Characterisation of the low affinity ammonium transporters AMF in maize

W Li1, J Dechorgnat1, SD Tyerman2 and BN Kaiser1

  1. Faculty of Agriculture and Environment, The University of Sydney, New South Wales 2570
  2. School of Agriculture Food and Wine, The University of Adelaide, South Australia 5064

Ammonium uptake and redistribution involves high and low affinity transport. The physiological and molecular activities of high-affinity ammonium transporters have been examined in numerous plant species. Conversely, molecular information about low-affinity ammonium transport systems is limited. Recently, a family of ammonium transport proteins called AMF1 (ammonium facilitator 1) were identified in both yeast and soybean by their genetic linkage with a membrane bound soybean transcription factor, bHLHm1. ScAMF1 and GmAMF3 homologs were shown to transport ammonium at high concentrations when expressed in yeast cells or Xenopus laevis oocytes. In maize two AMF1 genes, ZmAMF1;1 and ZmAMF1;2 were identified through sequence-based homology, and they will be characterised in this study. In contrast to ScAMF1 and GmAMF3, ZmAMF1;1 and ZmAMF1;2 failed to accumulate methyl ammonium in a yeast mutant defective in ammonium uptake. This result suggested ZmAMF1;1 and ZmAMF1;2 may have other transport activities besides that of ammonium. Currently ZmAMF1;1 and ZmAMF1;2 are being evaluated for potential roles in sugar transport in yeast mutant strains defective in sucrose and glucose uptake. In maize, the expression of both ZmAMF1;1 and ZmAMF1;2 were found to be upregulated by N starvation, and repressed by N resupply. A complimentary reverse genetics study is being conducted with a collection of Mu transposon insertions within the loci containing either ZmAMF1;1 or ZmAMF1;2 (B. Meeley, DuPont Pioneer). These maize lines have finished a backcrossing with the B73 and Gaspe backgrounds for three generations, and have been self-crossed and selected for homozygotic Mu insertions. The characterisation of these loss-of-function ZmAMF1;1 and ZmAMF1;2 lines will be presented.