Functional analysis and cell-specific expression of a phosphate transporter from tomato

For a better understanding of the molecular and biochemical processes involved in orthophosphate (Pi) uptake at the root/soil interface, we cloned a Pi-transporter cDNA (LePT1) from a root air-specific cDNA library of tomato (Lycopersicon esculentum Mill.). The corresponding protein belongs to the growing family of ion transporters with twelve putative transmembrane domains. It is highly homologous to recently isolated Pi transporters from higher plants, yeast and fungi. When expressed in a Pi-uptake-deficient yeast mutant, the L. esculentum phosphate transporter 1 (LePT1) protein exhibits an apparent K-m of 31 mu M. The transporter is still active at submicromolar Pi concentrations and mediates highest Pi uptake at pH 5. The activity of LePT1 is dependent on the electrochemical membrane potential mediated by the yeast P-type H+-ATPase. Transcript levels of LePT1 in tomato seedlings are detectable in all vegetative organs under Pi-sufficient conditions: with highest concentrations in root hairs. In situ hybridization studies demonstrate cell-specific expression of LePT1 in the tomato root. The LePT1 mRNA is detectable in peripheral cell layers such as rhizodermal and root cap cells. Under Pi-deprivation conditions, mRNA levels are also detectable in young stelar tissue. This work presents molecular and biochemical evidence for distinct root cells playing an important role in Pi acquisition at the root/soil interface.