Characterization of AMT-Mediated High-Affinity Ammonium Uptake in Roots of Maize (Zea mays L.)

Abstract

High-affinity ammonium uptake in plant roots is mainly mediated by AMT1-type ammonium transporters, and their regulation varies depending on the plant species. In this study we aimed at characterizing AMT-mediated ammonium transport in maize, for which ammonium-based fertilizer is an important nitrogen (N) source. Two ammonium transporter genes, ZmAMT1;1a and ZmAMT1;3, were isolated from a maize root-specific cDNA library by functional complementation of an ammonium uptake-defective yeast mutant. Ectopic expression of both genes in an ammonium uptake-defective Arabidopsis mutant conferred high-affinity ammonium uptake capacities in roots with substrate affinities of 48 and 33 μM for ZmAMT1;1a and ZmAMT1;3, respectively. In situ hybridization revealed co-localization of both ZmAMT genes on the rhizodermis, suggesting an involvement in capturing ammonium from the rhizosphere. In N-deficient maize roots, influx increased significantly while ZmAMT expression did not. Ammonium resupply to N-deficient or nitrate-pre-cultured roots, however, rapidly enhanced both influx and ZmAMT transcript levels, revealing a substrate-inducible regulation of ammonium uptake. In conclusion, the two rhizodermis-localized transporters ZmAMT1;1a and ZmAMT1;3 are most probably the major components in the high-affinity transport system in maize roots. A particular regulatory feature is their persistent induction by ammonium rather than an up-regulation under N deficiency.

An animal homolog of plant Mep/Amt transporters promotes ammonia excretion by the anal papillae of the disease vector mosquito,Aedes aegypti

“…While plants and microorganisms typically scavenge for ammonia in order to incorporate the nitrogen into proteins, animals excrete it as a toxic by-product of metabolic protein breakdown. Members of the methylammonium permease/ammonium transporter (Mep/Amt) family of transporters play a critical role in ammonia uptake by plants and microorganisms (Thornton et al, 2006;Nygaard et al, 2006;Willmann et al, 2007;Gu et al, 2013;Wu et al, 2015). The annotated genomes of vertebrate animals lack Mep/Amt homologs and excretion of ammonia in animals has been linked to members of a related family of glycoproteins, the Rhesus glycoproteins (RhGPs) (Nawata et al, 2007;Braun et al, 2009;Shih et al, 2013).…”

“…The RhGPs have also been implicated in conducting CO 2 across cellular membranes (Soupene et al, 2004;Kustu and Inwood, 2006;Peng and Huang, 2006;Perry et al, 2010). In contrast, evidence suggests that the plant and microorganism Mep/Amts transport NH 4 + (Willmann et al, 2007;Yuan et al, 2009;Gu et al, 2013;Wu et al, 2015). The exact mechanism of transport is the subject of debate; however, one study proposes that AmtB recruits NH 4 + and as the NH 4 + travels further into the channel it is deprotonated yielding NH 3 (Khademi et al, 2004).…”

Nitrogen Nutrition, Its Regulation and Biotechnological Approaches to Improve Crop Productivity

“…In tomato AMT1; 2 gene expression has been found to be strongly up-regulated by nitrate [30] [218]. Genes coding for AMT1; 2 and AMT1; 3 transporters were found to be induced by sucrose and diurnal changes [382].…”

“…However, in rice two additional proteins, AMT2; 1 and AMT5.1 were found to uptake ammonium [59] [218]. AMT1; 1, AMT1; 2, AMT1; 3 and AMT1; 5 are high affinity ammonium transporters while AMT2; 1 and AMT5; 1 are low affinity ammonium transporters.…”

“…AMT1; 1 and AMT2; 1 are constitutively expressed but ammonium increases expression of these genes in roots. AMT1; 2 is an ammonium inducible protein while AMT1; 3, and AMT1; 5 are repressed by nitrogen [162] [218]. In Arabidopsis all AMT proteins are constitutively expressed but in maize, AMT1; 1 and AMT1; 3 are ammonium inducible [219].…”