Institut für Kulturpflanzenwissenschaften

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Regulation of ammonium transport in Arabidopsis thaliana
(2022) Ganz, Pascal; Ludewig, Uwe
The overarching question of this thesis deals with how plants ensure the selective uptake of ammonium while maintaining ion and pH homeostasis. A key component of this is ammonium transporters (AMTs) with high affinity towards their substrate, which are at the same time part of a multilayered protection system against uncontrolled ammonium influx. Conserved protein sequences inside the transporter were analyzed as well as the regulatory system based on post-translational modification of the transporter.
Transporters mediating ammonium uptake in plants and their regulation by the abiotic stress signaling pathway
(2023) Porras Murillo, Romano; Ludewig, Uwe
Nitrogen nutrition refers to the uptake, assimilation, and utilization of ammonium, nitrate, and organic nitrogen sources. Ammonium is energetically a more cost-effective nitrogen source than nitrate but can be toxic for plants, and its use by plants is regulated at different levels. Ammonium transporters (AMTs) take up ammonium and are localized primarily in plant roots, working as trimers in the plasma membrane. Under high external ammonium concentrations, phosphorylation in AMTs C-termini shuts down transport to avoid toxicity. This phosphorylation is performed by CIPK23, a kinase shown to be inhibited by Clade A PP2Cs. This study aimed to characterize AMTs from wheat and analyze their transcriptional response to ammonium. Another aim was to determine the role of clade A PP2Cs and PYR/PYL receptor proteins for abscisic acid in ammonium nutrition. Chapter I describes the physiological responses of winter wheat to different nitrogen sources and ammonium concentrations. The plants mainly used root morphological responses to adapt to differences in the nitrogen source. High external concentrations of ammonium reduced plant growth, while these conditions induced the expression of TaAMT1;1 and TaAMT1;2. In Chapter II, we studied the capacity of TaAMT2s to transport ammonium and their transcriptional responsiveness to ammonium nutrition. From the six TaAMT2s, only TaAMT2;1 could transport ammonium in a yeast complementation line. Besides, its expression in roots is lower under ammonium than under nitrate. The expression pattern among the remaining TaAMT2s (TaAMT2;2-TaAMT2;6) is similar, with higher expression under ammonium, in both roots and leaves, compared to nitrate. Chapter III focused on the role of the PP2C phosphatase ABI1 (ABA-insensitive 1) in ammonium nutrition and the effect of external ammonium concentrations on ABA concentrations. Ammonium increased ABA concentrations in roots by activating ABA-GE, meaning ammonium toxicity could be sensed as abiotic stress through ABA. Without ammonium, ABI1 dephosphorylates AMTs and inhibits CIPK23; with ammonium, ABA-PYR/PYL complex-mediated inhibition of ABI1 releases CIPK23 to phosphorylate AMTs and avoids ammonium toxicity. Finally, in Chapter IV, we studied the role of AIP1 and its ammonium-dependent regulator, PYL8, in nitrogen nutrition. We described the function of AIP1, which was redundant to ABI1 in AMT regulation. Based on ammonium-dependent root architecture changes, and higher auxin accumulation in pyl8-1 root tips compared to the wild type, we suggest that PYL8 is involved in root-phenotype modulation in an ammonium-dependent manner.

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