TY  - THES
T1  - Plant ammonium transporter (AMT) integration in regulatory networks
A1  - Straub,Tatsiana
Y1  - 2016/08/15
N2  - Ammonium is a ubiquitous key nutrient in agricultural soils and the preferred nitrogen source for plants. However, excessive ammonium accumulation represses plant growth and development. Ammonium is taken up by plant cells via high-affinity ammonium transporters (AMTs). Six AMT genes were identified in Arabidopsis, which are separated in two distinct clades, five AMT1s and one AMT2. In the plasma membrane, AMT proteins form homo- and heterotrimers with extra-cytoplasmic N-termini and cytoplasmic C-termini. In addition to transcriptional and post-transcriptional control of AMTs by ammonium, phosphorylation in the C-terminus serves as a rapid allosteric switch of the AMT activity and prevents further internal ammonium accumulation.
In a physiological screen, a kinase (CIPK23) was identified, which directly regulates ammonium transport activity under high-NH4+ conditions. Interestingly, CIPK23 is already known to regulate nitrate and potassium uptake in roots. Lesion of the CIPK23 gene significantly increased ammonium uptake, but caused growth inhibition. As expected, cipk23 plants were also limited in potassium accumulation, but high potassium availability failed to rescue the cipk23 phenotype. Furthermore, cipk23 plants were more susceptible to methylammonium (MeA), a non-metabolizable analogue of ammonium. The sensitivity to MeA was lost upon genetic suppression of AMT1 genes in the cipk23 background.
The data suggest that CIPK23 directly phosphorylates AMT1s in a complex with CBL1 (calcineurin B-like protein) and thereby regulates transport activity. The expression of the CIPK23 and the CBL1 genes were ammonium-dependent and increased when N-starved plants were resupplied with ammonium. Furthermore, cbl1 mutants had enhanced NH4+ accumulation; this phenocopies the larger ammonium uptake in the cipk23 loss-of-function mutant. In vivo experiments demonstrated bimolecular interaction between CIPK23, AMT1;1, and AMT1;2, but not with AMT2;1, suggesting direct phosphorylation of AMT1-type ammonium transporters by CIPK23.
However, Western blot analysis with the cipk23 mutant suggested that the loss of the kinase was not sufficient to completely abolish AMT1;1 and AMT1;2 phosphorylation, indicating several independent pathways to regulate ammonium transport activity in AMT trimers.
The data identify complex post-translational regulation of ammonium transporters via the CBL1–CIPK23 pathway, which ensures reduction of AMT1 activity and suppression of ammonium uptake under high external NH4+ concentrations.
KW  - Pflanzen
KW  - Ernährung
KW  - Ammoniumverbindungen
KW  - Phosphorylierung
CY  - Hohenheim
PB  - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
AD  - Garbenstr. 15, 70593 Stuttgart
UR  - http://opus.uni-hohenheim.de/volltexte/2016/1247
ER  -