Regulatory function of PP2C phosphatases in plant ion homeostasis

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Ammonium is an ubiquitous key nutrient in agricultural soils that is taken up by microorganisms and plants via high-affinity transporters of the ammonium transporter / rhesus glycoprotein (AMT/Rh) family. The precise localization of each AMT member and the transcriptional, post-transcriptional and post-translational regulation contribute to the adequate ammonium nutrition. In Arabidopsis thalina, six AMT genes are identified, which separate in two distinct clades (AMT1 and AMT2) and code for transporters with distinct functional properties. Current knowledge suggests that the activity of individual AMTs is regulated in a complex, complementary manner. An allosteric switch by phosphorylation at the C-terminus has been identified that regulates the activity of AMT1s. The phosphorylation of a conserved threonine in the AMT carboxyl-terminus inactivates the transport of the AMT1 trimer. This phosphorylation is directly ammonium dependent and increases rapidly after ammonium resupply. Preliminary work in the frame of the project "Plant ammonium transporter (AMT) integration in regulatory Networks" identified CIPK23 as AMT1 phosphorylating and regulating kinase. The results further identified two PP2C phosphatases as potential regulators of AMT1 transporters. The complex AMT1 regulation cascade as a whole is not completely decoded yet. It is still unclear if and in which way the identified PP2Cs might act in this cascade. These candidates will therefore be characterized in this project. The results will yield a better comprehension of the molecular basis of NH4+, K+ and NO3- ion homeostasis in plants.