The function of OsAKT2 in K⁺ uptake and the aboveground K⁺ reflux of rice

Objectives The electrophysiological functions and regulatory characteristics of OsAKT2 were investigated to unravel its K⁺ uptake mechanisms and its potential influence on the aboveground K⁺ reflux .

Methods The Phylogenetic tree and sequence alignment were constructed to analyze the homology of Shaker potassium channel genes. The two-electrode voltage-clamp experiments were performed on OsAKT2-expressing Xenopus oocytes to study the electrophysiological characteristics of OsAKT2. The relationship between OsAKT2 gene expression and K⁺ concentration, NH₄⁺ concentration, and stress conditions in rice was studied.

Results The phylogenetic analysis showed that OsAKT2 was closest related to the weakly rectifying potassium channel with 56% to AtAKT2. The results showed that OsAKT2 function mainly as an inwardly rectifying K⁺ channel with strong voltage dependency. However, the outward activity of a typical leak-like AKT2 channel was notably suppressed. The electrophysiological results indicated that OsAKT2 was a typical low-affinity potassium channel (Km = 43 mmol/L, >1 mmol/L) with high selectivity for K⁺. Compared with the Shaker-type channel, OsAKT2 was inhibited by Ba²⁺, but the inhibition rate was lower (<78%). OsAKT2 had some permeability to NH₄⁺ (about 22% of K⁺ uptake). The gene expression abundance of OsAKT2 increased significantly under nitrogen deficiency, potassium deficiency, and sorbitol conditions, and had a circadian rhythm whereby the gene expression level was higher in the dark.

Conclusions This study suggests that OsAKT2 mainly mediate the K⁺ uptake in rice, which is sensitive to the change of external K⁺ concentration. OsAKT2 may play an important role in plant adaptation to stress, which also provides a way to improve the reflux of K⁺. Further, OsAKT2 has a potential contribution to ammonium transport in rice.