half of the roots suckers investigated showed the presence of the rbcS-rolC gene construct. We concluded that in addition to the widely accepted generative propagation, vegetative dispersal capacity of transgenic perennial plants is also important and must be included in risk assessment studies.