Sequencing of the Populus trichocarpa genome creates an opportunity to describe gene networks involved in Pi acquisition and homeostasis of a woody perennial species able to form mycorrhizal symbioses. Members of the PHT1 gene family play a key role in high affinity inorganic phosphate (Pi) transport. Here, we describe features of the 12 genes coding for the PHT1 transporters identified in the genome of P. trichocarpa, their evolutionary history and expression during major developmental stages (endo- and ectomycorrhizal symbiosis,senescence) and growth on Pi-depleted medium. Pi starvation triggered the upregulation of most members of the Pht1 family; PtrPht1;9 and PtrPht1;11 showed the highest upregulation. At least five PHT1 transporters were upregulated in senescing leaves. PtrPht1;9 and PtrPht1;12 showed a striking upregulation in ectomycorrhizas, whereas PtrPht1;10 was the only transcripts induced in endomycorrhizal roots. The present data suggest that individual PHT1 transporters may play a specific role in acquiring and translocating Pi in the different poplar tissues. This study provides a better understanding of the role this family plays in Pi uptake and partitioning in a perennial woody species as well as its evolutionary development. Poplar has developed adaptative mechanisms to cope with limiting concentrations of Pi available in forest soils through increased Pi uptake and fine tuning of the differential expression of the PHT1 family members in symbiosis.