Graduation date: 2006We isolated and characterized the expression of two genes from Populus trichocarpa that are homologous to the TERMINAL FLOWER 1 (TFL1) gene from the model annual plant Arabidopsis. In Arabidopsis, overexpression of the TFL1 gene extends the vegetative growth phase, and the homozygous mutant tfl1 allele causes early flowering and formation of a terminal flower. Overexpression of another TFL1 family member, FLOWERING LOCUS T (FT), inhibits the action of TFL1. The two homologs studied, poplar CENTRORADIALIS LIKE-1 (PtCENL-1) and poplar MOTHER OF FT AND TFL1 (PtMFT), encode proteins that are 52% identical to one another; and 72% and 50% identical to TFL1, respectively. Real-time RT-PCR studies revealed that PtCENL-1 was expressed in all stages of development studied, and was most strongly expressed in vegetative buds and shoot apices. PtMFT was expressed preferentially in inflorescence buds. Expression patterns suggest that PtCENL-1 promotes maintenance of the vegetative growth phase, and that PtMFT promotes the onset of flowering. We tested these hypotheses by overexpression of the PtCENL-1/PtMFT under the control of the CaMV 35S promoter (poplar and Arabidopsis), and via suppression of the endogenous genes via RNA interference (RNAi: poplar only). Some PtCENL-1 RNAi trees flowered during the second growing season in the field, several years earlier than expected. Floral buds were detected in four independent gene insertion events; in two of the four events, floral buds expanded into mature-appearing female catkins and dehisced, though seeds were not formed. All four events had native PtCENL-1 transcript levels lower than in non-flowering events, and below 50% of the level detected in non-transgenic poplar. These early flowering events had normal budflush, however, 35S::PtCENL-1 transcript levels were strongly and positively correlated with date of budflush. These results suggest that endogenous PtCENL-1 is a natural inhibitor of the onset of flowering, and may also retard release from vegetative dormancy. Correspondingly, ectopic expression of PtCENL-1 in Arabidopsis had delayed flowering. No phenotypic differences were observed in PtMFT overexpressing or RNAi transgenic trees. However, ectopic expression of PtMFT in Arabidopsis caused early flowering. Suppression of PtCENL-1 might be useful for inducing early flowering in Populus.