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 lib rary of tomato (Lycopersicon esculentum Mill.). The corresponding prot ein belongs to the growing family of ion transporters with twelve puta tive transmembrane domains. It is highly homologous to recently isolat ed Pi transporters from higher plants, yeast and fungi. When expressed in a Pi-uptake-deficient yeast mutant, the L. esculentum phosphate tr ansporter 1 (LePT1) protein exhibits an apparent K-m of 31 mu M. The t ransporter is still active at submicromolar Pi concentrations and medi ates 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 demonstra te cell-specific expression of LePT1 in the tomato root. The LePT1 mRN A is detectable in peripheral cell layers such as rhizodermal and root cap cells. Under Pi-deprivation conditions, mRNA levels are also dete ctable in young stelar tissue. This work presents molecular and bioche mical evidence for distinct root cells playing an important role in Pi acquisition at the root/soil interface.