PtAAP11, a high affinity amino acid transporter specifically expressed in differentiating xylem cells of poplar

Amino acids are the currency of nitrogen exchange between source and sink tissues in plants and constitute a major source of the components used for cellular growth and differentiation. The characterization of a new amino acid transporter belonging to the amino acid permease (AAP) family, AAP11, expressed in the perennial species Populus trichocarpa is reported here. PtAAP11 expression analysis was performed by semi-quantitative RT-PCR and GUS activity after poplar transformation. PtAAP11 function was studied in detail by heterologous expression in yeast. The poplar genome contains 14 putative AAPs which is quite similar to other species analysed except Arabidopsis. PtAAP11 was mostly expressed in differentiating xylem cells in different organs. Functional characterization demonstrated that PtAAP11 was a high affinity amino acid transporter, more particularly for proline. Compared with other plant amino acid transporters, PtAAP11 represents a novel high-affinity system for proline. Thus, the functional characterization and expression studies suggest that PtAAP11 may play a major role in xylogenesis by providing proline required for xylem cell wall proteins. The present study provides important information highlighting the role of a specific amino acid transporter in xylogenesis in poplar.