MADS-box genes have been shown to be important to flower and vegetative tissue development, senescence and winter dormancy in many plant species. Heterologous overexpression of known MADS-box genes has also been used for unravelling gene regulation mechanisms in forest tree species. The constitutive expression of the BpMADS4 gene from birch in poplar, known to induce early flowering in birch and apple, induced broad changes in senescence and winter dormancy but no early flowering. Other analyses revealed that 35S::BpMADS4 poplars maintained photosynthetic activity, chlorophyll and proteins in leaves under winter conditions. BpMADS4 may be influencing transcription factors regulating the senescence and dormancy process due to homology with poplar proteins related to both traits. Little is known of the regulatory genes that co-ordinate senescence, dormancy, chlorophyll/ protein degradation, and photosynthesis at the molecular level. Dissecting the molecular characteristics of senescence regulation will probably involve the understanding of multiple and novel regulatory pathways. The results presented here open new horizons for the identification of regulatory mechanisms related to dormancy and senescence in poplar and other temperate tree species. They confirm recent reports of common signalling intermediates between flowering time and growth cessation in trees (Böhlenius et al. in Science 312:1040-1043, 2006 and additionally indicate similar connections between flowering time signals and senescence.

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.

A major concern over the use of transgenic trees is the potential for transgene dispersal through pollen and seeds. The incorporation of sterility inducing genes into transgenic lines of trees has been proposed to reduce or even avoid gene flow of transgenes into non-transgenic interbreeding species. The evaluation of strategies for the induction of sterility in transgenic forest tree species has been hindered by their long vegetative phases. In this study an early flowering 35S::Leafy poplar line was used for the faster evaluation of the sterility construct C-GPDHC::Vst1. The combination of two transgenic approaches, one to induce early flowering and a second for the induction of sterility, allowed evaluation of this sterility strategy two years after transformation. This is a very short period of time considering the long vegetative period of seven to twenty years common in forest tree species. This approach opens opportunities for the assessment of sterility mechanisms for this plant group.

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