The role of OsAKT1 in ammonium uptake and osmotic stress tolerance in relation to nitrogen supply

Nitrogen is an essential plant macronutrient: it not only affects the growth, development and yield of plants, but also contributes to stress tolerance. OsAKT1, a rice homolog of a K+ channel AKT1 at the plasma membrane of root hair cells, is permeable to ammonium and therefore potentially plays a role in the uptake of this nutrient. Beside taking part mainly in K+ uptake in the roots, OsAKT1 is also involved in drought and osmotic stress tolerance in rice. Therefore, I investigated the role of NH4+ uptake via OsAKT1 and the function of the channel in osmotic stress tolerance related to NH4+. Growth experiments which compared the growth of wild-type (WT) and akt1 knockout rice (KO) in media with different predominant form of nitrogen (either NO3- or NH4+) under control and osmotic stress of -0.21 MPa (8% PEG 4,000) showed that the OsAKT1 role in osmotic stress tolerance is not associated with NH4+. Nevertheless, NH4+ depletion experiments illustrated that OsAKT1 functions in NH4+ uptake at low external NH4+ concentrations of 0.2 mM. Depletion assays at 2 mM NH4+ and the complementation growth studies in yeast mutants lacking NH4+-specific uptake systems (Δmep1-3) also suggested the possibility of NH4+ uptake at high NH4+ concentrations via OsAKT1. In all, the data imply that OsAKT1 takes up small amounts of NH4+ irrespective of external NH4+ concentrations.