Characterization of ammonium absorption by ammonium-preferential cassava

Cassava plants can adapt to poor soils where most other crops are unable to grow normally, suggesting that they are able to efficiently uptake and utilize nutrient elements from the soils. However, little is known about the mechanism of nutrient efficiency in the crop. Herein, we report that cassava grows better under low concentration of mixed nitrogen sources (0.15 mM NH₄NO₃) than under normal nitrogen levels. Furthermore, a low concentration of ammonium (NH₄⁺) was sufficient for cassava plants, suggesting that cassava may efficiently absorb NH₄⁺ in the high-affinity concentration range. AMT1 transporters are involved in high-affinity NH₄⁺ uptake in plants. Four AMT1-type genes were cloned from cassava plants, and all four MeAMT1 transporters (MeAMT1; 1-MeAMT1; 3, MeAMT1; 5) were found to localize at the plasma membrane. Of them, expression of MeAMT1; 1, MeAMT1; 3 and MeAMT1; 5 restored growth of a yeast mutant strain and an Arabidopsis mutant line lacking primary ammonium transporters under ammonium deficiency. More interestingly, both NH₄⁺ absorption mediated by MeAMT1; 5 in transgenic yeast cells and NH₄⁺ influx at cassava roots displayed a two-phase pattern characterized by high- and low-affinity. In particular, the constant of high-affinity ammonium uptake mediated by MeAMT1; 5 is similar to the Km value of high-affinity ammonium absorption at cassava roots, but also close to the ammonium concentration of most soils, suggesting that cassava can efficiently capture low amounts of NH₄⁺ from soils via plasma membrane-bound AMT1-type ammonium transporters, allowing the crop to grow and develop very well in low-nitrogen soils.