Characterization of the ammonium transporter family AMF1 in maize

Abstract

Ammonium is an important inorganic nitrogen source utilized by plants. The uptake of ammonium by plant roots involves two types of ammonium transport pathways, the high-affinity ammonium transporters (HATS) and the low-affinity ammonium transporters (LATS). Genes encoding HATS proteins have been studied well in plants while the LATS pathway is not sufficiently understood. Recently, through a transcriptional linkage to a membrane bound transcription factor, bHLHm1, a novel family of ammonium transporters, AMF1 (ammonium facilitator 1) were discovered. Heterologous expression of yeast and soybean AMF1 orthologs in Xenopus laevis oocytes, indicates AMF1 proteins increase the level of low-affinity ammonium transport. In this study the functional activity of two orthologs of ScAMF1 in maize (ZmAMF1;1 and ZmAMF1;2) were investigated. This study has provided an initial understanding of AMF1’s function in maize through the following outcomes: 1) In yeast, both ZmAMF1;1 and ZmAMF1;2 were capable to rescue growth of a K+-transport mutant CY162 when external ammonium concentration was high but potassium supply was limiting. It is predicted that ZmAMF1;1 and ZmAMF1;2 act as putative NH4+/ H+ antiporters in yeast to facilitate ammonium efflux in the vacuole to the cytoplasm, but the mechanism behind this remains unclear. 2) In maize, loss of ZmAMF1;1 and ZmAMF1;2 activity resulted in a significant increase in short-term ammonium uptake, increased shoot development and root nitrogen content. 3) ZmAMF1;1 and ZmAMF1;2 were both localized in the vascular cylinder in maize roots starved of nitrogen. Both genes were significantly upregulated (around 3.3 to 3.7-fold) in shoot tissues under nitrogen starvation but not downregulated with nitrogen resupply (4 hrs). 4) ZmAMF1;1 and ZmAMF1;2 had different expression profiles in maize where ZmAMF1;1 was more expressed in shoot tissues especially during the reproductive stage, while ZmAMF1;2 expression was primarily in the roots.