Ammonium uptake in plant roots is mediated by AMT/MEP/Rh-type ammonium transporters.Out of five AMTs being expressed in Arabidopsis roots,four AMT1-type transporters contribute to ammonium uptake,whereas no physiological function has so far been assigned to the only homolog belonging to the MEP subfamily,AMT2;1.Based on the observation that under ammonium supply,the transcript levels of AMT2;1 increased and its promoter activity shifted preferentially to the pericycle,we assessed the contribution of AMT2;1 to xylem loading.When exposed to 15N-labeled ammonium,amt2;1 mutant lines translocated less tracer to the shoots and contained less ammonium in the xylem sap.Moreover,in an amt1;1 amt1;2 amt 1;3 amt2;1 quadruple mutant (qko),co-expression of AMT2;1 with either AMT1;2 or AMT1;3 significantly enhanced 15N translocation to shoots,indicating a cooperative action between AMT2;1 and AMT1 transporters.Under N deficiency,proAMT2;1-GFP lines showed enhanced promoter activity predominantly in cortical root cells,which coincided with elevated ammonium influx conferred by AMT2;1 at millimolar substrate concentrations.Our results indicate that in addition to contributing moderately to root uptake in the low-affinity range,AMT2;1 functions mainly in root-to-shoot translocation of ammonium,depending on its cell-type-specific expression in response to the plant nutritional status and to local ammonium gradients.