Influence of over-expression of the FLOWERING PROMOTING FACTOR 1 gene (FPF1) from Arabidopsis on wood formation in hybrid poplar (Populus tremula L. x P. tremuloides Michx.)

Constitutive expression of the FPF1 gene in hybrid aspen (Populus tremula L. x P. tremuloides Michx.) showed a strong effect on wood formation but no effect on flowering time. Gene expression studies showed that activity of flowering time genes PtFT1, PtCO2, and PtFUL was not increased in FPF1 transgenic plants. However, the SOC1/TM3 class gene PTM5, which has been related to wood formation and flowering time, showed a strong activity in stems of all transgenic lines studied. Wood density was lower in transgenic plants, despite significantly reduced vessel frequency which was overcompensated by thinner fibre cell walls. Chemical screening of the wood by pyrolysis GC/MS showed that FPF1 transgenics have higher fractions of cellulose and glucomannan products as well as lower lignin content. The latter observation was confirmed by UV microspectrophotometry on a cellular level. Topochemical lignin distribution revealed a slower increase of lignin incorporation in the developing xylem of the transgenics when compared with the wild-type plants. In line with the reduced wood density, micromechanical wood properties such as stiffness and ultimate stress were also significantly reduced in all transgenic lines. Thus, we provide evidence that FPF1 class genes may play a regulatory role in both wood formation and flowering in poplar.